Pharmacy & Pharmacology Free guide

Veterinary Drug Calculations: Doses, Volumes, Percent Solutions and CRIs

Pharmacy and Pharmacology (D1 C; K7, K13) 15 min read, 3,381 words Last verified

Work every VTNE drug calculation the same way: convert the weight, read the concentration, cancel the units. Covers doses, tablets per course, percent solutions, dilutions, drip rates, CRIs and insulin syringes.

Introduction

Every shift, a veterinary technician turns a veterinarian's order into something real: a volume in a syringe, a number of tablets in a vial, a rate on a fluid pump or a bag of fluids with a drug added. The math is simple, but one slipped decimal can give a patient ten times the intended dose. The VTNE expects you to do these calculations quickly and correctly, and so does every clinic you work in.

This guide covers unit conversions, dimensional analysis, mg/kg doses, tablet counts for a course of treatment, percent solutions, dilutions, fluid rates in mL/h and drops per minute, constant rate infusions (CRIs) and U-40 versus U-100 insulin syringes. Each section ends with a method you can use on the exam and at the treatment table.

Key facts

  • 1 kg = 2.2 lb. Divide pounds by 2.2 to get kilograms.2
  • Dose (mg) = body weight (kg) x dosage (mg/kg). Volume (mL) = dose (mg) / concentration (mg/mL).7
  • A 1% solution contains 1 g in 100 mL, which is 10 mg/mL. A 2% lidocaine injection is 20 mg/mL.78
  • Dilutions follow C1 x V1 = C2 x V2: the amount of drug does not change when diluent is added.711
  • Drops per minute = mL/h x drop factor (gtt/mL) / 60 min/h. Macrodrip sets deliver 10, 12, 15 or 20 drops/mL; microdrip sets deliver 60 drops/mL, so mL/h equals drops/min.612
  • A CRI is solved in four steps: drug per hour, hours per bag, total drug, then mL of stock to add.7
  • U-40 insulin is 40 units/mL and U-100 is 100 units/mL. Each must be drawn up with its matching syringe.16
  • Write a leading zero (0.5 mL, never .5 mL) and no trailing zero (3, never 3.0).13

How the VTNE tests this

  • Calculate medications, fluid therapy rates and CRIs from the veterinarian's order (Domain 1 Task C). Pharmacy and Pharmacology carries 20 of the 150 items, or 13%.1
  • Applied mathematics: metric weights and measures, percentage solutions and dosage calculations (K7).1
  • Fluid balance and therapy, including calculation (K13): mL/h, drops per minute and infusion time.1
  • Expect short word problems: a weight in pounds, a dosage in mg/kg, a concentration in mg/mL or percent, and four answers that differ by a factor of 10 or by a skipped conversion.
  • Expect a dispensing item: how many tablets for a full course, with half tablets allowed or not.

Core concepts

Units and conversions

Veterinary drugs are dosed per kilogram of body weight, usually as mg/kg, mcg/kg or g/kg.3 Each metric step between kilogram, gram, milligram and microgram is a factor of 1,000: 1 kg = 1,000 g, 1 g = 1,000 mg and 1 mg = 1,000 mcg (micrograms, also written ug or with the Greek letter mu).323 For volume, 1 L = 1,000 mL and 1 mL = 1 cc.13 For fluid, 1 mL weighs about 1 g, so a patient given 500 mL of fluid IV should gain about 0.5 kg.2

Some US labels dose in pounds. The carprofen chewable label, for example, gives 2 mg/lb once daily or 1 mg/lb twice daily for dogs.19 Multiply by 2.2 to change a per-pound dosage into a per-kilogram one: 2 mg/lb x 2.2 lb/kg = 4.4 mg/kg. Always check which unit the order and the label use before you start.

Dimensional analysis

Dimensional analysis (also called the factor-label method) arranges conversion factors as fractions so that unwanted units cancel and only the answer's unit remains.411 A nursing math text lists five steps: identify the units you want, gather the needed information, set up the equation and check the units, calculate, then round and verify.4 Do not round in the middle of a problem, only at the end. Drop rates are usually rounded to whole drops.4

Units are your safety net. If the units in your setup do not cancel to the unit the question asks for, the setup is wrong, even if the arithmetic is perfect.

