Period Calculator

How to predict your next period

1. Enter the first day of your last period.
2. Set your average cycle length.
3. Set how many days your period usually lasts.
4. Read your next expected period and the upcoming cycles.

How the prediction works

This period calculator projects future periods by repeating your cycle length forward from the first day of your last period. If your cycle is 28 days, the next period is expected 28 days later, and so on for the cycles after that. It also marks each cycle's ovulation, about 14 days before the following period, and the fertile window around it. It is a simple menstrual cycle calculator built on the average length you give it, working as a period tracker and period predictor from your cycle average.

What the cycle looks like

A menstrual cycle is counted from the first day of one period to the first day of the next. The period itself is the bleeding at the start; ovulation falls later, roughly 14 days before the next period. Cycle length varies from person to person, commonly between 21 and 35 days, so tracking a few cycles gives a better average to predict from.

Source: menstrual-cycle timing as described by the American College of Obstetricians and Gynecologists (ACOG) and the UK National Health Service (NHS).

When predictions are less reliable

The forecast assumes your cycle stays close to the length you entered. Real cycles shift with stress, illness, travel, and life stage, and some people have naturally irregular cycles that a calendar cannot predict well. The fertile-window dates are estimates and are not a method of contraception. Use the prediction as a rough guide, not a guarantee.

An estimate, not medical advice

These dates are estimates that vary by cycle and by person. This is not medical advice and cannot diagnose anything. If your periods are very irregular, very painful, or suddenly change, speak to a doctor.

FAQ

When is my next period?

Add your cycle length to the first day of your last period. Enter those and this tool shows the next expected date and several cycles ahead.

How long is a normal menstrual cycle?

Commonly between 21 and 35 days, counted from the first day of one period to the first day of the next. Yours may be shorter or longer, and tracking a few gives a better average.

Can this predict irregular periods?

Not reliably. The prediction assumes a regular cycle, so irregular cycles are hard to forecast from a calendar. Track several cycles or speak to a doctor.

Does it show my fertile window?

Yes, as an estimate for each cycle. It is not a method of contraception and should not be used to avoid or plan pregnancy on its own.

Is my information private?

Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.