Statistics & Probability

Number Needed to Treat Calculator

Calculate number needed to treat from event rates in two groups, with the absolute and relative reductions.

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Number needed to treat
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One step, one exact factorNumber Neededwhat you haveconvertTreat Calculatorwhat you getBoth units are defined against the same base unit, so the factor is exact — never an approximation.

The most honest single number in clinical evidence

An NNT of 12.5 means twelve or thirteen people take the treatment for one to benefit. It converts a percentage into people, which is why patients understand it better than a relative risk reduction of 40% — the same result described in the way that sounds most impressive.

What this page needs from you

This page asks for event rate, control group as a percentage and event rate, treatment group as a percentage. From those it returns number needed to treat in patients, together with absolute risk reduction, relative risk reduction, relative risk and events prevented per 1,000 treated. Every figure updates as you type, and nothing you enter leaves your browser.

Significance is not the same as importance

With a large enough sample almost any difference becomes statistically significant, because significance measures how confident you can be that a difference exists at all — not how big it is or whether it matters. Effect size answers the second question, and the two should always be read together.

The formula

NNT = 1 / absolute risk reduction

Every result on this page comes from this formula. Nothing is rounded until the final display, so figures match a spreadsheet to the cent.

How to use the Number Needed to Treat Calculator

  1. Fill in each field with your own figures — the defaults are only there as an example.
  2. The result updates as you type, so there is no button to press.
  3. Check the breakdown under the main result for the supporting numbers.
  4. Use the copy button to take the result with you, or share the link to reload the same inputs.

Number Needed to Treat Calculator — frequently asked questions

Is this number needed to treat calculator free to use?

Yes. Every calculator on the site is free, needs no sign-up, and runs entirely in your browser, so your figures never leave your device.

How accurate is the result?

The maths is exact for the formula shown on the page. Accuracy in the real world depends on your inputs and on assumptions the formula cannot see, so treat the output as a well-grounded estimate rather than a quote.

Can I use it on my phone?

Yes. The page is responsive, weighs a few kilobytes and works offline once loaded, so it behaves the same on a phone as on a desktop.

What people search for

This page answers each of these searches. They are the exact phrases visitors use to find the number needed to treat calculator.

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About this calculator

Written and reviewed by The Samsung Calculator editorial team. Every calculator is written against a published formula, reviewed against at least one independent reference implementation, and dated when it changes.

Last reviewed September 14, 2026 · Calculation runs client-side · No data collected · Report an error