When do I ovulate on a 21-day cycle?
Ovulation usually happens about 14 days before the next period starts. On a 21-day cycle, the next period starts on day 22, so ovulation is most likely around day 7 (counting the first day of your period as day 1). A 21-day cycle is a short cycle; cycles from about 21 to 35 days are common in adults.
Your fertile window on a 21-day cycle
Sperm can live for up to five days and an egg for about a day, so the fertile window is the five days before ovulation plus ovulation day: cycle days 2 to 7. The best days to try are days 5 to 7.
| Cycle day | Fertility |
|---|---|
| Day 1 | Low chance |
| Day 2 | Fertile |
| Day 3 | Fertile |
| Day 4 | Fertile |
| Day 5 | Most fertile |
| Day 6 | Most fertile |
| Day 7 | Ovulation (most fertile) |
| Day 8 | Fertility falls quickly |
| Day 9 | Fertility falls quickly |
| Day 10 | Fertility falls quickly |
Does my period length matter?
No — whether your period lasts 3, 5 or 7 days, ovulation on a 21-day cycle is still around day 7. With a 5-day period, that is about 2 days after your period ends — so on a cycle this short the fertile window can start very soon after your period.
How sure is this?
The 14-day rule is an average: the second half of the cycle usually lasts 12 to 16 days, so ovulation on a 21-day cycle could be anywhere from about day 5 to day 9. Ovulation tests, which detect the LH surge, or temperature charting can confirm your own pattern. If your cycles vary a lot, enter your average cycle in the Ovulation Date Calculator and treat the dates as approximate.
Other cycle lengths
22-day cycle · 23-day cycle · 24-day cycle · 25-day cycle · 26-day cycle · 27-day cycle