Heparin drip calculations: units per hour to mL per hour

Step 1: the concentration

Premixed heparin bags are commonly 25,000 units in 250 mL or 25,000 units in 500 mL. Divide units by mL: 100 units/mL and 50 units/mL. Write it down before anything else, because every later step depends on it and the two common bags differ by a factor of two.

Read the bag every time. A question that shows a 25,000 units / 500 mL bag after several 250 mL questions is testing whether you looked.

Step 2: the dose per hour

Some orders give units/hr directly: heparin 1,200 units/hr. Others are weight-based: 18 units/kg/hr for an 80 kg patient is 18 × 80 = 1,440 units/hr. Weight must be in kilograms; divide pounds by 2.2 first.

Weight-based protocols also give a bolus, a one-time dose in units/kg, for example 80 units/kg. For 80 kg that is 6,400 units, drawn from a vial such as 1,000 units/mL: 6,400 ÷ 1,000 = 6.4 mL. The bolus vial and the infusion bag are different concentrations; do not mix them.

Step 3: mL per hour

Divide the units per hour by the units per mL. 1,200 units/hr from a 100 units/mL bag is 12 mL/hr. 1,440 units/hr from a 50 units/mL bag is 28.8 mL/hr.

The reverse question is common too: the pump reads 15 mL/hr on a 25,000 units / 250 mL bag; how many units/hr is the patient receiving? Multiply: 15 mL/hr × 100 units/mL = 1,500 units/hr. Titration questions then adjust the units/hr per the protocol and convert back to mL/hr.

Safety around the math

Heparin is on the ISMP list of high-alert medications because small errors cause bleeding or clotting. In practice, facilities require an independent double-check of the bag, the dose, the rate and the pump settings, and titrate to aPTT or anti-Xa results per protocol.

On exams, the classic error is inverting the concentration: dividing units/mL by units/hr instead of the other way round. 50 ÷ 1,440 gives 0.03, and 1,440 ÷ 50 gives 28.8. Keep the units on every number and the right division is the only one where units/hr cancels.

Titration questions

Weight-based heparin protocols adjust the rate from lab results, typically the aPTT or anti-Xa level. A protocol might say: aPTT below range, give a bolus of 40 units/kg and increase the rate by 2 units/kg/hr. For an 80 kg patient on 1,440 units/hr, the increase is 2 × 80 = 160 units/hr, so the new rate is 1,600 units/hr. From a 50 units/mL bag that is 32 mL/hr.

The numbers in titration questions come from the protocol table printed in the question; they vary between facilities. Read the table, apply it to the weight, and convert to mL/hr exactly as for the starting rate.

Reading the units

Heparin is measured in units, not milligrams, and units never convert to mg. Write units in full: the abbreviation U is on the ISMP list of error-prone abbreviations because it has been misread as a zero, turning 10 U into 100.

Subcutaneous heparin for prophylaxis, such as 5,000 units from a 10,000 units/mL vial, is an ordinary mg to mL style calculation: 5,000 ÷ 10,000 × 1 mL = 0.5 mL. The infusion steps on this page apply to the continuous IV drip.

How exams test it

Expect at least one heparin item on most dosage exams, often with a bag label exhibit. Common forms are mL/hr from units/hr, units/hr from a running rate, a weight-based start with a bolus, and a titration from a protocol table. All four use the same concentration from step 1.

Where the numbers come from

Weight-based heparin nomograms, such as the widely used 80 units/kg bolus and 18 units/kg/hr starting rate for venous thromboembolism, come from published protocols; cardiology protocols often use lower doses. Facilities adopt their own, and many cap the bolus and the hourly rate for heavier patients. On an exam, the question gives you the protocol to apply. Use its numbers, not ones you remember, and check whether it states a maximum.

Check before you submit

Did you calculate the bag's concentration from this question's bag? Is the weight in kilograms? Did you divide units/hr by units/mL, so that units cancel and mL/hr remains? Did you keep the bolus vial and the infusion bag separate? Did you round to the rule the question states, often the tenth for heparin pumps? Is the rate plausible, usually under about 40 mL/hr for a 100 units/mL bag at adult doses?

Heparin questions reward slow, written steps more than any other type. Write the concentration, the units/hr and the mL/hr on three separate lines, each with its unit.