CoaglyPT to INR conversion: the formula and a free calculator
Some labs report a prothrombin time in seconds and leave you to work out the INR, which is awkward when the INR is the number your clinic actually uses. The conversion is a single formula — but it needs two values from your own lab report, and using anyone else's will give you the wrong answer.
PT and INR are not the same thing
Prothrombin time (PT) is the raw measurement: how many seconds a sample of your plasma takes to clot after a reagent called thromboplastin is added. A typical untreated PT is somewhere around 11 to 13 seconds.
The problem is that thromboplastin reagents differ between manufacturers and batches. The same blood tested in two labs can produce noticeably different PT values — which made a raw PT useless for steering warfarin doses across different services.
INR was created to fix that. It is a mathematical correction that expresses your PT as though every lab used one standard reference reagent. It is why an INR of 2.5 means the same thing everywhere.
The conversion formula
The formula
INR = (Patient PT ÷ Normal mean PT) ISI
Where ISI is the International Sensitivity Index of the reagent the lab used, and normal mean PT is that lab's own geometric mean PT for healthy people.
The ratio alone is not the INR — it has to be raised to the power of the ISI. That exponent is what corrects for reagent sensitivity, and skipping it is the most common mistake. When the ISI is exactly 1.0 the two happen to coincide, which is why the error often goes unnoticed until it is applied to a lab with a different reagent.
Finding your ISI and normal mean
Both are properties of the specific lab and reagent, and both are usually printed on the report:
- ISI — typically between 0.9 and 1.7 for modern reagents; many sit close to 1.0
- Normal mean PT — usually 11 to 13 seconds, sometimes shown as the reference interval
If either is missing, ring the lab and ask. Do not substitute values from a website, including this one — the defaults in the calculator below are placeholders to demonstrate the maths, not values for your report.
Free PT to INR calculator
Our free PT to INR converter applies the formula above, tells you whether the result falls inside your target range, and runs entirely in your browser. Enter your PT in seconds along with your lab's normal mean and ISI.
A worked example
Patient PT 24.0 s · normal mean 12.0 s · ISI 1.2
- Ratio = 24.0 ÷ 12.0 = 2.0
- INR = 2.0 1.2 = 2.30
Notice that the ratio was 2.0 but the INR is 2.30. Reporting the ratio as the INR would have understated the result by a clinically meaningful amount — enough to look in range when it was drifting toward the upper half.
Where this goes wrong
- Using the ratio instead of the INR. Only identical when the ISI is exactly 1.0.
- Borrowing an ISI from elsewhere. It belongs to the reagent lot, not to you.
- Mixing labs. A PT from one lab with a normal mean from another produces a number that means nothing.
- Point-of-care meters. Home meters such as CoaguChek report INR directly and apply their own internal correction — there is nothing to convert.
- Treating a converted INR as a dosing instruction. It is a translation of a result, not a recommendation. Dose changes come from your clinic.
Also worth knowing: INR is only validated for monitoring vitamin K antagonists such as warfarin. A raised INR in liver disease or vitamin K deficiency is real, but it does not carry the same meaning as the same number in someone on warfarin.
Logging a PT in Coagly
Coagly handles this on the entry screen. When you add a result there is a "Only have a PT in seconds?" option — enter the seconds along with your lab's normal mean and ISI, and Coagly converts it, stores the INR, and keeps the original PT with it so the source of the number is never lost.
The converted value joins the rest of your history: same trend chart, same in-range labelling, same contribution to your time in therapeutic range, and included in the CSV export you take to your appointment.
Common questions
How do you convert PT to INR?
Divide your patient PT in seconds by your lab’s normal mean PT, then raise that ratio to the power of the reagent’s ISI. In full: INR = (patient PT / normal mean PT) ^ ISI. Both the normal mean and the ISI are specific to the lab that ran your test and are usually printed on the report.
What is a normal PT in seconds?
Most labs report a normal prothrombin time of roughly 11 to 13 seconds, though the exact reference interval varies by lab and reagent. Your own lab’s normal mean is the value you should use in the conversion.
What is ISI in an INR calculation?
The International Sensitivity Index describes how sensitive a particular thromboplastin reagent is compared with an international reference preparation. It is the exponent in the INR formula and typically falls between 0.9 and 1.7. Without it, a PT ratio cannot be turned into a comparable INR.
Is the PT ratio the same as the INR?
Only when the ISI is exactly 1.0. For any other reagent the ratio has to be raised to the power of the ISI, and the difference can be clinically meaningful — a ratio of 2.0 with an ISI of 1.2 gives an INR of 2.30.