Dosing
Warfarin dose calculator
Enter the INR and the current weekly dose — get the adjusted weekly dose, the range behind it, and the hold or vitamin K plan when it applies.
A patient on 35 mg/week with a target of 2.0–3.0 returns an INR of 3.8, without bleeding.
Hold 0–1 dose, then ~30.6 mg/week (protocol range 29.8–31.5, −12.5%)
A 10–15% weekly-dose reduction with an INR recheck in one to two weeks — and the tool shows the daily average for tablet planning.
Who this is for
For clinicians running anticoagulation reviews. Maintenance adjustments follow published anticoagulation-clinic percentages (Horton & Bushwick, AFP 1999) applied to the weekly dose; INR 4.5–10, INR above 10, and bleeding scenarios follow CHEST 2012 — including when the right answer is to hold and give no new dose at all. Splits the week using 1 mg, 2.5 mg, and scored 5 mg tablets.
How it works
What it calculates
The maintenance warfarin dose from the INR: enter the target range, the INR, and the current weekly dose, and it computes the suggested new weekly dose (with the protocol's full range), the daily average, and the percentage change — plus hold and vitamin K guidance for high INRs per CHEST 2012.
Key principles
Adjust the weekly dose, not single daily doses, and split the week using the available strengths — 1 mg, 2.5 mg, and scored 5 mg tablets — by alternating different daily amounts, never by quartering tablets. Recheck in 1–2 weeks after any change, and look for a cause first (missed doses, new drugs, alcohol, acute illness) before changing a previously stable dose. Adjustment percentages follow published anticoagulation-clinic protocols (Horton & Bushwick, AFP 1999).
High INR essentials
INR 4.5–10 without bleeding: hold 1–2 doses, no routine vitamin K. INR above 10 without bleeding: hold and give oral vitamin K. Major bleeding at any INR: IV vitamin K plus four-factor PCC. Initiation is a different problem — use a starting nomogram (e.g. Kovacs 2003), not maintenance adjustments.
Built on published evidence
- maintenance adjustment percentages per anticoagulation-clinic protocols (Horton & Bushwick, AFP 1999); high-INR and bleeding management per CHEST 2012 (Holbrook)
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
Why adjust the weekly dose rather than the daily dose?
Warfarin responds over days, and anticoagulation-clinic protocols express changes as a percentage of the total weekly dose — alternating different daily amounts across the week gives finer control than changing every day by the same amount.
What happens at INR 4.5–10 without bleeding?
Hold one to two doses; routine vitamin K is NOT recommended (CHEST 2012). The calculator shows the resume dose — 10–20% below current — and is explicit that it starts only once the INR is back in range.
Does this cover starting warfarin?
No — initiation follows a starting nomogram (for example Kovacs 2003), which is a different problem from maintenance adjustment. The tool says so rather than extrapolating.
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