How to use the calculator
- Pick the first day of your last period.
- Enter your usual cycle length, counted from the first day of one period to the first day of the next.
- You’ll see your estimated ovulation date, your most fertile days, your next period, and dates for the next three cycles.
How the dates are worked out
- Next period = first day of last period + cycle length
- Ovulation ≈ next period − 14 days
- Most fertile days = the 5 days before ovulation, plus ovulation day
Example: with a period starting on 1 September 2026 and a 28-day cycle, ovulation is estimated for 15 September, the most fertile days are 10 to 15 September, and the next period is due on 29 September.
| Cycle length | Ovulation (day of cycle) |
|---|---|
| 24 days | Day 11 |
| 28 days | Day 15 |
| 32 days | Day 19 |
Things that can shift ovulation
Stress, illness, travel, breastfeeding and some medicines can all change when you ovulate. If your cycles are irregular, the estimate will be less precise.
If you think you might be pregnant, a home test is most reliable from the day your period is due. To estimate a due date, use the pregnancy calculator. For any concerns about your cycle, talk to your doctor.