Last updated: 2026-06-22

Period Predictor

A period predictor estimates the start dates of upcoming menstrual periods from two pieces of information: the first day of the most recent period and the average length of the menstrual cycle. It is used to anticipate when bleeding is likely to begin, to plan around travel or events, and to identify the fertile window indirectly by knowing when the next cycle starts. The estimate is a projection, not a measurement, and it carries the same regularity as the cycle it is built on.

How the projection works

The menstrual cycle is counted from the first day of one period to the first day of the next. Adding one cycle length to the last period's start date gives the expected start of the next period. Adding two and three cycle lengths projects the two periods after that:

Here $L$ is the average cycle length in days, $d$ is the period (bleeding) length in days, and $n$ is 1, 2, or 3 for the first, second, and third upcoming periods. The period end subtracts one day because the start day itself counts as the first day of bleeding: a five-day period that begins on the 29th runs through the 2nd, not the 3rd.

Worked example

Suppose the last period began on 3 March 2026, the average cycle is 31 days, and bleeding typically lasts 4 days. The next period is projected as:

So the next period is expected to begin around 3 April and end around 6 April, with the two following periods near 4 May and 4 June.

Typical cycle figures

Cycle length varies between people and from month to month. The figures below describe common ranges rather than fixed values.

Interpretation and limits

The projection assumes the cycle length stays roughly constant. Because real cycles drift by a few days, the estimated dates are best read as a window of a day or two on either side rather than an exact calendar entry. Stress, illness, travel across time zones, significant weight change, perimenopause, and hormonal contraception can all shift the timing, sometimes by more than the usual variation. For cycles that swing widely from month to month, an average is a weak predictor and the dates should be treated with caution.

The tool projects period start dates only; it does not estimate ovulation precisely or assess fertility. For an ovulation-focused estimate see the Ovulation Predictor, and for pregnancy dating see the Pregnancy Due Date Calculator.