Continuous infusions: concentration and rate per kilogram per minute
- Calculation
For continuous infusions, work out the concentration of the mixture and convert the ordered dose (for example mcg/kg/min) into mL/h for the pump.
What it is
Many critical-care drugs are ordered as a dose over time (mg/h, units/h) or per weight and time (mcg/kg/min). The pump, however, is set in mL/h. The bridge between the two is the concentration of the mixture: the amount of drug divided by the bag volume. With it you calculate the rate in mL/h or, the other way round, what dose the patient gets at the current rate. These are high-alert drugs: always on a pump with an independent double check. The examples use fictional drugs.
Formulas
- Concentration (mg/mL) = mg in the bag ÷ mL in the bag; × 1000 for mcg/mL.
- Per weight and minute: mL/h = mcg/kg/min × kg × 60 ÷ mcg/mL.
- Per hour: mL/h = (mg/h) ÷ (mg/mL), or (units/h) ÷ (units/mL).
- Current dose: mcg/kg/min = (mL/h × mcg/mL) ÷ 60 ÷ kg.
How to calculate it
- Work out the bag's concentration and express it in the order's unit — mixing mg and mcg causes thousandfold errors.
- Multiply the dose by the weight if the order is per kilogram — gives the patient's dose per minute or hour.
- Multiply by 60 if the order is per minute — the pump works in hours.
- Divide by the concentration — gives mL/h.
- Do the reverse calculation to check — it must return the ordered dose.
- Programme the pump and ask for a double check — catches programming errors.
Worked examples (fictional drugs; educational, not a clinical order)
| Data | Set-up | Answer |
|---|---|---|
| Drug A 250 mg in 250 mL. Order: 5 mcg/kg/min. Weight: 80 kg. | Concentration 1 mg/mL = 1000 mcg/mL; 5 × 80 × 60 = 24 000 mcg/h; 24 000 ÷ 1000 | 24 mL/h |
| Drug B 4 mg in 250 mL. Order: 0.1 mcg/kg/min. Weight: 70 kg. | 4000 mcg ÷ 250 mL = 16 mcg/mL; 0.1 × 70 × 60 = 420 mcg/h; 420 ÷ 16 | 26.25 ≈ 26.3 mL/h |
| Drug C 400 mg in 250 mL running at 15 mL/h. Weight: 60 kg. What dose is the patient getting? | 1.6 mg/mL = 1600 mcg/mL; 15 × 1600 = 24 000 mcg/h; ÷ 60 = 400 mcg/min; ÷ 60 kg | 6.67 mcg/kg/min |
| Drug D 25 mg in 250 mL. Order: 2 mg/h. | 25 ÷ 250 = 0.1 mg/mL; 2 ÷ 0.1 | 20 mL/h |
| Drug E 25 000 units in 500 mL. Order: 1200 units/h. | 25 000 ÷ 500 = 50 units/mL; 1200 ÷ 50 | 24 mL/h |
Common mistakes
- Not converting mg to mcg when working out the concentration.
- Forgetting to multiply by 60 when the order is per minute.
- Using a weight different from the one recorded for the order.
- Programming the pump in the wrong unit (mL/h instead of dose, or the reverse).
Double-checking
- Do the reverse calculation and compare with the order.
- Ask another person to check the concentration, weight, rate and pump programming.
- Check the bag label: concentration, date and time of preparation.
Test yourself
Think of the answer, then tap the question to see it.
How is the concentration of an infusion mixture calculated?
Divide the amount of drug by the bag volume (for example mg ÷ mL), and convert to mcg/mL if the order is in mcg.
Why multiply by 60 when the order is in mcg/kg/min?
Because the pump is set in mL per hour and an hour has 60 minutes.
Educational example: 100 mg in 100 mL and an order of 5 mg/h. What rate in mL/h?
Concentration 1 mg/mL; 5 ÷ 1 = 5 mL/h (made-up numbers, educational, not a clinical order).
Connections
Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.
Sources: Nursing Skills (Open RN), 5.17 Multi-Step Calculations (Open RN, Chippewa Valley Technical College (LibreTexts), 2022); Nursing Skills (Open RN), 5.15 IV Infusion by Pump (Open RN, Chippewa Valley Technical College (LibreTexts), 2022); Medication safety in high-risk situations (World Health Organization, 2019).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.