How the ovulation date and fertile window are calculated
Enter the first day of your last period and your average cycle length, and the calculator shows your next expected period, estimated ovulation day and fertile window. It uses the standard calendar method:
| Item | Formula | Example (period started Oct 1, 28-day cycle) |
|---|---|---|
| Next period | Start date + cycle length | Oct 29 |
| Ovulation day | Next period − 14 days | Oct 15 |
| Fertile window | 5 days before to 1 day after ovulation | Oct 10 – Oct 16 |
Because ovulation is counted back 14 days from the next period, a longer cycle moves it later and a shorter cycle moves it earlier. With a 35-day cycle ovulation falls around day 21; with a 24-day cycle, around day 10.
Three-cycle forecast and calendar
The results include a table of the current cycle and the next two: expected period dates, fertile window and ovulation day. The mini calendar color-codes period days, fertile days and the ovulation day, and you can page through months. It also tells you which phase today falls in. Changing the period length updates how many days are marked as period days.
To count days between dates for anything else, try the date calculator.
Important limitations
- If your cycle varies a lot from month to month, the estimate can be well off. Averaging your last few cycles gives a better input.
- Stress, sleep, weight changes and health conditions can all shift ovulation.
- This tool is for general information only. Do not rely on it as contraception or as the sole basis for trying to conceive; talk to a doctor if you have concerns.
- Everything runs in your browser — the dates you enter are never sent to a server.