Concentration: mg/mL, percent and units

Concentration is the amount of drug in each mL. Injectable labels usually give mg/mL. Many solutions are labeled in percent, which for drugs means weight in volume (w/v): grams of drug per 100 mL of solution.7 So 1% is 1,000 mg in 100 mL, or 10 mg/mL.7 The shortcut is percent x 10 = mg/mL. Labels confirm it: lidocaine 1% is 10 mg/mL and 2% is 20 mg/mL; dextrose 5% is 5 g/100 mL or 50 mg/mL.89

Insulin is measured in units, not milligrams. U-40 means 40 units in each mL and U-100 means 100 units in each mL.16

Dilution

Adding diluent lowers the concentration but does not change the amount of drug. That is why C1 x V1 = C2 x V2 works, where C1 and V1 are the stock concentration and the volume of stock you use, and C2 and V2 are the final concentration and final volume.11 Any units can be used as long as both sides match, for example percent and mL.117

Ratio wording is a trap. One feline guideline says to dilute 50% dextrose "1:2" to make a 25% solution, which is 1 part stock in 2 parts total.22 A pediatric fluid review calls 12.5% dextrose "a 1:3 dilution of 50% dextrose with sterile water", which is 1 part stock plus 3 parts water.20 When a ratio is given, confirm the final concentration with C1 x V1 = C2 x V2.

Fluid rates

Fluid orders are written as a volume over time. Volume to be infused (mL) divided by hours gives mL/h.12 A fluid pump is the safest and most efficient way to run IV fluids; gravity flow is the fallback when no pump is available.12 Gravity sets have a calibrated drip chamber, and the drop factor (drops per mL) is printed on the package.612

Maintenance needs are often estimated with 30 x body weight (kg) + 70 = mL per 24 hours for dogs and cats between 2 and 70 kg. Outside that range, Merck gives 132 x BW0.75 for dogs and 80 x BW0.75 for cats.14 The veterinarian sets the rate; your job is to turn it into a pump setting or a drip rate and to check it.

Constant rate infusions

A CRI delivers a drug continuously at a dose per unit of time, such as mcg/kg/min or mg/kg/h. To find the pump rate, you need the patient's weight, the dosage, the drug concentration, the fluid volume and the fluid rate.7 The drug is either added to a bag of fluids that runs at a set rate, or diluted in a syringe for a syringe pump. A syringe pump is ideal for very small volumes, such as a CRI in a neonate.20

Procedure or technique

Use the same routine for every calculation, on paper or on the exam.

  1. Read the whole order. Note the drug, dosage and units (mg/kg, mcg/kg/min, units), route, frequency and duration.
  2. Weigh the patient and convert to kg. Divide pounds by 2.2.2 Use the weight recorded for this visit.
  3. Read the label for concentration. Change percent to mg/mL (percent x 10) before you calculate a volume.7
  4. Decide what the answer's unit is. mg, tablets, mL, mL/h, drops/min or mL of drug to add to a bag. Figure 1 shows which formula fits.
  5. Set up with units and cancel them. Keep full numbers until the last step.4
  6. Check that the answer is reasonable. A volume of 30 mL for a cat injection, or 0.003 mL for a large dog, signals a decimal error. If a dose range is given, work out both ends and choose what can be measured or dispensed.3
  7. Double-check before you start. Recheck every CRI calculation before the infusion begins.7 If your hospital requires a second person to check high-risk drugs, do that check now. A correct calculation does not by itself prove that the order is right for the patient.4
  8. Record it. Write fluid orders as mL/h, mL/day and drops/min so that others can spot discrepancies, and record the volume actually delivered.12
Unit ladder for mass, volume, weight and percent conversions kg g mg mcg L mL (1 mL = 1 cc) lb / 2.2 = kg 1% = 10 mg/mL down: x 1000 up: / 1000 down: x 1000 up: / 1000 down: x 1000 up: / 1000 x 1000 / 1000 Smaller unit, bigger number. Larger unit, smaller number.
Figure 2. The unit ladder. Each step between kg, g, mg and mcg is a factor of 1,000, and so is the step between L and mL. Moving down to a smaller unit always makes the number larger. Pounds divided by 2.2 give kilograms, and a percent solution times 10 gives mg/mL.
Table 1. Conversion factors you will use most
ConvertFactorExample
Pounds to kilograms1 kg = 2.2 lb (divide lb by 2.2)33 lb / 2.2 = 15 kg
Kilograms to grams1 kg = 1,000 g0.35 kg = 350 g
Grams to milligrams1 g = 1,000 mg0.005 g = 5 mg
Milligrams to micrograms1 mg = 1,000 mcg0.5 mg = 500 mcg
Liters to milliliters1 L = 1,000 mL; 1 mL = 1 cc0.25 L = 250 mL
Fluid volume to weight1 mL of fluid weighs about 1 g500 mL IV, about 0.5 kg gained
Hours to minutes1 h = 60 min15 mL/h = 0.25 mL/min
Per pound to per kilogrammg/lb x 2.2 = mg/kg2 mg/lb = 4.4 mg/kg

