Heparin Weight-Based Dose Calculator
Weight-based heparin protocols dose intravenous unfractionated heparin as units per kilogram body weight to provide more predictable therapeutic anticoagulation than fixed-dose approaches. The standard protocol (Raschke nomogram) begins with an IV bolus of 80 units/kg followed by an infusion of 18 units/kg/hr, with serial aPTT measurements every 6 hours to guide titration. This calculator computes the initial bolus (units) and infusion rate (units/hr and mL/hr given a specified concentration) from patient weight. It is for educational illustration only.
Heparin weight-based dosing formula
Bolus (units) = Weight (kg) x Bolus rate (units/kg)
Infusion (units/hr) = Weight (kg) x Infusion rate (units/kg/hr)
Infusion (mL/hr) = Infusion (units/hr) / Concentration (units/mL)
These are starting doses only. Actual heparin dosing is titrated based on aPTT results measured at 6-hour intervals. Institutional heparin nomograms specify dose adjustments for sub-therapeutic, therapeutic, and supra-therapeutic aPTT values.
Heparin anticoagulation monitoring
- Therapeutic aPTT target: typically 60 to 100 seconds (1.5 to 2.5 x control), varies by indication and laboratory.
- Anti-Xa assay (heparin level 0.3 to 0.7 units/mL) is an alternative monitoring parameter used in some institutions.
- HIT (heparin-induced thrombocytopenia) must be monitored: platelet counts should fall by less than 50% and not drop below 100 x 10^9/L.
- Bleeding is the primary complication; reversal with protamine sulfate (1 mg per 100 units heparin) is available.
- Weight caps: many protocols limit bolus to 10,000 units maximum regardless of calculated dose.
Heparin weight-based dosing: frequently asked questions
What is weight-based heparin dosing?
Weight-based heparin dosing calculates the bolus and continuous infusion doses as units per kilogram of body weight, then adjusts based on serial aPTT measurements. Common starting protocols use 80 units/kg IV bolus followed by 18 units/kg/hr infusion for therapeutic anticoagulation.
What is aPTT and why does it matter?
Activated partial thromboplastin time (aPTT) measures the intrinsic coagulation pathway and is prolonged by heparin. Therapeutic heparin anticoagulation typically targets an aPTT of 60 to 100 seconds (or 1.5 to 2.5 x the control value). Serial aPTT monitoring drives dose titration.
Why is weight-based dosing used for heparin?
Fixed-dose heparin protocols produce highly variable anticoagulation. Weight-based protocols (pioneered by Raschke et al., 1993) more consistently achieve therapeutic aPTT within 24 hours, reducing thromboembolic complications and over-anticoagulation.
What is the maximum safe bolus dose?
Many institutional protocols cap the bolus at 10,000 units regardless of weight to limit bleeding risk in obese patients. Ideal or adjusted body weight may be used for obese patients. Always follow your institution's specific heparin protocol.
Can this calculator be used for patient care?
No. This tool is for educational and informational purposes only. Heparin is a high-alert medication requiring aPTT monitoring, physician orders, pharmacist review, and institutional protocol adherence. Never use this calculator to administer heparin without appropriate clinical oversight.
Official sources
- U.S. Food and Drug Administration: FDA Drug Safety Communication: Heparin.
- NIH National Library of Medicine: Heparin (StatPearls).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.