Heparin Drip Rate
Bolus, units per hour, and the pump rate for a weight-based heparin order.
1440 units/hr at a bag concentration of 50 units/mL.
18 units/kg/hr × 80 kg.
Drawn from the same concentration that is 128 mL — most protocols use a separate concentrated vial, so check yours.
Multiply the units per kilogram per hour by the weight, then divide by the bag concentration. For an 80 kg patient at 18 units/kg/hr the infusion is 1440 units/hr; with 25,000 units in 500 mL — that is 50 units/mL — the pump runs at 1440 ÷ 50 = 28.8 mL/hr.
Three numbers, three steps
A weight-based heparin order has a familiar shape: a bolus in units/kg, an infusion in units/kg/hr, and a bag mixed to a stated concentration. The calculation runs in the same order every time — work out the bag concentration, turn the order into units per hour, then convert units per hour into millilitres per hour for the pump.
The bag concentration is the step students skip. A pump is programmed in millilitres, but the order is written in units, and only the concentration links them. Two hospitals using 25,000 units in 500 mL and 25,000 units in 250 mL will run the same patient at very different millilitre rates for the same dose.
Titration against the aPTT
Heparin infusions are rarely left at their starting rate. A nomogram specifies how the rate changes in response to the aPTT or anti-Xa result — typically a rate increase or decrease expressed in units/kg/hr, sometimes with a further small bolus. Each adjustment is the same calculation with a new units/kg/hr figure, which is why getting the method fluent matters more than memorising any one rate.
The bolus is a separate calculation: units/kg × weight in kg. Boluses are usually drawn from a concentrated vial rather than the infusion bag — follow the protocol.
Worked example: 80 kg patient, 80 units/kg bolus, 18 units/kg/hr, 25,000 units in 500 mL
Concentration first, then the hourly dose, then the pump rate:
- 1 Work out the bag concentration. 25,000 units ÷ 500 mL = 50 units/mL.
- 2 Calculate the bolus. 80 units/kg × 80 kg = 6400 units. Draw this from the vial the protocol specifies, not from the infusion bag.
- 3 Calculate the hourly dose. 18 units/kg/hr × 80 kg = 1440 units/hr.
- 4 Convert to a pump rate. 1440 units/hr ÷ 50 units/mL = 28.8 mL/hr.
- 5 Program and double-check. Heparin is a high-alert medication — most units require an independent second check of the rate before it starts.
- 6 Recalculate at each aPTT. When the nomogram changes the order to a new units/kg/hr, repeat steps three and four with the new figure.
Pump rate at 18 units/kg/hr for a 25,000 units in 500 mL bag
Bag concentration 50 units/mL. Rate = (18 × weight) ÷ 50, rounded to one decimal place.
| Weight | Dose (units/hr) | Pump rate (mL/hr) | Bolus at 80 units/kg |
|---|---|---|---|
| 50 kg | 900 | 18 | 4000 units |
| 60 kg | 1080 | 21.6 | 4800 units |
| 70 kg | 1260 | 25.2 | 5600 units |
| 80 kg | 1440 | 28.8 | 6400 units |
| 90 kg | 1620 | 32.4 | 7200 units |
| 100 kg | 1800 | 36 | 8000 units |
Why heparin gets extra checks
Heparin sits on most institutions’ high-alert medication lists, because an error of one decimal place is the difference between a therapeutic infusion and a bleeding event. That is the reason for the independent double-check, the standardised premixed bags, and the dedicated pump libraries with hard limits.
Two practical cautions. Protocols often cap the bolus and the hourly dose regardless of weight, so a heavy patient can hit the ceiling that weight-based arithmetic alone would miss. And the weight used may be actual or dosing weight depending on the protocol — check which before multiplying, because every number downstream inherits that choice.
This tool is a study aid for practising calculations. Heparin is a high-alert medication: doses and pump rates must follow local protocol and be independently verified by a qualified professional.