Infusion rates and drip rates: five worked examples
Infusion questions chain several steps together, and each step is a chance to slip on units. This guide shows the four types you are likely to meet and a habit that keeps the units straight.
Not affiliated with the General Pharmaceutical Council. This guide is for revision only. It is not clinical or legal advice, and its worked examples use invented numbers. Check a current source before you rely on any point.
The four types
- Volume over time. A bag of fluid runs over a set time. The answer is in mL/hour.
- Drip rate. A gravity set delivers a known number of drops per mL (the drop factor). The answer is in drops/minute.
- Drug dose rate to pump rate. The prescriber wants a dose per hour or per minute. You turn it into mL/hour.
- Pump rate to dose. The pump is running. You work out the dose the patient is getting.
Real products give you timings to work with. The SmPC for a ceftriaxone 1 g powder says it can be given by intravenous infusion over at least 30 minutes. The SmPC for a paracetamol and ibuprofen infusion product says to give 100 mL as a 15-minute infusion. Those are the sort of facts a question turns into a rate.
One habit that keeps the units straight
Write every line with its unit, and convert time to a single unit before anything else. Minutes to hours, or hours to minutes, whichever suits the answer. Most lost marks in this topic are right arithmetic in the wrong unit.
Example 1: volume over time
1,000 mL of fluid is to run over 8 hours. What is the rate in mL/hour?
1,000 ÷ 8 = 125 mL/hour.
The same method turns the SmPC timings above into rates. The ceftriaxone SmPC's 2 g row uses 40 mL of diluent. As a practice sum, 40 mL run over 30 minutes is 40 ÷ 0.5 = 80 mL/hour. The paracetamol and ibuprofen product gives 100 mL as a 15-minute infusion, which is 100 ÷ 0.25 = 400 mL/hour. Notice that the shorter the time, the faster the rate, so a 15-minute infusion needs a pump set well above 125.
Example 2: drip rate
500 mL is to run over 4 hours through a set that delivers 20 drops/mL. What is the rate in drops/minute?
Total drops = 500 × 20 = 10,000. Total minutes = 4 × 60 = 240. 10,000 ÷ 240 = 41.67, which is 42 drops/minute to the nearest whole drop. You can only count whole drops, so a question like this asks for a whole number. The drop factor comes from the question or the giving set label.
Example 3: dose per hour to pump rate
Drug D is prescribed at 4 mg/hour. A bag holds 100 mg in 250 mL. What pump rate in mL/hour?
Concentration = 100 ÷ 250 = 0.4 mg/mL. Rate = 4 ÷ 0.4 = 10 mL/hour.
Example 4: a dose per kg per minute
Drug E is prescribed at 3 micrograms/kg/minute for a patient weighing 80 kg. The syringe holds 250 mg in 50 mL. What pump rate in mL/hour?
Dose per minute = 3 × 80 = 240 micrograms. Dose per hour = 240 × 60 = 14,400 micrograms = 14.4 mg. Concentration = 250 ÷ 50 = 5 mg/mL. Rate = 14.4 ÷ 5 = 2.88 mL/hour.
The micrograms-to-milligrams step is where marks go. 14,400 micrograms is 14.4 mg because you divide by 1,000.
Example 5: from the pump back to the dose
A pump runs at 12 mL/hour with a solution of 2 mg/mL. The patient weighs 60 kg. What dose in micrograms/kg/minute?
Dose per hour = 12 × 2 = 24 mg = 24,000 micrograms. Per minute = 24,000 ÷ 60 = 400 micrograms. Per kg = 400 ÷ 60 = 6.667, which is 6.7 micrograms/kg/minute.
Both sides of this question use "60", once for minutes in an hour and once for kilograms. Label them so you do not divide by the wrong one.
Checks before you answer
- Direction: if you double the concentration, the pump rate should halve.
- Size: would this rate be sensible for this fluid and this time?
- Rounding: keep the full figure through the working and round once, in the form the question wants.
For the reconstitution step that often comes first, see the displacement values guide. The calculations guide covers the other calculation types.
Sources
- Ceftriaxone 1g Powder for Solution for Injection or Infusion, Summary of Product Characteristics (emc). https://www.medicines.org.uk/emc/product/1361/smpc. Accessed 22 October 2026.
- Combogesic IV 10 mg/ml + 3 mg/ml solution for infusion, Summary of Product Characteristics (emc). https://www.medicines.org.uk/emc/product/15054/smpc. Accessed 22 October 2026.
Not affiliated with the General Pharmaceutical Council. Revision only, not clinical or legal advice. Worked examples use invented numbers and products. Every source above was read on 22 October 2026; check anything that matters against a current source. See our terms of use and privacy notice.