Sources: OSU CVM Clinical Skills Handbook; Kirk and Bistner's Handbook, Charts and Tables; WisTech Open Nursing Dosage Calculations; Rimadyl label.213519

Table 2. Percent solution equivalents (w/v, grams per 100 mL)
PercentGrams per 100 mLmg/mLExample product
0.9%0.9 g9 mg/mL0.9% sodium chloride
1%1 g10 mg/mLLidocaine 1%
2%2 g20 mg/mLLidocaine 2%
5%5 g50 mg/mLDextrose 5%
20%20 g200 mg/mLAcetylcysteine 20%
50%50 g500 mg/mLDextrose 50%

Sources: Mages, Medical Math Mentality; DailyMed labels for lidocaine, dextrose 5% and veterinary dextrose 50%; Kirk and Bistner's drug table (acetylcysteine). The 0.9% row is the same arithmetic.7891013

Table 3. Drip set factors and the drops-per-minute shortcut
Drop factorTypeShortcut: gtt/min =At 120 mL/h
10 gtt/mLMacrodripmL/h / 620 gtt/min
15 gtt/mLMacrodripmL/h / 430 gtt/min
20 gtt/mLMacrodripmL/h / 340 gtt/min
60 gtt/mLMicrodripmL/h (same number)120 gtt/min

Sources: WisTech Open Nursing Skills 2e (drop factors); Kirk and Bistner's Handbook, Emergency Care (60 drops/mL means mL/h equals drops/min). Each shortcut is 60 divided by the drop factor.612

Flowchart: which formula do I need?

Decision tree: choosing the dose, volume, dilution, rate or CRI formula Yes Yes Yes Yes Yes No No No No No Weigh, convert lb to kg, read the label What unit must the answer be in? Answer in mg or tablets? Answer in mL of a drug? Making a weaker solution? Answer in mL/h or drops/min? Drug dosed per min or per hour? DOSE kg x mg/kg = mg mg / mg per tab = tablets VOLUME mg / (mg/mL) = mL % x 10 = mg/mL first DILUTION C1 x V1 = C2 x V2 solve V1, fill to V2 RATE mL / h = mL/h mL/h x gtt/mL / 60 CRI mg/h, then hours per bag, total mg, then mL to add Stop: ask the veterinarian to clarify
Figure 1. Which formula do I need? Start from the unit the answer must be in. Each "yes" leads to one formula. If the order fits none of them, the order is unclear: ask the veterinarian before you draw anything up.

Species differences

  • Dogs: weights span from under 2 kg to over 70 kg, so the same mg/kg order can mean 0.05 mL or 5 mL. The 30 x BW + 70 maintenance formula applies from 2 to 70 kg; use 132 x BW0.75 outside that range.14
  • Cats: small volumes make decimal errors more likely. The cat maintenance formula is 80 x BW0.75.14 Some CRI ranges are lower than in dogs: for lidocaine as an antiarrhythmic, Merck lists 25 to 75 mcg/kg/min in dogs and 10 to 20 mcg/kg/min in cats.25 Merck's feline analgesic table does not recommend a lidocaine CRI for pain, because cats are sensitive to the drug's nervous system and cardiovascular effects.15 So never reuse a dog CRI recipe for a cat. Vetsulin and ProZinc are both U-40 insulins.1718
  • Neonates and very small patients: pediatric patients have higher fluid requirements than adults but are more easily volume overloaded. A syringe pump is ideal for very small volumes; even microdrip sets are prone to error.20
  • Exotic and small mammals are often weighed in grams. Convert to kg first: 350 g = 0.35 kg.
  • Horses and cattle: the same math applies, but volumes are large. A decimal error in a 450 kg horse's CRI involves a very large quantity of drug, so a second check matters just as much.

