Enter the patient’s prothrombin time, mean normal prothrombin time, and the international sensitivity index into the calculator to determine the International Normalized Ratio (INR).

International Normalized Ratio (INR) Calculator

Calculate INR from your PT, or check whether an INR value is in therapeutic range.

Calculate INR
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Found on your lab report or reagent insert. Most modern reagents are close to 1.0.
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Medical safety notice: Educational use only—do not change warfarin/anticoagulant dose based on this calculator. INR target ranges vary by condition and person; discuss results and your target range with your clinician/lab. Seek urgent care for signs of serious bleeding (e.g., vomiting/coughing blood, black stools, severe headache, or any fall/head injury) or possible clotting (e.g., chest pain, shortness of breath, one-sided weakness).

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INR Formula

The following formula is used to calculate the International Normalized Ratio (INR) for a given patient’s prothrombin time. 

INR = (PT_{patient} / PT_{normal})^{ISI}

Variables:

  • INR is the International Normalized Ratio
  • PTpatient is the patient’s prothrombin time in seconds
  • PTnormal is the mean normal prothrombin time in seconds
  • ISI is the International Sensitivity Index

To calculate the INR, divide the patient’s prothrombin time by the mean normal prothrombin time. Raise the result to the power of the International Sensitivity Index (ISI).

Learn more: Look up “PT/INR test” on MedlinePlus or “prothrombin time test” on the Mayo Clinic website for patient education and interpretation guidance. Last updated: January 2026.

What is INR?

The International Normalized Ratio (INR) is a standardized number that’s calculated in the laboratory. It measures how long it takes for blood to form a clot. The INR is used to monitor patients who are on anticoagulant medications, such as warfarin, to ensure that their blood is clotting within a safe and effective range. The INR is particularly useful because it standardizes the results of prothrombin time tests, regardless of the testing method or location, making it easier to compare results across different laboratories and settings. Accurate INR reporting depends on the correct ISI for the specific lab reagent/instrument system. INR monitoring is primarily used for warfarin and is not used to routinely monitor direct oral anticoagulants (DOACs) such as apixaban or rivaroxaban.

How to Calculate INR?

The following steps outline how to calculate the International Normalized Ratio (INR).

  1. First, determine the patient’s prothrombin time (PTpatient).
  2. Next, determine the mean normal prothrombin time (PTnormal).
  3. Next, determine the International Sensitivity Index (ISI).
  4. Finally, calculate the INR using the formula INR = (PTpatient / PTnormal)ISI.
  5. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Patient’s Prothrombin Time (PTpatient) = 20 seconds

Mean Normal Prothrombin Time (PTnormal) = 12 seconds

International Sensitivity Index (ISI) = 1.2