Heparin Weight-Based Dosing Calculator: Bolus, Infusion mL/h and aPTT Adjustment
Calculate a weight-based unfractionated heparin bolus and starting infusion in units, mL and mL/hour (VTE nomogram 80 units/kg + 18 units/kg/h, ACS 60 units/kg + 12 units/kg/h with maximums, or your protocol's doses and caps), then convert an aPTT result into the nomogram rate change.
How it works
Bolus (units) = weight × units/kg (capped if the protocol sets a maximum) · Infusion (units/h) = weight × units/kg/h (capped) · mL/h = units/h ÷ bag concentration (units/mL) · Bolus mL = bolus units ÷ concentration of the bag or vial used · aPTT adjustment: new units/h = current units/h + nomogram change (units/kg/h) × weight
Weight-based protocols for unfractionated heparin give a bolus in units per kilogram and a starting infusion in units per kilogram per hour, then adjust the rate against the aPTT or anti-Xa level. The widely used VTE nomogram (Raschke, 1993) starts at 80 units/kg and 18 units/kg/h; acute coronary syndrome protocols use 60 units/kg (maximum 4,000 units) and 12 units/kg/h (maximum 1,000 units/h) because heparin is combined with antiplatelet drugs. Multiply each dose by the dosing weight, apply any cap, then divide by the bag concentration to get the mL/h to program. The bolus volume depends on where it is drawn from: the bag, or a 1,000 or 5,000 units/mL vial. In aPTT adjustment mode the calculator applies the original nomogram: below 35 s re-bolus 80 units/kg and increase by 4 units/kg/h; 35–45 s re-bolus 40 units/kg and increase by 2 units/kg/h; 46–70 s no change; 71–90 s decrease by 2 units/kg/h; above 90 s hold for 1 hour and decrease by 3 units/kg/h, with an aPTT about 6 hours after each change. Those seconds belong to the original laboratory reagent, so hospitals publish their own ranges. Your institution's nomogram, its therapeutic range and its double-check policy always take precedence over this tool.
Worked example
With the defaults, an 80 kg patient on 80 units/kg and 18 units/kg/h, with 25,000 units in 250 mL (100 units/mL): bolus = 80 × 80 = 6,400 units = 64.00 mL from the bag; infusion = 80 × 18 = 1,440 units/h = 1,440 ÷ 100 = 14.4 mL/h, or 34,560 units per 24 hours. Drawn from a 5,000 units/mL vial instead, the bolus would be 1.28 mL. If the 6-hour aPTT is 40 s, the nomogram says re-bolus 40 units/kg (3,200 units) and increase by 2 units/kg/h: 1,440 + 160 = 1,600 units/h = 16.0 mL/h.
Sources: Heparin: dosing (80 units/kg bolus, 18 units/kg/h), monitoring and contraindications (StatPearls, NCBI Bookshelf) · Heparin: indication-specific dosing including ACS maximums of 4,000 units and 1,000 units/h (WikEM) · Weight-based heparin order set and aPTT adjustment table after Raschke et al., Ann Intern Med 1993 (Perinatology.com) · 2014 AHA/ACC NSTE-ACS guideline UFH dosing: 60 IU/kg (max 4,000 IU) then 12 IU/kg/h (max 1,000 IU/h) (WikiDoc summary) · NIH/NLM DailyMed — heparin sodium injection labeling
Frequently asked questions
What is the standard weight-based heparin dose?
For venous thromboembolism the classic nomogram is an 80 units/kg IV bolus followed by 18 units/kg/h. For acute coronary syndromes it is lower: 60 units/kg (maximum 4,000 units) then 12 units/kg/h (maximum 1,000 units/h). Always use your institution's protocol and the dose actually ordered.
Is heparin dosed on actual or adjusted weight?
It depends on the protocol. Many weight-based nomograms use actual body weight but cap the bolus (for example at 10,000 units) or switch to an adjusted dosing weight in significant obesity. Enter the weight and caps your institution's nomogram specifies rather than assuming actual weight.
How do I convert heparin units/h to mL/h?
Divide the units per hour by the bag concentration. 1,440 units/h with a 100 units/mL bag is 14.4 mL/h; with a 50 units/mL bag (25,000 units in 500 mL) it is 28.8 mL/h. Using the wrong concentration doubles or halves the delivered dose.
How is the infusion adjusted after an aPTT?
In the original weight-based nomogram: aPTT below 35 s, re-bolus 80 units/kg and increase 4 units/kg/h; 35–45 s, re-bolus 40 units/kg and increase 2 units/kg/h; 46–70 s, no change; 71–90 s, decrease 2 units/kg/h; above 90 s, hold 1 hour and decrease 3 units/kg/h. Recheck about 6 hours after each change. Your lab's therapeutic range may differ.
How often should the aPTT be checked?
Typically about 6 hours after starting and 6 hours after each rate change, then once daily once two consecutive results are therapeutic. Platelets, haemoglobin and signs of bleeding are also monitored, and a falling platelet count raises concern for heparin-induced thrombocytopenia.
Related calculators
Educational estimate only — not medical advice, and no substitute for a clinician, pharmacist or the product label. Heparin is a high-alert medication: every dose, rate and volume must be independently double-checked against the actual order, bag label and patient before administration. Your institution's heparin protocol and its laboratory's therapeutic aPTT or anti-Xa range take precedence over this tool. Do not use it to self-treat or to bypass a double-check policy.