Calculation

Worked example 1: tablets for a course (dog, carprofen). A 75 lb dog is prescribed carprofen at the label dose of 2 mg/lb once daily for 14 days. Tablets are 25, 75 or 100 mg and scored.19

  1. Dose: 75 lb x 2 mg/lb = 150 mg per day (lb cancels). In kg: 75 lb / 2.2 = 34.1 kg; 34.1 kg x 4.4 mg/kg = 150 mg.
  2. Tablets per dose: 150 mg / 75 mg per tablet = 2 tablets (mg cancels).
  3. Course: 2 tablets/day x 14 days = 28 tablets of 75 mg.

Worked example 2: mL of a percent solution. The veterinarian orders lidocaine 2 mg/kg IV for a 20 kg dog. You have 2% lidocaine.

  1. Dose: 20 kg x 2 mg/kg = 40 mg.
  2. Concentration: 2% x 10 = 20 mg/mL.8
  3. Volume: 40 mg / 20 mg/mL = 2 mL (mg cancels, leaving mL).

Worked example 3: CRI added to a fluid bag. A 20 kg dog is to receive lidocaine at 50 mcg/kg/min (Merck lists 20 to 50 mcg/kg/min as an analgesic CRI in dogs) in a 500 mL bag of fluids running at 50 mL/h. Stock is 2% lidocaine (20 mg/mL).15

  1. Drug per hour: 50 mcg/kg/min x 20 kg x 60 min/h = 60,000 mcg/h = 60 mg/h.
  2. Hours per bag: 500 mL / 50 mL/h = 10 h.
  3. Total drug: 60 mg/h x 10 h = 600 mg.
  4. mL of stock to add: 600 mg / 20 mg/mL = 30 mL of 2% lidocaine.7
  5. Check backward: 600 mg / 500 mL = 1.2 mg/mL; 1.2 mg/mL x 50 mL/h = 60 mg/h = 1,000 mcg/min; 1,000 / 20 kg = 50 mcg/kg/min.

Adding 30 mL to the 500 mL bag makes 530 mL, which lowers the concentration by about 6%. Some hospitals remove the same volume from the bag first. Follow your protocol and label the bag with drug, amount added, final concentration, date, time and initials.

Worked example 4: dilution and drip rate. Make 10 mL of 25% dextrose from 50% dextrose. C1 x V1 = C2 x V2: 50% x V1 = 25% x 10 mL, so V1 = 5 mL of 50% dextrose plus 5 mL of sterile diluent. Each 2 mL then holds 500 mg (0.5 g) of dextrose, which matches the feline guideline's 2 to 4 mL of 25% for 0.5 to 1.0 g.22

Drip rate: 1,000 mL ordered over 8 h is 125 mL/h. With a 15 gtt/mL set: 125 mL/h x 15 gtt/mL / 60 min/h = 31.25, rounded to 31 gtt/min, or about 1 drop every 2 seconds.64

Table 4. Worked examples at a glance
ProblemSetupAnswer
Metoclopramide 0.3 mg/kg, 25 kg dog, 1 mg/mL liquid25 kg x 0.3 mg/kg = 7.5 mg; 7.5 mg / 1 mg/mL7.5 mL (or 1.5 tablets of 5 mg)
Carprofen 2 mg/lb daily, 75 lb dog, 75 mg tablets, 14 days150 mg / 75 mg = 2 tablets/day x 1428 tablets
Lidocaine 2 mg/kg, 20 kg dog, 2% stock40 mg / 20 mg/mL2 mL
10 mL of 25% from 50% dextrose50 x V1 = 25 x 105 mL stock + 5 mL diluent
1,000 mL over 8 h, 15 gtt/mL set125 mL/h x 15 / 6031 gtt/min
Lidocaine CRI 50 mcg/kg/min, 20 kg, 500 mL at 50 mL/h60 mg/h x 10 h = 600 mg; 600 / 20 mg/mL30 mL of 2%
Dobutamine 400 mg in 250 mL at 15 mL/h, 93.18 kg patient1,600 mcg/mL x 0.25 mL/min / 93.18 kg4.3 mcg/kg/min
10 units of U-40 insulin10 units / 40 units/mL0.25 mL

