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Period calculator
Your next period usually starts one cycle after your last one began: add your usual cycle length (21 to 35 days is common) to the first day of your last period. Enter yours to see your next three periods as honest ranges. Nothing is stored.
- No sign-up, no email needed
- Nothing you enter is stored
- Ranges that fit real, changing cycles
Your next periods
Enter your dates above and your estimates will show here. Nothing you enter leaves this page.
An estimate, not medical advice or birth control. Don’t use these dates to avoid pregnancy.
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How the calculator works
Simple math, shown honestly.
We count forward from the first day of your last period using your usual cycle length. Ovulation usually happens 12 to 16 days before the next period (NHS), so we use 14. The fertile window is the five days before ovulation plus the day itself (ACOG). The ovulation calculator shows those days in more detail.
Why ranges, not dates
Most cycles move by a few days from month to month. A single date looks precise but is often wrong, so we show the days your period is likely to start. Tell us how much your cycle varies and the range widens to match.
Typical dates by cycle length
Where ovulation, the fertile window and the next period usually fall for common cycle lengths.
| Cycle length | Likely ovulation | Fertile window | Next period starts |
|---|---|---|---|
| 21 days | day 7 | days 2–7 | day 22 |
| 24 days | day 10 | days 5–10 | day 25 |
| 26 days | day 12 | days 7–12 | day 27 |
| 28 days | day 14 | days 9–14 | day 29 |
| 30 days | day 16 | days 11–16 | day 31 |
| 32 days | day 18 | days 13–18 | day 33 |
| 35 days | day 21 | days 16–21 | day 36 |
Days are counted from the first day of your last period (day 1). Real cycles move, so treat each as the middle of a range.
Sources: NHS: Periods · ACOG: Abnormal uterine bleeding
Is my data saved anywhere?
No. The calculator runs in your browser. The dates you enter are never sent to us, never put in the page address and never stored. Close the tab and they’re gone.
How do I calculate my next period?
Add your usual cycle length to the first day of your last period. A cycle is counted from the first day of one period to the day before the next. If your last period started on 2 November and your cycle is 28 days, your next period would be due around 30 November. Because cycles vary, it’s more useful to expect a range of a few days either side.
What is a normal cycle length?
The average is about 28 days, but anything from 21 to 35 days is common in adults, and the NHS describes up to 40 days as normal. Cycles often vary by a few days from month to month.
What if my cycle is irregular?
Choose “It varies a lot” and we’ll widen each range. If you know the start dates of your last few periods, the irregular period calculator uses them to work out your average and how much it moves.
How long does a period usually last?
Most periods last between 2 and 7 days. Bleeding is usually heaviest in the first two days.
Why is my period late?
Stress, illness, travel, weight changes, exercise, breastfeeding, hormonal contraception and conditions like PCOS can all change your cycle. If there’s a chance you could be pregnant, take a pregnancy test. If your periods change suddenly or stop, see a doctor.
Can I use this as birth control?
No. These are estimates from a calendar, and ovulation can move from cycle to cycle. Don’t use them to avoid pregnancy. If you need contraception, talk to a doctor, nurse or pharmacist about methods that are tested for it.