Period Calculator

This period calculator works out when your next period is likely to start, using the first day of your last period and the length of your menstrual cycle. It also estimates your ovulation day and fertile window, and shows the next 3 months at once, so you can plan holidays, exams or trips.

There is no app to install and no sign up. The dates are worked out on your own device and nothing you type is saved or sent anywhere, so you can use it as a free period tracker without giving your health details to anyone.

Everything here is an estimate. Bodies are not clocks, and even regular cycles move by a few days. Please don't use this calculator as a way to avoid pregnancy.

The maths is simple, and you can do it yourself with a calendar. Count the first day of your period as day 1. Your next period is due one cycle length later. Ovulation usually falls on the cycle day you get by taking 14 away from your cycle length, and the fertile window is the 5 days before ovulation, ovulation day itself and the day after.

Here is an example with a 28 day cycle. Say your last period started on 1 March. Add 28 days and your next period is due on 29 March. Take 14 away from 28 and you get day 14, so ovulation is likely around 14 March. Your fertile window is roughly 9 March to 15 March.

For a 30 day cycle, everything shifts by two days: next period on 31 March, ovulation around day 16 (16 March), fertile window about 11 to 17 March. The second half of the cycle, from ovulation to your period, stays about the same length for most people. It is the first half that stretches or shrinks.

Menstrual phase (about days 1 to 5): the lining of the womb comes away and you bleed. A period usually lasts 2 to 7 days.

Follicular phase (from day 1 to ovulation): hormones help an egg grow in one of the ovaries and the womb lining builds up again. This is the part of the cycle that changes most in length, which is why your cycle might be 26 days one month and 31 the next.

Ovulation (around the middle): the ovary releases an egg. It lives for about a day, but sperm can survive for up to 5 days, which is why the fertile window starts before ovulation.

Luteal phase (ovulation to your next period, often about 14 days): if the egg is not fertilised, hormone levels drop and the next period begins. PMS, such as bloating, sore breasts or mood changes, often shows up in the last few days of this phase.

The average cycle is about 28 days, but anywhere from 21 to 35 days is common in adults. In the first few years after periods start, and in the years before menopause, cycles of up to 45 days are also usual.

It is normal for your cycle to change by a few days from month to month. To find your own cycle length, count from the first day of one period to the day before the next one starts. Doing this for three or four months gives you a much better number than guessing.

Irregular periods are very common. Stress, travel and time zone changes, a big change in weight, intense exercise, breastfeeding, and starting or stopping hormonal birth control can all move your period. So can conditions like polycystic ovary syndrome (PCOS) and thyroid problems.

After stopping the pill or another hormonal method, it can take a few months for your natural cycle to settle. After having a baby, periods can take weeks or months to come back, especially if you are breastfeeding, and the first few cycles are often uneven. In both cases, use the irregular option and enter a range.

Please see a doctor if your periods stop for 3 months or more (and you are not pregnant), if your cycles are often shorter than 21 days or longer than 35, if you bleed between periods or after sex, or if your periods are very heavy or very painful. These are common and usually treatable, and you don't need to wait.