Sources: OSU CVM Drug Dose Calculation (metoclopramide); Rimadyl label; DailyMed lidocaine label; ISFM diabetes guidelines; WisTech Open (drop rate and mcg/kg/min examples); Illinois insulin handling.319822516

Insulin syringes: U-40 versus U-100

The 1-unit mark on a U-40 syringe and on a U-100 syringe each deliver 1 unit of the matching insulin, but the volumes differ.16 On a U-100 syringe, 1 unit is 1/100 mL (0.01 mL). On a U-40 syringe, 1 unit is 1/40 mL (0.025 mL). The Vetsulin label states that using a syringe other than a U-40 syringe will result in incorrect dosing,18 and a practice review states that both Vetsulin and ProZinc must be given with U-40 syringes.17 A "correction factor" to use the wrong syringe confuses owners and risks under- or overdosing.17

See the size of the error. Ten units of U-40 insulin is 0.25 mL. If U-40 insulin is drawn to the 10 mark of a U-100 syringe, the patient gets 0.1 mL, which is only 4 units. If U-100 insulin is drawn to the 10 mark of a U-40 syringe, the patient gets 0.25 mL x 100 units/mL = 25 units, two and a half times the dose.

The same 0.25 mL read on a U-100 syringe, a U-40 syringe and a 1 mL syringe U-100 insulin syringe: 1 mL = 100 units U-40 insulin syringe: 1 mL = 40 units Standard 1 mL syringe 020406080100 010203040 00.250.51 mL Shaded 0.25 mL reads 25 units Same 0.25 mL reads 10 units Same 0.25 mL reads 0.25 mL
Figure 3. One volume, three scales. The shaded 0.25 mL is 25 units on a U-100 syringe, 10 units on a U-40 syringe and 0.25 mL on a standard 1 mL syringe. Insulin syringes are not interchangeable: always match the syringe to the insulin's U number. Schematic only; real syringes carry more graduation marks.
  • Write numbers safely. Use a leading zero (0.5 mL) and avoid trailing zeros (3, not 3.0). Write "units" in full, because "u" can be read as 0, 4 or a mu. Write "per" instead of a slash, which can look like the number 1.1323
  • Errors harm patients. A 2026 review of Finnish malpractice claims found 148 medication errors in 87 cases; 21.8% of those cases ended in death. The drugs most often involved were antibacterials, anti-inflammatory drugs and anesthetics, and most errors arose at the decision-making stage.21
  • Prefer a pump. Fluid pumps are the safest way to run IV fluids, and syringe pumps are ideal for very small volumes.1220 If you must use gravity flow, recheck the drip rate often.
  • Insulin CRIs need priming. One protocol adds regular insulin to a 250 mL bag of 0.9% saline and runs 50 mL through the drip set and lines first, because insulin binds to plastic.24
  • Teach insulin owners. Explain that insulin syringes are not interchangeable: a U-40 syringe for U-40 insulin and a U-100 syringe for U-100 insulin.16 Use a new needle and syringe for every dose.16
  • Teach tablet splitting. Scored tablets such as carprofen chewables are dosed in half-tablet steps, and the label shows how to break them on the score.19 Count the tablets you dispense against the full course.
  • Stay within your role. You calculate and check; the veterinarian decides the drug, dose and rate. If an order looks wrong after you have checked your math, ask.

Common mistakes and exam traps

  • Forgetting to convert pounds. Using 66 lb as if it were 66 kg makes the dose 2.2 times too large.
  • Treating percent as mg/mL. A 2% solution is 20 mg/mL, not 2 mg/mL. That error gives ten times the volume.
  • Mixing mcg and mg. 500 mcg is 0.5 mg. Convert before dividing by a concentration in mg/mL.
  • Forgetting the 60 in drip rates. mL/h must be changed to mL/min. With a 60 gtt/mL set the numbers match, but with 10, 15 or 20 gtt/mL sets they do not.
  • Reading dilution ratios two ways. "1:3" can mean 1 part in 3 total or 1 part plus 3 parts. Check with C1 x V1 = C2 x V2.
  • Counting doses, not tablets. Twice-daily for 7 days is 14 doses. If each dose is 1.5 tablets, the course is 21 tablets.

