Calculate INR from patient prothrombin time, control time, and ISI.
Copy PT and ISI values from the applicable laboratory context.
Educational estimate only. It is not medical advice, a diagnosis, or a substitute for assessment by a qualified healthcare professional. Never change anticoagulant medicine from this page; use the target and instructions supplied by the treating clinician.
The INR Calculator helps you calculate an International Normalized Ratio from patient PT, mean normal PT and reagent ISI. INR standardizes prothrombin-time results for differences in thromboplastin sensitivity. The inputs should come from the same laboratory context and a calibrated reagent.
Use it as a transparent arithmetic check. The worked example below follows the same inputs and rounding as the live form, while the notes explain what the result can and cannot support.
Write down the source and date of a real laboratory PT result before entering it. That small habit makes the result reproducible and helps another reader spot a unit or timing mismatch.
INR = (patient PT ÷ mean normal PT)ISI. Both PT values must use the same time unit; seconds cancel in their ratio.
| Input or result | Meaning |
|---|---|
| Patient PT | Measured patient prothrombin time, greater than zero. |
| Control PT | Laboratory mean normal PT for the method, greater than zero. |
| ISI | International Sensitivity Index for the thromboplastin and method. |
| INR | Dimensionless standardized ratio, rounded here to two decimals. |
With patient PT 15 seconds, control PT 12 seconds and ISI 1.0, INR = (15/12)¹ = 1.25.
The arithmetic does not decide whether 1.25 is appropriate for a particular person.
INR results are used most commonly in monitoring vitamin K antagonist therapy, but a target is chosen for a clinical reason. Different methods can still show variation. Contact the treating service for an unexpected result or dose question; do not change medication from this page. Urgent bleeding or other emergency symptoms require urgent clinical care.
Practical use: People usually open this page when they need to calculate an International Normalized Ratio from patient PT, mean normal PT and reagent ISI without building a spreadsheet for a single question. INR standardizes prothrombin-time results for differences in thromboplastin sensitivity. The inputs should come from the same laboratory context and a calibrated reagent. For a real decision, calculate a central case and then change the most uncertain input to a reasonable lower and upper value. Seeing that range is often more informative than treating one rounded answer as permanent.
A useful result should be repeatable: keep the original inputs, unit choices and relevant date beside the calculated INR. If a second source uses another convention, reconcile that difference before comparing the numbers.
When sharing the answer, include a short label such as “calculated INR from the stated inputs.” That label keeps the number attached to its meaning. It also prevents a reader from mistaking a converted value, model output or planning estimate for a direct observation.
Scope: It does not supply a therapeutic target or medication advice.
Do a quick reasonableness check after calculating. A correct-looking decimal can still answer the wrong question when the source laboratory PT result uses a different definition.
It is designed to calculate an International Normalized Ratio from patient PT, mean normal PT and reagent ISI. The output is a calculated INR based only on the values and conventions described on this page.
Check the unit, time period, sign and source definition for every entry. For this page in particular, confirm the meaning of the laboratory PT result rather than copying an unlabeled number from another source.
Another tool may use different rounding, units, timing or assumptions. Compare its stated formula with this page’s formula and scope. For INR Calculator, even a valid alternative convention can produce a different result without either arithmetic operation being mistyped.
Keep the displayed precision for an ordinary check, but retain source precision when the result feeds another calculation. Do not imply more measurement certainty than the original laboratory PT result provides.
The result is an educational estimate. It does not supply a therapeutic target or medication advice. Use the governing document, qualified professional or official service when a decision has financial, medical, legal or safety consequences.
Repeat each substitution in the displayed formula, keep units beside the numbers, and reverse the operation where possible. The example is also checked against the working calculator shown on the INR Calculator.
A legitimate unit conversion changes the numeral while preserving the quantity. A selector that merely labels currency or a model convention does not fetch outside data; the instructions above identify which behavior applies to this INR Calculator.