Drug Calculation Formulas for Nurses: Every Formula With Worked Examples (Tablets, Liquids, IV Rates, Drips)

Drug calculations appear in every nursing licensing exam, from the NCLEX and NMC CBT to Gulf Prometric exams and local board exams. On the ward they are a matter of life and death. A misplaced decimal point can mean a tenfold overdose. This guide covers every formula you need, with worked examples.

Golden rules before you calculate

1. Convert to the same units first. You cannot divide milligrams by grams.
2. Estimate the answer before you calculate, so you notice when the result is wildly wrong.
3. Never use a trailing zero (write 5 mg, not 5.0 mg), and always use a leading zero (0.5 mg, not .5 mg).
4. Double-check high-risk drugs (insulin, heparin, opioids, potassium, paediatric doses) with a second nurse.

Unit conversions you must know

1 gram (g)	= 1,000 milligrams (mg)
1 milligram (mg)	= 1,000 micrograms (mcg)
1 litre (L)	= 1,000 millilitres (mL)
1 kilogram (kg)	= 1,000 grams = 2.2 pounds (lb)
1 hour	= 60 minutes

To go from a bigger unit to a smaller one, multiply by 1,000. To go from smaller to bigger, divide by 1,000.

1. Tablets and capsules

Formula: Number of tablets = Dose wanted ÷ Dose in each tablet

Example: Prescribed paracetamol 1 g. Tablets are 500 mg. Convert first: 1 g = 1,000 mg. Then 1,000 ÷ 500 = 2 tablets.

2. Liquids and injections

Formula: Volume to give = (Dose wanted ÷ Dose you have) × Volume it comes in

Example 1: Prescribed amoxicillin 250 mg. Stock is 125 mg in 5 mL. (250 ÷ 125) × 5 = 10 mL.

Example 2: Prescribed morphine 2.5 mg. Ampoule is 10 mg in 1 mL. (2.5 ÷ 10) × 1 = 0.25 mL.

3. Weight-based doses

Formula: Dose = Dose per kg × Weight in kg

Example: A child weighing 18 kg is prescribed 15 mg/kg of paracetamol. 15 × 18 = 270 mg. If the suspension is 120 mg in 5 mL: (270 ÷ 120) × 5 = 11.25 mL.

If weight is given in pounds, divide by 2.2 to get kilograms first: 44 lb ÷ 2.2 = 20 kg. Always check the calculated dose against the maximum safe dose.

4. IV infusion rate on a pump (mL/hour)

Formula: Rate (mL/hr) = Total volume (mL) ÷ Time (hours)

Example: 1,000 mL of 0.9% sodium chloride over 8 hours. 1,000 ÷ 8 = 125 mL/hr.

If the time is in minutes: Rate (mL/hr) = Volume (mL) ÷ Time (minutes) × 60. For example, 100 mL over 30 minutes = 100 ÷ 30 × 60 = 200 mL/hr.

5. Gravity drip rate (drops per minute)

Without a pump, common in many hospitals in Africa and Asia, you set the drip rate by counting drops.

Formula: Drops/min = (Volume in mL × Drop factor) ÷ Time in minutes

• Standard (macro-drip) giving sets: usually 20 drops/mL (some are 15 or 10). Check the packet.
• Blood giving sets: usually 15 drops/mL.
• Micro-drip (paediatric burette) sets: 60 drops/mL.

Example: 1,000 mL over 8 hours with a 20 drops/mL set. Time = 8 × 60 = 480 minutes. (1,000 × 20) ÷ 480 = 41.7, so about 42 drops per minute.

Shortcut: with a 60 drops/mL micro-drip set, drops per minute equals mL per hour. So 50 mL/hr = 50 drops/min.

6. Infusions in mcg/kg/min (critical care)

Drugs such as dopamine, dobutamine and noradrenaline (norepinephrine) are often prescribed per kilogram per minute.

Formula: Rate (mL/hr) = (Dose in mcg/kg/min × Weight in kg × 60) ÷ Concentration in mcg/mL

Example: Dopamine 5 mcg/kg/min for a 70 kg patient. The bag is 400 mg in 250 mL.

1. Concentration: 400 mg = 400,000 mcg, and 400,000 ÷ 250 = 1,600 mcg/mL.
2. Dose per minute: 5 × 70 = 350 mcg/min. Per hour: 350 × 60 = 21,000 mcg/hr.
3. Rate: 21,000 ÷ 1,600 = 13.1 mL/hr.

7. Infusions in units per hour (heparin, insulin)

Formula: Rate (mL/hr) = Units per hour wanted ÷ Units per mL

Example: Heparin 1,000 units/hr. The bag is 25,000 units in 500 mL, which is 50 units/mL. 1,000 ÷ 50 = 20 mL/hr.

8. Time to complete an infusion

Formula: Time (hours) = Volume (mL) ÷ Rate (mL/hr)

Example: 500 mL running at 125 mL/hr. 500 ÷ 125 = 4 hours.

Practice questions (answers below)

1. Prescribed furosemide 40 mg orally. Tablets are 20 mg. How many tablets?
2. Prescribed gentamicin 80 mg IV. Stock is 40 mg/mL. How many mL?
3. Run 500 mL over 4 hours on a pump. What rate?
4. Run 1 L over 12 hours with a 20 drops/mL set. How many drops per minute?
5. A child of 12 kg needs 50 mg/kg/day of amoxicillin in 3 divided doses. How many mg per dose?

Answers: 1) 2 tablets; 2) 2 mL; 3) 125 mL/hr; 4) (1,000 × 20) ÷ 720 = 27.8, so about 28 drops/min; 5) 50 × 12 = 600 mg a day, ÷ 3 = 200 mg per dose.

Exam tips

• NCLEX: calculation questions are fill-in-the-blank. Follow the rounding instructions exactly.
• NMC CBT: includes a numeracy section, so practise without a calculator first, then with the on-screen calculator.
• Prometric (Gulf): expect several calculation questions. Speed comes from practice.

Related: Normal vital signs · How to pass the Prometric exam · NMC CBT and OSCE preparation

Educational content. Always check doses against your local formulary, prescription and policy, and double-check high-risk calculations with a colleague.