Quick review

  1. 1 kg = 2.2 lb; divide pounds by 2.2.
  2. kg, g, mg and mcg are each 1,000 apart; 1 L = 1,000 mL; 1 mL = 1 cc.
  3. Dose (mg) = kg x mg/kg.
  4. Volume (mL) = mg / (mg/mL).
  5. Tablets = mg per dose / mg per tablet, then x doses per day x days.
  6. Percent (w/v) = grams per 100 mL; percent x 10 = mg/mL.
  7. C1 x V1 = C2 x V2; the amount of drug stays the same.
  8. mL/h = total mL / hours.
  9. gtt/min = mL/h x drop factor / 60; a 60 gtt/mL microdrip makes gtt/min equal mL/h.
  10. CRI: mg/h, hours per bag, total mg, mL of stock to add; then check backward.
  11. U-40 = 40 units/mL and U-100 = 100 units/mL; match the syringe to the insulin.
  12. Leading zero yes, trailing zero no, and write "units" in full.

Test yourself

5 questions written for this guide. Pick an answer, then check it.

Question 1A 22 lb dog is ordered a drug at 4 mg/kg IV. The vial is labeled 5%. What volume should the technician draw up?

Show answer

Answer: B. 0.8 mL

22 lb / 2.2 = 10 kg; 10 kg x 4 mg/kg = 40 mg. A 5% solution is 5 g/100 mL = 50 mg/mL, so 40 mg / 50 mg/mL = 0.8 mL. A (8 mL) treats 5% as 5 mg/mL. C (1.76 mL) skips the pound to kilogram conversion (22 x 4 = 88 mg). D (0.08 mL) treats 5% as 500 mg/mL.

Source Medical Math Mentality (M3), Mastering Percentage Solutions, IDEXX/BSAVA 2025

Question 2The veterinarian orders 600 mL of lactated Ringer's solution IV over 6 hours. No pump is free, so you use a 20 gtt/mL administration set. What drip rate should you set?

Show answer

Answer: D. 33 gtt/min

600 mL / 6 h = 100 mL/h. 100 mL/h x 20 gtt/mL / 60 min/h = 33.3, rounded to 33 gtt/min. A (100) would be right only for a 60 gtt/mL microdrip set. B (20) is the drop factor, not the rate. C (17) uses a 10 gtt/mL set.

Source 5.14 IV Infusion by Gravity, WisTech Open Nursing Skills 2e

Question 3A cat's owner gives 2 units of ProZinc, a U-40 insulin, twice daily. The owner ran out of U-40 syringes and drew the insulin to the 2-unit mark of a U-100 syringe. How much insulin did the cat receive?

Show answer

Answer: A. 0.8 units

The 2-unit mark on a U-100 syringe holds 2/100 = 0.02 mL. U-40 insulin has 40 units/mL, so 0.02 mL x 40 units/mL = 0.8 units, an underdose. B assumes syringes are interchangeable; they are not. C is the result of the opposite error (U-100 insulin in a U-40 syringe would give 2.5 times the dose). D confuses the volume with the dose.

Source Pharmacist's Corner: Storage and Handling of Insulins, University of Illinois CVM

Question 4You need 20 mL of 1% lidocaine for a local block, but only 2% lidocaine is in stock. How do you prepare it?

Show answer

Answer: C. 10 mL of 2% lidocaine plus 10 mL of diluent

C1 x V1 = C2 x V2: 2% x V1 = 1% x 20 mL, so V1 = 10 mL of stock, made up to a final volume of 20 mL with 10 mL of diluent. A makes 0.2%. B makes 0.5%. D gives 30 mL total, which is about 0.67%, because the diluent was added to the wrong total.

Source 3.3 Molarity, Dilution of Solutions, OpenStax Chemistry 2e

Question 5A 10 kg dog is to receive lidocaine at 30 mcg/kg/min, added to a 250 mL bag of fluids that will run at 25 mL/h. The stock is 2% lidocaine. How much stock should be added to the bag?

Show answer

Answer: B. 9 mL

Drug per hour: 30 mcg/kg/min x 10 kg x 60 min/h = 18,000 mcg/h = 18 mg/h. Hours per bag: 250 mL / 25 mL/h = 10 h. Total drug: 18 mg/h x 10 h = 180 mg. Volume: 180 mg / 20 mg/mL (2%) = 9 mL. D (18 mL) reads 2% as 10 mg/mL. A and C are decimal errors by a factor of 10.

