Warfarin has a narrow therapeutic window, so keeping the INR in range means making small, structured changes to the weekly dose rather than guessing. This calculator applies a widely used maintenance nomogram to turn a current INR and weekly dose into a concrete recommended change.
How it works
The tool first places the INR into a band, then applies the percentage change that band specifies to the current total weekly dose, and finally divides by seven to suggest a daily figure.
new weekly dose = current weekly dose × (1 + percentage change / 100)
new daily dose = new weekly dose / 7For a target INR of 2.0 to 3.0, the bands work as follows:
| INR result | Action |
|---|---|
| Below 1.5 | Increase weekly dose by 10–20% |
| 1.5–1.9 | Increase weekly dose by 5–10% |
| 2.0–3.0 (in range) | No change |
| 3.1–3.9 | Decrease weekly dose by 5–10% |
| 4.0–9.0 | Hold 1–2 doses, decrease by 10–20%, consider vitamin K |
| Above 9.0 | Stop warfarin, give vitamin K, urgent clinical review |
For the 2.5–3.5 target (used for some mechanical valves and other indications), the in-range band shifts accordingly.
Why weekly dose, not daily dose
Warfarin tablets come in small increments (typically 0.5 mg, 1 mg, 2 mg, 5 mg) and daily doses are often split across the week rather than being uniform each day. Working in weekly totals allows fine-grained changes — for example, raising from 35 mg/week to 37.5 mg/week by adding half a milligram on five days — which would be difficult to express as a simple daily dose change.
Worked example
A patient on 35 mg per week has an INR of 1.7 with a target of 2.0 to 3.0. The nomogram puts this in the 1.5–1.9 band: increase by 5–10%.
- 5% increase: 35 × 1.05 = 36.75 mg/week (about 5.25 mg/day)
- 10% increase: 35 × 1.10 = 38.5 mg/week (about 5.5 mg/day)
A reasonable choice is 37.5 mg/week. Plan a recheck INR in one to two weeks.
Before acting on any out-of-range INR
A nomogram tells you what to do mechanically — but clinical judgement decides whether to act at all. Consider:
- Missed doses — a single missed dose is often the explanation and may require no permanent change.
- Drug interactions — antibiotics, antifungals, amiodarone, and many other drugs significantly affect INR.
- Dietary vitamin K — large servings of green leafy vegetables can drop the INR; a sudden reduction in intake can raise it.
- Illness — fever and reduced oral intake both affect warfarin metabolism.
If a clear reversible cause is found, recheck the INR in a few days rather than adjusting the maintenance dose. This nomogram assumes no active bleeding and stable adherence; always work within your local anticoagulation protocol and under clinician oversight.