Menstrual cycle calculator

The calculator needs two things: the first day of your last period and your average cycle length. A cycle is counted from the first day of one period up to the day before the next one starts, so the first day of bleeding is always day 1 of a new cycle.

From that day 1, the calculator maps out your current cycle and the two that follow. Each new cycle starts one cycle length after the previous one, so the estimated period start dates are spaced evenly by the number you entered. Within each cycle, it places the estimated ovulation day 14 days before the next expected period, because 14 days is the luteal phase it assumes. The fertile window is the five days before ovulation plus ovulation day itself, six days in total. If you leave the cycle length at its default, the calculator uses 28 days.

Say your last period started on 1 March and your cycles are usually 30 days long. Cycle 1 begins on 1 March. Its estimated ovulation day is 17 March, and its fertile window runs from 12 March to 17 March.

Cycle 2 is estimated to begin 30 days later, on 31 March, with ovulation around 16 April and a fertile window from 11 April to 16 April. Cycle 3 is estimated to begin on 30 April, with ovulation around 16 May and a fertile window from 11 May to 16 May. Every date in the plan comes from the same two numbers, so changing either one moves the whole plan.

Very few people have cycles that are exactly the same length every month. It is common for cycle length to shift by a few days from one month to the next, which is why the calculator asks for an average rather than the length of a single cycle.

Because each projected cycle is built on top of the one before it, a difference between your average and your real cycles adds up. In the example above, if your cycles actually ran 33 days instead of 30, cycle 2 would begin on 3 April instead of 31 March, and cycle 3 on 6 May instead of 30 April. A three-day gap in one cycle becomes a six-day gap two cycles later. That is why the first cycle in the plan is usually the closest estimate and the last one is the roughest.

If you do not know your average, count the days from the first day of one period up to the day before the next one starts, for a few cycles, then take the average. Using your own number instead of the 28-day default is the single biggest improvement you can make to the plan.

The calculator assumes your cycles follow a steady pattern and that your luteal phase lasts 14 days. Neither is true for everyone. When your luteal phase is a little shorter or longer, the real ovulation day sits away from the date shown, even if your cycle length is exactly right.

Cycles are also often less predictable in the first years after periods begin, in the years leading up to menopause, after pregnancy and while breastfeeding, and when starting or stopping hormonal contraception. Stress, illness, travel and changes in sleep, weight or exercise can shift a single cycle. If your cycles often fall outside the 20 to 45 day range the calculator accepts, or vary widely from month to month, treat the plan as a rough guide only.

If your periods are often very irregular, stop for several months, or change suddenly, speak to a healthcare professional. This calculator cannot tell you why a cycle is irregular.

A single calculation is a snapshot of the numbers you entered today. Tracking turns it into a record. When you note the first day of every period, you can see your real cycle lengths instead of guessing, notice whether they stay steady or vary, and spot changes worth mentioning to a healthcare professional.

With a few months of history you can replace the 28-day default with your own average and recalculate after each new period, so the plan always starts from your most recent cycle. The Evecare app is built for this: you log your periods as they happen, and your upcoming cycles are estimated from your own history rather than a textbook average.

Whichever way you track, the results on this page are for information only. They cannot, and should not, be used as a conception aid or to prevent pregnancy.