Source Medical Math Mentality (M3), CRIs Calculated in Four Simple Steps, IDEXX/BSAVA 2025

0 of 5 answered

Practise this topic

Two questions from our VTNE practice bank on this topic.

Question 1A 500 mL bag of lactated Ringer's solution must infuse over 4 hours using a 15 gtt/mL drip set. What is the drop rate in gtt/min?

Show answer

Answer: B. 31 gtt/min

For a gravity drip, multiply the volume by the drop factor and divide by the time in minutes. Four hours is 240 minutes, so 500 mL times 15 gtt/mL divided by 240 minutes equals 31.25 gtt/min. Drop rates are rounded to whole drops, which gives 31 gtt/min. The drop factor is printed on the administration set package; macrodrip sets have drop factors such as 10, 12, 15, or 20 gtt/mL and microdrip sets are 60 gtt/mL.

A rate of 25 gtt/min is what you would get with a 12 gtt/mL set, so it uses the wrong drop factor for this set. A rate of 42 gtt/min matches a 20 gtt/mL set. A rate of 125 gtt/min comes from dividing by 60 minutes instead of 240 minutes, which would empty the bag in about 1 hour.

Source 5.14 IV Infusion by Gravity (Nursing Skills 2e), WisTech Open
4.1 Introduction to IV Flow Rates and Infusion Time (Nursing Dosage Calculations), WisTech Open

Question 2A 2.5% solution of a drug contains how many mg per mL?

Show answer

Answer: C. 25 mg/mL

A weight per volume percent means grams per 100 mL. A 2.5% solution therefore holds 2.5 g in 100 mL, which is 2,500 mg in 100 mL, or 25 mg/mL. The shortcut is to move the decimal one place to the right: any percent multiplied by 10 gives mg/mL. For example, mannitol 20% is labeled as 0.2 g/mL, which is 200 mg/mL.

The value 2.5 mg/mL assumes the percent equals mg/mL directly and is ten times too low. The value 0.25 mg/mL is a hundred times too low. The value 250 mg/mL would be a 25% solution, ten times stronger than stated.

Source OSMITROL (mannitol) injection label, DailyMed, US National Library of Medicine
Chapter 7: Solutions and concentrations (Foundations of Biomedical Science), La Trobe University, CAUL OER Collective
5% Dextrose Injection, USP label, DailyMed, US National Library of Medicine

Practise all 25 questions on this topic Free account. Or try the free practice quiz.

Figure credits

  • Figure 1 Original diagram, VTNE Exam Prep
  • Figure 2 Original diagram, VTNE Exam Prep
  • Figure 3 Original diagram, VTNE Exam Prep

References

  1. AAVSB. 2023 Veterinary Technician National Examination Content Outline. aavsb.org (accessed 2026-10-30) aavsb.org/wp-content/uploads/2025/04/AAVSB-2023-Veterinary-Technician-National-Examination-Content-Outline-2.pdf
  2. The Ohio State University College of Veterinary Medicine. English and Metric Conversion. OSU CVM Veterinary Clinical and Professional Skills Center Handbook. ohiostate.pressbooks.pub (accessed 2026-10-30) ohiostate.pressbooks.pub/osuvcpslhandbook/chapter/cp_basics_calchandlingrx_english-and-metric-measurement-conversion/
  3. The Ohio State University College of Veterinary Medicine. Drug Dose Calculation. OSU CVM Veterinary Clinical and Professional Skills Center Handbook. ohiostate.pressbooks.pub (accessed 2026-10-30) ohiostate.pressbooks.pub/osuvcpslhandbook/chapter/cp_basics_calchandlingrx_drug-dose-calculation/
  4. WisTech Open. 4.1 Introduction to IV Flow Rates and Infusion Time. Nursing Dosage Calculations (CC BY 4.0). wtcs.pressbooks.pub (accessed 2026-10-30) wtcs.pressbooks.pub/nursingdosagecalculations/chapter/introduction-to-iv-flow-rates-infusion-time/
  5. WisTech Open. 8.1 Introduction to mcg/kg/min. Nursing Dosage Calculations (CC BY 4.0). wtcs.pressbooks.pub (accessed 2026-10-30) wtcs.pressbooks.pub/nursingdosagecalculations/chapter/introduction-to-mcg-kg-min/
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