How your ovulation date is estimated
Ovulation usually happens 10 to 16 days before your next period, so this calculator counts backwards from your next expected period rather than forwards from your last one. It uses 14 days unless you change the luteal phase setting, and this is why a 35-day cycle gives a very different answer from a 28-day one.
The likely fertile days cover the five days before ovulation and the day itself. We widen that window according to how much your cycle varies, because a window that ignored variation would be too narrow to be useful.
Body signs to watch for
- Discharge: around ovulation it becomes thinner, clear and stretchy, similar to raw egg white. The discharge decoder can help you identify this.
- Temperature: a small, sustained rise in your morning temperature suggests ovulation has already happened, so it’s more useful for learning your pattern than for predicting this month. You can log it in the tracker.
Estimated ovulation by cycle length
Day 14 is only a typical answer for a 28-day cycle. This table uses the default 14-day luteal phase, and shows why counting back from your next period gives a better estimate:
| Cycle length | Estimated ovulation | Likely fertile days |
|---|---|---|
| 24 days | Around day 10 | Days 5 to 10 |
| 28 days | Around day 14 | Days 9 to 14 |
| 32 days | Around day 18 | Days 13 to 18 |
| 35 days | Around day 21 | Days 16 to 21 |
The calculator widens these days according to how much your cycle varies, so your own result may cover more days.
Why the luteal phase setting matters
The luteal phase is the time between ovulation and your next period. It’s usually 10 to 16 days, so moving the setting moves your estimate by the same number of days. Unless you’ve measured it using temperature or ovulation tests over several cycles, it’s best to leave it at 14.
Why calendar estimates have limits
A calendar can only work from past dates. It can’t detect what your body is doing this month. Sperm can survive for up to 7 days after sex, and an early ovulation in a short cycle can make pregnancy possible soon after a period, so the days outside the window shown can’t be treated as safe.
If you’re trying to conceive, use the estimate as a starting point and combine it with body signs or ovulation tests. If you need to avoid pregnancy, please use a proven contraceptive method.
To see how ovulation fits into the whole month, read our guide to the period cycle and its phases.