Menstrual cycle lengths from 21 to 40 days
How many periods a year each cycle length gives, how the start day moves through the week, and an example calendar for each.
| Cycle | Periods a year | Next start, in the week | Calendar-method mid-cycle estimate |
|---|---|---|---|
| 21 days | 17.4 | same weekday | day 7 |
| 22 days | 16.6 | 1 day later | day 8 |
| 23 days | 15.9 | 2 days later | day 9 |
| 24 days | 15.2 | 3 days later | day 10 |
| 25 days | 14.6 | 3 days earlier | day 11 |
| 26 days | 14.0 | 2 days earlier | day 12 |
| 27 days | 13.5 | 1 day earlier | day 13 |
| 28 days | 13.0 | same weekday | day 14 |
| 29 days | 12.6 | 1 day later | day 15 |
| 30 days | 12.2 | 2 days later | day 16 |
| 31 days | 11.8 | 3 days later | day 17 |
| 32 days | 11.4 | 3 days earlier | day 18 |
| 33 days | 11.1 | 2 days earlier | day 19 |
| 34 days | 10.7 | 1 day earlier | day 20 |
| 35 days | 10.4 | same weekday | day 21 |
| 36 days | 10.1 | 1 day later | day 22 |
| 37 days | 9.9 | 2 days later | day 23 |
| 38 days | 9.6 | 3 days later | day 24 |
| 39 days | 9.4 | 3 days earlier | day 25 |
| 40 days | 9.1 | 2 days earlier | day 26 |
The NHS describes periods every 21 to 35 days as common; the US Office on Women's Health calls 24 to 38 days regular. Every date here is an estimate from a typical cycle length. It is not birth control and it is not medical advice.
Isla works out your typical cycle length from the periods you log, so the calendar follows your cycles rather than a number you typed once. No account, and the log stays on your iPhone.
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