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Cycle length calculator
Your cycle length is the number of days from the first day of one period to the first day of the next. The average is about 28 days, and 21 to 35 is common in adults. Add your last few start dates to measure yours.
- No sign-up, no email needed
- Nothing you enter is stored
- Ranges that fit real, changing cycles
Your cycle
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 cycle length is counted
From one period to the next.
A cycle starts on the first day of your period and ends the day before your next period starts. Enter two start dates and we can measure one cycle; enter four and we can measure three. The more dates, the better the average.
What we do with it
We average your three most recent cycles and use your shortest and longest of those to set the range for your next period. If your cycle moves a lot, the range is wider, which is more honest than a single date. If yours varies by more than about a week, the irregular period calculator explains what that can mean.
What your cycle length means
How ovulation and your next period line up with 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
How do I calculate my cycle length?
Count the days from the first day of one period to the first day of the next. Don’t include the second start day. For example, from 1 October to 29 October is a 28-day cycle.
What is a normal cycle length?
About 28 days on average. Anything from 21 to 35 days is common in adults, and the NHS describes up to 40 days as normal. Cycles are often longer and less regular in the first years after periods start and in the years before menopause.
How much can my cycle vary?
A few days from month to month is normal. Cycles that differ by more than about 7 to 9 days are usually called irregular.
Is a 35-day cycle normal?
Yes, it’s within the usual range for adults. What matters more is whether it’s typical for you. A sudden change is worth mentioning to a doctor.
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.
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.