Period calculator

The calculator needs three things: the first day of your last period, your average cycle length and the number of days your period usually lasts. 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.

To estimate your next period, it adds your cycle length to that day 1. To estimate the one after, it adds the cycle length again, and it repeats this for your next three periods. Each predicted period then lasts for the number of days you entered as your period length. If you leave the fields at their defaults, the calculator assumes a 28-day cycle and a 5-day period.

Say your last period started on 1 March, your cycles are usually 30 days long and your period lasts 5 days. The calculator adds 30 days to 1 March and predicts that your next period will start on 31 March and end on 4 April. Adding another 30 days gives 30 April to 4 May, and a third step gives 30 May to 3 June. The arithmetic is simple, which is why the estimate is only as good as the cycle length you give it.

Very few people have a cycle that is 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.

Small differences add up. Because each prediction builds on the one before it, an average that is two days too short moves your next predicted period two days too early, the second four days too early and the third six days too early. That is why the first date in your results is usually the most useful one, and why it makes sense to recalculate each time a new period starts rather than relying on dates several months ahead.

If you do not know your average, count the days from the first day of one period to the first day of the next 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 estimate.

The calculator assumes your cycles follow a steady pattern. When they do not, the predicted dates are less likely to match what actually happens. Cycles are 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.

Everyday life can play a part too. Stress, illness, travel and changes in sleep, weight or exercise can all make a period arrive earlier or later than expected. If your cycles often fall outside the 20 to 45 day range the calculator accepts, or vary widely from month to month, treat the results as a rough guide only.

A late period is not something this calculator can explain. If your period is later than you expected and you could be pregnant, a pregnancy test will give you a clearer answer. If you are worried about irregular, very heavy or painful periods, speak to a healthcare professional.

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 with more confidence. The Evecare app is built for this: you log your periods as they happen, so your next prediction starts from your own cycles 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.