Warfarin Dosing Calculator

Estimate a starting weekly and daily warfarin maintenance dose from age, height, weight, race, and interacting medications, using the published IWPC clinical (non-genetic) dosing algorithm.

Quick Facts

Formula
IWPC clinical dosing algorithm (2009)
A published regression equation that predicts weekly warfarin dose without genetic test results.
Narrow therapeutic index
Estimate only, not a prescription
Real maintenance dose is set and adjusted by INR monitoring, not by this equation alone.

Your Results

Calculated
Estimated weekly dose
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IWPC clinical algorithm output
Estimated average daily dose
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Weekly dose divided by 7
Dose-requirement category
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Low / typical / high band
Practical daily reference
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Rounded to nearest 0.5 mg

Ready

Enter patient details, then calculate the estimated warfarin maintenance dose.

About the Warfarin Dosing Calculator

Warfarin is an anticoagulant with a narrow therapeutic index: too little dose leaves a patient under-protected against clotting, and too much raises bleeding risk, so clinicians typically start from an estimated dose and then adjust using International Normalized Ratio (INR) blood tests. This calculator implements the clinical (non-genetic) dosing algorithm published by the International Warfarin Pharmacogenetics Consortium (IWPC) in 2009, which predicts an average weekly maintenance dose from routinely available patient characteristics when CYP2C9/VKORC1 genetic testing is not available.

The formula

The IWPC clinical algorithm predicts the square root of the weekly dose in milligrams, then squares that value to get the dose itself:

sqrt(weekly dose in mg) = 4.0376 − 0.2546 × (age in decades) + 0.0118 × (height in cm) + 0.0134 × (weight in kg) − 0.6752 × (Asian race) + 0.4060 × (Black or African American race) + 0.0443 × (mixed or missing race) + 1.2799 × (enzyme-inducer use) − 0.5695 × (amiodarone use)

Each race, enzyme-inducer, and amiodarone term is either 0 or 1 depending on whether it applies. Squaring the result gives the predicted weekly dose in milligrams; dividing by 7 gives an average daily dose. Enzyme-inducing drugs (such as rifampin, carbamazepine, or phenytoin) speed up warfarin clearance and push the estimate higher, while amiodarone slows clearance and pushes the estimate lower.

Reading the dose-requirement category

Predicted weekly doses are commonly grouped into practical bands: under about 21 mg/week (roughly 3 mg/day) is considered a low dose requirement, 21 to 49 mg/week is a typical range, and above 49 mg/week is a high dose requirement. Patients in the low or high bands often warrant closer initial INR monitoring, since they are further from the average response seen in the derivation population.

When to consult a professional

This tool performs published-formula arithmetic only; it does not prescribe or adjust medication. Warfarin dosing must always be individualized and monitored by a licensed clinician using serial INR results, and this estimate should never replace that process or be used to start, stop, or change a dose on your own.

Frequently Asked Questions

What formula does this warfarin dosing calculator use?
It uses the clinical (non-genetic) dosing algorithm published by the International Warfarin Pharmacogenetics Consortium (IWPC) in 2009. The square root of the predicted weekly dose in mg is calculated from age in decades, height in cm, weight in kg, race category, enzyme-inducer use, and amiodarone use, then squared to give mg per week. This is the same style of equation used when genetic test results (CYP2C9 and VKORC1) are not available.
Is this the actual dose I should take?
No. The result is a population-based starting estimate from a published regression equation, not a prescription. Warfarin has a narrow therapeutic index, and the real maintenance dose is set and adjusted by a clinician using serial INR measurements, not by a single formula.
Why do race and interacting medications affect the estimate?
In the IWPC derivation cohort, average warfarin requirements differed by self-reported race category, and by use of enzyme-inducing drugs (such as rifampin, carbamazepine, or phenytoin) or amiodarone, both of which change how the body clears warfarin. The formula includes coefficients for these factors to reduce average prediction error across the population studied.

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