Ovulation Calculator — Fertile Days
Enter the first day of your last period and your cycle length to estimate when you ovulate and which days you are most fertile.
Ovulation Calculator
Fertile window and next period
Estimated ovulation2026-01-15
Fertile window starts2026-01-10
Fertile window ends2026-01-16
Next period expected2026-01-29
How it works
- Ovulation usually happens about 14 days before the next period.
- The fertile window is the 5 days before ovulation plus the day itself and one after.
- Irregular cycles make estimates less reliable.
- This is an estimate, not a method of contraception or medical advice.
Worked example: period started 1 March, 28-day cycle
- 1Ovulation: day 28 − 14 = day 14 → 15 March.
- 2Fertile window: 10–16 March.
- 3Next period: 29 March.
Most fertile around 13–15 March
Estimated ovulation day by cycle length
| Cycle length | Ovulation day |
|---|---|
| 24 days | Day 10 |
| 26 days | Day 12 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 32 days | Day 18 |
Frequently asked questions
When am I most fertile?
In the two or three days up to and including ovulation.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable enough as contraception.
What if my cycle is irregular?
Use your average cycle length and consider ovulation tests or temperature tracking for more precision.
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