🧠 The theory: it is all rate = amount over time
Every infusion question is a rate, which is an amount divided by a time. For a simple bag it is volume over time. For drops it is the same volume over time, multiplied by how many drops make a millilitre. For a weight-based drug it is a dose per minute that you scale to per hour, then convert from an amount per hour to a volume per hour using the concentration. Once you see them all as amount over time, the different formulas are just the same idea in different units.
Rate in mL per hour
rate (mL/h) = volume / time in hours
Worked example
1000 mL is to be infused over 8 hours. What is the rate in mL/hour?
- Step 1 — Rate is volume divided by time, and the time is already in hours.
- Step 2 — Divide: 1000 / 8 = 125.
Answer: 125 mL/hour
Worked example
250 mL is to be infused over 30 minutes. What is the rate in mL/hour?
- Step 1 — The rate must be per hour, so convert the time: 30 minutes = 0.5 hours.
- Step 2 — Divide the volume by the time in hours: 250 / 0.5 = 500.
Answer: 500 mL/hour
Drops per minute
Gravity giving sets are set by counting drops. You need the drop factor of the set, in drops per mL: a standard (macro) set is 20 drops/mL, a paediatric (micro) set is 60 drops/mL, and a blood set is 15 drops/mL.
drops/min = (volume in mL x drop factor) / time in minutes
Worked example
Infuse 1 litre over 8 hours with a set delivering 20 drops/mL. What is the drip rate?
- Step 1 — Turn the volume into a number of drops: 1000 mL x 20 drops/mL = 20,000 drops.
- Step 2 — Turn the time into minutes: 8 hours x 60 = 480 minutes.
- Step 3 — Divide drops by minutes: 20,000 / 480 = 41.7, which rounds to 42.
Answer: 42 drops per minute
From mg per hour to mL per hour
Worked example
Aminophylline 500 mg in 500 mL is prescribed at 30 mg/hour. What is the rate in mL/hour?
- Step 1 — Find the concentration: 500 mg in 500 mL is 1 mg/mL.
- Step 2 — You need 30 mg each hour, and each mL carries 1 mg, so divide the amount per hour by the concentration: 30 / 1 = 30.
Answer: 30 mL/hour
Harder: weight-based (micrograms/kg/min)
These look hard but are just a chain of conversions: dose per minute, then per hour, then divide by the concentration.
Worked example
Dobutamine 250 mg in 50 mL runs at 5 micrograms/kg/min for a 70 kg patient. What is the rate in mL/hour?
- Step 1 — Dose per minute: multiply the rate by the weight, 5 x 70 = 350 micrograms/min.
- Step 2 — Scale to per hour: 350 x 60 = 21,000 micrograms/hour.
- Step 3 — Find the concentration in the same units: 250 mg in 50 mL is 5 mg/mL, which is 5000 micrograms/mL.
- Step 4 — Convert the amount per hour to a volume per hour by dividing by the concentration: 21,000 / 5000 = 4.2.
Answer: 4.2 mL/hour
Units per hour
Worked example
Heparin 25,000 units in 50 mL runs at 1.5 mL/hour. How many units per hour?
- Step 1 — Find the concentration in units per mL: 25,000 / 50 = 500 units/mL.
- Step 2 — Multiply the rate by the concentration: 1.5 x 500 = 750.
Answer: 750 units/hour
💡 Identify the type first
Before reaching for a formula, decide which type you have: a simple mL/hour, a drops/min, an amount per hour, a weight-based infusion, or a units/hour. Naming the type tells you the method.
Test yourself
1 litre over 12 hours. What is the rate in mL/hour?
Answer: 1000 / 12 = 83.3 mL/hour
500 mL over 4 hours with a 15 drops/mL set. What is the drip rate?
Answer: (500 x 15) / (4 x 60) = 7500 / 240 = 31 drops/min
Noradrenaline 4 mg in 50 mL at 0.1 micrograms/kg/min for an 80 kg patient. What rate in mL/hour?
Answer: 0.1 x 80 = 8 mcg/min; x 60 = 480 mcg/h; concentration 4 mg/50 mL = 80 mcg/mL; 480 / 80 = 6 mL/hour
Insulin 50 units in 50 mL at 4 mL/hour. How many units per hour?
Answer: Concentration 1 unit/mL; 4 x 1 = 4 units/hour