A math aid for students and nurses. It does not check whether a dose is appropriate for a patient. Verify every order and follow your facility's policies. Reviewed September 30, 2026.
Dosage calculation formulas
Oral and injectable doses
DH × Q = amount to give
D = desired (ordered) dose, H = dose on hand, Q = quantity it comes in (1 tablet or the mL on the label).
Weight-based dose
dose/kg × weight (kg) = dose
Convert pounds first: kg = lb ÷ 2.2. For a daily dose divided into several doses, divide by the number of doses per day.
Infusion by dose (mcg/kg/min)
mL/hr = dose × kg × 60concentration (mcg/mL)
Concentration = drug in the bag ÷ bag volume. Match units: convert mg to mcg (× 1,000) before dividing.
Infusion by units or mg per hour
mL/hr = ordered units/hrunits/mL
The same logic works for mg/hr with a concentration in mg/mL.
Conversions every nurse needs
| 1 g = 1,000 mg | 1 mg = 1,000 mcg |
| 1 kg = 2.2 lb | 1 L = 1,000 mL |
| 1 tsp = 5 mL | 1 tbsp = 15 mL |
| 1 fl oz = 30 mL | 1 hr = 60 min |
To go from a larger unit to a smaller one (g to mg), multiply. To go from a smaller unit to a larger one (mcg to mg), divide. Convert the order and the supply to the same unit before you use the formula.
Rounding rules
- Scored tablets: round to the nearest half tablet. Do not split unscored tablets.
- Capsules: round to the nearest whole capsule.
- Liquids: round to the nearest tenth of a mL (7.46 mL becomes 7.5 mL); use the hundredth for small doses measured in a 1 mL syringe when your policy says so.
- Drops: round to the nearest whole drop.
- When rounding: 4 or less rounds down, 5 or more rounds up. Round only the final answer, not the steps in between.
Leading and trailing zeros. Write 0.5 mg, never .5 mg, and 5 mg, never 5.0 mg. A missed decimal point turns either one into a tenfold error.
How to solve any dosage problem
- Read the order and the label. Write down what is ordered and what you have, with units.
- Convert so the order and the supply use the same unit (and the weight is in kg).
- Estimate. Is the answer more or less than one tablet or one mL? This catches most setup errors.
- Calculate with the formula or dimensional analysis.
- Check that it makes sense and round by the rules above. An answer such as 15 tablets or 30 mL IM is a signal to recheck.
Dosage calculation practice problems
Try each one, then open it to check your answer. Medications are left unnamed on purpose: the math is what you are practicing.
The order is 500 mg by mouth. Tablets on hand are 250 mg each. How many tablets do you give?
500 ÷ 250 × 1 tablet = 2 tablets.
The order is 0.25 mg. Tablets on hand are 0.125 mg, scored. How many tablets?
0.25 ÷ 0.125 × 1 = 2 tablets.
The order is 375 mg of an oral suspension. The label reads 250 mg per 5 mL. How many mL?
375 ÷ 250 × 5 mL = 7.5 mL.
The order is 40 mg IM. The vial reads 80 mg/mL. How many mL?
40 ÷ 80 × 1 mL = 0.5 mL.
The order is 1.5 g. Capsules on hand are 500 mg. How many capsules?
Convert first: 1.5 g = 1,500 mg. 1,500 ÷ 500 × 1 = 3 capsules.
A child weighs 44 lb. The order is 10 mg/kg per dose. What is the dose in mg?
44 ÷ 2.2 = 20 kg. 20 kg × 10 mg/kg = 200 mg per dose.
An order is 20 mg/kg/day divided every 8 hours for a 15 kg child. How much per dose?
20 × 15 = 300 mg/day. Every 8 hours = 3 doses a day. 300 ÷ 3 = 100 mg per dose.
An infusion is ordered at 5 mcg/kg/min for a 70 kg patient. The bag contains 400 mg in 250 mL. What is the rate in mL/hr?
Concentration: 400 mg ÷ 250 mL = 1.6 mg/mL = 1,600 mcg/mL. Dose per hour: 5 × 70 × 60 = 21,000 mcg/hr. 21,000 ÷ 1,600 = 13.1 mL/hr.
An order is 1,000 units/hr. The bag contains 25,000 units in 250 mL. What is the rate?
Concentration: 25,000 ÷ 250 = 100 units/mL. 1,000 ÷ 100 = 10 mL/hr.
Give 1,000 mL over 10 hours with 20 gtt/mL tubing. What is the drip rate?
1,000 × 20 ÷ 600 min = 33.3, so 33 gtt/min. Check it with the drip rate calculator.
Frequently asked questions
What is the formula for dosage calculation?
The basic formula is Desired ÷ Have × Quantity. D is the dose ordered, H is the dose in each tablet or in the stated volume, and Q is that tablet or volume. The answer is the number of tablets or mL to give.
What is dimensional analysis?
Dimensional analysis sets the problem up as a chain of fractions whose units cancel until only the unit you want is left, such as mL/hr. It is useful for multi-step problems, such as weight-based infusions, because you can see every conversion in one line.
How do I convert pounds to kilograms?
Divide pounds by 2.2. A patient who weighs 154 lb weighs 70 kg. Most dosing uses kilograms, so convert before you calculate.
How should I round in dosage calculations?
Common textbook rules: scored tablets to the nearest half, capsules to the nearest whole number, liquids to the nearest tenth of a mL, and drops to the nearest whole drop. Small volumes, such as pediatric doses under 1 mL, are often rounded to the hundredth with a syringe that can measure it. Your program and facility policy take priority.
Why do nurses avoid trailing zeros?
A trailing zero (5.0 mg) can be misread as 50 mg if the decimal point is missed, which is a tenfold error. Always write a leading zero for doses under 1 (0.5 mg) and never a trailing zero (5 mg). ISMP lists these among error-prone dose designations.
Is it safe to rely on a calculator?
Use it to check your math, not to replace it. Estimate the answer first, confirm it makes sense (for example, 12 tablets is almost never a real dose), and follow your facility's double-check policy for high-alert medications.
More tools
Sources: OpenStax, Pharmacology for Nurses 2.4: Dosage Calculations · WisTech Open, Nursing Dosage Calculations · ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations. Practice problems written by our editorial team. Reviewed September 30, 2026.