This calculator implements a weight-based unfractionated heparin protocol — the kind printed on hospital anticoagulation nomograms — to give the initial IV bolus, starting infusion, and the corresponding pump rate. It is built for nurses, pharmacists, and anticoagulation clinics initiating a heparin drip.
How it works
Two simple per-kilogram rules drive the starting dose:
Bolus (units) = bolus per kg × weight
Infusion (units/hr) = rate per kg × weight
A widely used venous thromboembolism (VTE) nomogram uses an 80 units/kg bolus and an 18 units/kg/hr infusion; acute coronary syndrome protocols often use lower figures (60 units/kg bolus, 12 units/kg/hr) with explicit caps. If you enter optional caps, the tool clamps the bolus or infusion to that maximum and labels it as capped — protecting very heavy patients from excessive dosing.
The pump rate then follows from the bag concentration:
Rate (mL/hr) = infusion units/hr ÷ concentration (units/mL)
A common premixed bag of 25,000 units in 250 mL is 100 units/mL.
Worked example
Patient: 85 kg, VTE protocol
- Bolus per kg: 80 units/kg
- Infusion rate: 18 units/kg/hr
- Maximum bolus cap: 10,000 units
Step 1 — Calculate bolus: 80 × 85 = 6,800 units (below the 10,000-unit cap, so no capping)
Step 2 — Calculate infusion: 18 × 85 = 1,530 units/hr
Step 3 — Calculate pump rate: Bag: 25,000 units in 250 mL = 100 units/mL 1,530 ÷ 100 = 15.3 mL/hr
So: give a 6,800-unit bolus IV, then start the drip at 15.3 mL/hr. Recheck aPTT in 6 hours and adjust per the nomogram.
Contrast with ACS protocol (same patient): 60 units/kg bolus = 5,100 units (cap commonly 4,000–5,000 units — capped) 12 units/kg/hr infusion = 1,020 units/hr → 10.2 mL/hr
Common nomogram protocols and when to use them
| Indication | Typical bolus | Typical infusion | Target aPTT |
|---|---|---|---|
| Venous thromboembolism (DVT/PE) | 80 units/kg | 18 units/kg/hr | 60–100 sec |
| Acute coronary syndrome (UA/NSTEMI) | 60 units/kg | 12 units/kg/hr | 50–70 sec |
| Mechanical heart valve | per protocol | per protocol | higher range |
These are common published starting points, not universal standards. Always use your institution’s approved nomogram, which may differ.
Adjusting the infusion
The starting infusion is only the beginning. Heparin is titrated to a target aPTT or anti-Xa level using your institution’s adjustment table. Recheck the level about 6 hours after starting and after every rate change, then step the rate up or down per the nomogram.
Typical aPTT-based titration steps move the rate by 1–2 mL/hr increments at a time, with a repeat aPTT after each change and a steady-state recheck at 6 hours when no adjustment was needed in the prior 24 hours.
Choosing the correct weight
Most VTE protocols specify actual body weight (ABW). Some centres cap the dosing weight for patients with a body mass index above a threshold to limit bleeding risk, using an adjusted or ideal body weight calculation instead. When in doubt, follow your institution’s written policy — do not assume one weight type without checking.
This tool plans the initial dose only. Always verify calculations with a pharmacist or anticoagulation service before administering. All calculations run locally in your browser.