Irregular period calculator

When your cycles are not the same length each month, one number is the wrong input. Enter the days your last few periods started and you get your own typical length, how much your cycles vary, and the next six estimated periods with a range that fits how much they vary.

This calculator runs entirely in your browser. The dates you enter are not sent anywhere, not stored and not seen by anyone, including us. Close the tab and they are gone.

Every date here is an estimate from a typical cycle length. It is not birth control and it is not medical advice.

What counts as irregular

Cycle length is counted from the first day of one period to the day before the next one starts. The NHS calls periods irregular when the gap between them is less than 21 days or more than 35 days. The US Office on Women's Health counts periods as regular if they usually come every 24 to 38 days, and notes that cycle length can differ from month to month.

So there are two different things people mean by irregular: cycles that are usually outside those ranges, and cycles that are inside them but differ a lot from one month to the next. This calculator handles the second: it does not need your cycles to be the same, it uses how much they differ.

Sources: NHS, Irregular periods (last reviewed 26 June 2026); Office on Women's Health, Your menstrual cycle (updated 13 January 2025).

How the estimate is worked out

From your dates, the calculator measures each cycle and takes the middle value of the last six as your typical length. The middle value is used instead of the average so that one very short or very long cycle does not pull every estimate with it. In the example, the average is 31.0 days and the middle value is 29.

When your shortest and longest cycles differ by more than a week, a single date would mislead, so each estimate is shown as a range: from your shortest cycle to your longest. The next period gets that range once, the one after twice as wide, and the third and later three times as wide, because the uncertainty adds up.

A gap of more than 45 days that is also much longer than your other cycles (more than 1.6 times the middle value) is left out, because it most often means a period was not entered. Gaps under 15 days or over 90 days are also left out.

This is the same rule the Isla app uses on the periods you log, so the calculator and the app give the same dates from the same history.

Why one number is the wrong input

A calculator that asks for one cycle length assumes the next period is exactly that many days away. With the example cycles above, a 29-day calculator would give Friday, February 5, 2027 with a 3-day margin. The real cycles in the example ran from 24 to 41 days, so the honest answer is a range of Jan 31 to Feb 17.

When to talk with a clinician

The NHS suggests seeing a GP if your periods are irregular, if they last longer than 7 days, or if you bleed between periods. The Office on Women's Health suggests talking with a doctor or nurse if your cycles are longer than 38 days or shorter than 24 days, if you have not had a period for 90 days and are not pregnant or breastfeeding, or if you are worried about your cycle. Starting or stopping hormonal birth control, pregnancy, breastfeeding, the years around perimenopause, stress, weight change and a lot of exercise can all change cycles.

This page estimates dates. It cannot say why your cycles vary.

Related

If your cycles are steady, the period date calculator needs only one start date and a length. Each length from 21 to 40 days has its own page under cycle lengths.