Predict when your next periods will start and end, how many days are left, and your likely fertile days, from your last period and cycle length.
How it works
Each period is predicted one cycle length after the one before: if your last period started on 20 September and your cycle is 28 days, the next ones start on 18 October, 15 November, 13 December. With a 5-day period, the next one runs from 18 October to 22 October. On 2 October that would be day 13 of the cycle, with 16 days to go.
The fertile window before each period is the 5 days before ovulation plus the day after, with ovulation estimated 14 days before the period (ACOG). For more on fertile days, see the ovulation calculator.
When to see a doctor
- Your periods are very heavy, very painful, or last longer than 7 days.
- They come more often than every 21 days, or stop for no clear reason.
- You bleed between periods or after sex.
Pregnant? Use the due date calculator.
Sources, checked 2 October 2026: NHS, Periods, Fertility in the menstrual cycle and Stopped or missed periods; ACOG, Your changing body and Fertility awareness-based methods. Predictions only, not medical advice or contraception.
Frequently asked questions
What's a normal menstrual cycle?
The NHS says the average cycle is 28 days and regular cycles from 21 to 35 days are normal, with periods lasting 2 to 7 days (usually about 5). In teenagers, cycles of 21 to 45 days are normal while periods settle down (ACOG).
How do I work out my cycle length?
Count from the first day of one period to the day before the next one starts. Note it for a few months and use the average: if your last three cycles were 27, 30 and 28 days, use 28.
Why is my period late?
The NHS lists pregnancy, stress, the start of menopause, PMOS (previously called PCOS), sudden weight loss, being overweight, doing too much exercise and some hormonal contraception as common causes. Take a pregnancy test if you could be pregnant, and see a GP if your periods stop or change for no clear reason.
Is my information saved?
No. The dates you enter stay in your browser and aren't stored or sent anywhere, so you'll need to enter them again next time.