INR Calculator (from prothrombin time, mean normal PT and ISI)

Understanding the INR calculator — concept and principle

The international normalised ratio (INR) expresses a prothrombin time (PT) blood test in a standard way, so results from different laboratories and reagents can be compared. It is the main test used to monitor warfarin and similar vitamin K antagonists. This calculator works out INR from the patient's PT, the laboratory's mean normal PT and the reagent's ISI.

PT ratio to the power ISI: with the default inputs the result is INR 2.14 (within the 2.0–3.0 target).

How it is calculated

INR = (patient PT ÷ mean normal PT)^ISI

PT ratio = patient PT ÷ mean normal PT

PT is in seconds.
The mean normal PT is the laboratory's average for healthy people using the same reagent.
The International Sensitivity Index (ISI) is printed with each reagent lot.

The principle

Prothrombin time measures how long plasma takes to clot after a tissue-factor reagent (thromboplastin) is added. Reagents differ in how sensitive they are to low clotting factors, so the same blood can give different PT values in different labs. Each reagent is calibrated against a World Health Organization reference thromboplastin, and the ISI is the exponent that converts its PT ratio to the ratio the reference reagent would have given. Sensitive reagents have an ISI near 1.0.

Worked example

Default inputs: patient PT 24 s, mean normal PT 12 s, ISI 1.1.

PT ratio = 24 ÷ 12 = 2.
INR = 2^1.1 = e^(1.1 × 0.6931) = 2.14.
2.14 is in 2.0–3.0 → Within 2.0–3.0.

Each point is the calculator re-run with only “Mean normal prothrombin time (seconds)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Mean normal prothrombin time (seconds)	INR (international normalised ratio)
3	9.85
6	4.59
9	2.94
12	2.14
15	1.68
18	1.37
24	1

How to read the result

Where does your result fall?

People not taking anticoagulants usually have an INR around 1. For most reasons to take warfarin, such as atrial fibrillation or venous thrombosis, the target range is 2.0–3.0; some mechanical heart valves need higher targets. An INR above the target raises bleeding risk and one below it raises clot risk. INR is not used to monitor direct oral anticoagulants.

Common mistakes

Using a mean normal PT or ISI from a different reagent lot.
Comparing PT seconds between labs instead of INR.
Changing a dose based on one result without the clinic's advice.

Keep in mind: This is not medical advice. Never change an anticoagulant dose on the basis of this page; the treating clinic decides.

Frequently asked questions

What is a normal INR?

About 0.8–1.2 in people not taking anticoagulants, depending on the lab.

Why does the target differ between patients?

It depends on the reason for treatment and bleeding risk; some valves need higher targets.

Do foods affect INR?

Yes. Large changes in vitamin K intake, such as leafy greens, can change INR in people taking warfarin.

Sources: University of Iowa Health Care, Department of Pathology Laboratory Services Handbook, "International Normalized Ratio (INR)" · World Health Organization, Expert Committee on Biological Standardization, guidelines for thromboplastins and plasma used to control oral anticoagulant therapy

Checked 9 Nov 2026. University of Iowa Health Care, Department of Pathology Laboratory Services Handbook, “International Normalized Ratio (INR)” — INR = (patient PT/mean normal PT)^ISI, WHO-calibrated ISI, INR 2.0–3.0 for most indications.

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