Weight-Based Heparin Drip Calculator
Last reviewed: by Jayson Minagawa, BSN, RN
Free weight-based heparin drip calculator. Enter weight (kg) and indication (DVT/PE vs. ACS) — returns the standard initial bolus and continuous infusion rate based on the most common nursing-aware protocol (Raschke nomogram). Clinical reference only — always follow your facility's protocol.
Heparin is one of the top medications I see causing nursing-level errors because the math is weight-based AND the protocols vary by indication. ACS protocols use lower bolus doses than DVT/PE because the bleeding risk profile is different. Always use the order set, not generic dosing — but knowing the standard math lets you catch wrong orders.
— Jayson Minagawa, BSN, RNCalculate
Standard weight-based heparin protocols
DVT/PE (full anticoagulation): bolus 80 units/kg IV (max 10,000 units), then continuous infusion at 18 units/kg/hr (max 2,000 units/hr). ACS/NSTEMI: bolus 60 units/kg IV (max 4,000 units), then continuous infusion at 12 units/kg/hr (max 1,000 units/hr). These are the Raschke nomogram doses, the most-cited protocol in U.S. hospitals.
Worked example
75 kg patient with PE: bolus 80 × 75 = 6,000 units IV (under cap of 10,000). Drip rate 18 × 75 = 1,350 units/hr (under cap of 2,000). At standard concentration (25,000 units/250 mL = 100 units/mL), drip rate = 13.5 mL/hr.
aPTT monitoring
Check aPTT 6 hours after starting the drip and 6 hours after each rate change. Goal aPTT is institution-specific but typically 1.5-2.5 × control (50-80 seconds in many labs). Adjust per nomogram: subtherapeutic → increase rate by 1-3 units/kg/hr + bolus; supratherapeutic → hold and decrease. Document each adjustment with the corresponding aPTT value.
Bleeding risk and reversal
Bleeding is the major adverse effect — monitor for hematomas, bleeding gums, hematuria, melena, dropping H/H. Reversal: protamine sulfate 1 mg per 100 units of heparin given in the last 4 hours, IV slow push, max 50 mg single dose. Watch for hypotension, bradycardia, anaphylaxis (especially in fish-allergic and protamine-allergic patients). HIT (heparin-induced thrombocytopenia) is a separate risk requiring monitoring of platelet count.
Running a heparin drip safely on a med-surg or telemetry unit
Heparin is one of the most error-prone medications nurses manage. The Institute for Safe Medication Practices has heparin on its list of high-alert medications for a reason: dosing errors land patients in ICU. The two errors I see most often: rate confusion between units/hr and mL/hr (especially when concentrations vary across pharmacy batches), and overlapping bolus doses when transferring between RNs at shift change. Always verify the concentration on the bag matches the pump library entry before starting or after every bag change. The most common in-house concentration is 25,000 units in 250 mL D5W (100 units/mL), but some facilities use 20,000 in 500 mL (40 units/mL) or 25,000 in 500 mL (50 units/mL). A patient on 1,800 units/hr is running 18 mL/hr at 100 units/mL but 36 mL/hr at 50 units/mL — same dose, different volume.
Independent double-check is required at every rate change, every bag swap, and every bolus dose. The ISMP-recommended double-check sequence: weight in kg, ordered units/kg/hr, calculated units/hr, programmed concentration on the pump, calculated mL/hr, actual pump display, line traced from bag to patient. Document the second nurse's name and the time. This adds 90 seconds to each adjustment and prevents tenfold dosing errors that have killed patients.
Anti-Xa vs aPTT monitoring
Many institutions are transitioning from aPTT to anti-Xa heparin assay because anti-Xa is more reproducible and less affected by lupus anticoagulant, factor deficiencies, and acute-phase reactants. Anti-Xa target for therapeutic UFH is typically 0.3–0.7 units/mL. The clinical bottom line for the bedside nurse: know which assay your facility uses, know its therapeutic range, and never compare aPTT and anti-Xa values directly — they measure different things. If your patient is bouncing between supratherapeutic and subtherapeutic on aPTT despite stable rates, ask the team about switching to anti-Xa monitoring; this is increasingly the standard for ICU and high-acuity patients.
HIT screening and the 4Ts score
Heparin-induced thrombocytopenia (HIT) is a delayed immune-mediated reaction that can paradoxically cause life-threatening clots. Onset is typically 5–10 days after heparin exposure (or within 24 hours if the patient was exposed in the previous 100 days). Watch for: platelet count drop ≥50% from baseline, new thrombosis on heparin, skin necrosis at injection sites, anaphylactoid reactions to IV heparin bolus. The 4Ts score (Thrombocytopenia severity, Timing, Thrombosis, oTher causes) stratifies probability. A score of ≥4 mandates stopping all heparin (including LMWH and flushes), starting a non-heparin anticoagulant (argatroban, bivalirudin, or fondaparinux per local protocol), and sending HIT antibody and serotonin release assay testing. Never restart heparin in a confirmed HIT patient — the antibodies persist for months and can trigger acute thrombosis on re-exposure.
Frequently asked
Why use weight-based heparin instead of fixed-dose?
Weight-based dosing achieves therapeutic aPTT in 80-90% of patients within 24 hours; fixed-dose only achieves it in 60% (Raschke 1993). Weight-based reduces both clotting and bleeding events by tighter target attainment.
Should I use actual or adjusted body weight?
Use actual body weight up to a cap (typically 130-150 kg). For patients above the cap, use adjusted body weight or facility protocol. Obese patients have variable heparin clearance; closer monitoring is required.
What if aPTT doesn't respond to heparin?
Heparin resistance: the patient requires high doses to achieve therapeutic aPTT. Causes include antithrombin deficiency, accelerated clearance (DKA, fever), drug interactions. Consider switching to a direct thrombin inhibitor (argatroban, bivalirudin) or fondaparinux. Consult hematology.