INR Calculator

Use this INR Calculator to calculate INR, patient prothrombin time (PT), control PT, or ISI when the other required values are known.

Important: This calculator is for formula calculation and education only. Do not use it to change a warfarin or other anticoagulant dose. Clinical decisions should use official laboratory results and healthcare-professional guidance.

Calculation Method

The standard INR relationship is:

INR = (Patient PT ÷ Control PT)^ISI

The formula can also be rearranged:

Patient PT = Control PT × INR^1/ISI

Control PT = Patient PT ÷ INR^1/ISI

ISI = ln(INR) ÷ ln(Patient PT ÷ Control PT)

Example

Suppose:

• Patient PT = 24 seconds
• Control PT = 12 seconds
• ISI = 1.0

First calculate the PT ratio:

24 ÷ 12 = 2

Then apply the ISI:

INR = 2^1.0 = 2.0

Important Limitations

An online calculation is not a substitute for an official laboratory INR result. INR interpretation depends on why the test was ordered, anticoagulant treatment, target range, symptoms, medications, diet, illness, and clinical history.

Do not start, stop, skip, double, or change anticoagulant medication based on this calculator.

Sources

• World Health Organization — Guidelines for Thromboplastins and Plasma Used to Control Oral Anticoagulant Therapy
• MedlinePlus — Prothrombin Time Test and INR