Ovulation usually happens about 14 days before your next period, not a fixed number of days after the last one. In a 28-day cycle that is day 14; in a 32-day cycle it is day 18. Your fertile window is the 5 days before ovulation plus ovulation day itself, because sperm can survive for several days and the egg for about a day.
Enter the first day of your last period and your usual cycle length. The calculator shows your next ovulation date, your fertile days and the same dates for the following cycles.
How to calculate your ovulation day
- Find your cycle length: count from the first day of one period to the first day of the next. Use the average of your last few cycles.
- Take away 14 from your cycle length. That is the cycle day you are most likely to ovulate.
- Your fertile window is that day and the 5 days before it.
Worked example
Last period started on 1 September 2026, cycles of 28 days.
| Step | Result |
|---|---|
| Next period: 1 September + 28 days | 29 September 2026 |
| Ovulation: cycle day 28 − 14 = day 14 | 14 September 2026 |
| Fertile window: day 9 to day 14 | 9 to 14 September 2026 |
With a 32-day cycle and the same start date, the next period is due on 3 October, ovulation is around day 18, 18 September, and the fertile window runs from 13 to 18 September.
Ovulation day by cycle length
Cycle day 1 is the first day of your period. These are averages; your own ovulation day can move from cycle to cycle.
| Cycle length | Likely ovulation | Fertile window |
|---|---|---|
| 21 days | Day 7 | Days 2 to 7 |
| 24 days | Day 10 | Days 5 to 10 |
| 26 days | Day 12 | Days 7 to 12 |
| 28 days | Day 14 | Days 9 to 14 |
| 30 days | Day 16 | Days 11 to 16 |
| 32 days | Day 18 | Days 13 to 18 |
| 35 days | Day 21 | Days 16 to 21 |
Why 14 days before the next period
The part of the cycle after ovulation, the luteal phase, is fairly steady at around 12 to 14 days in most women. The part before ovulation is what changes when a cycle is long or short. That is why a calendar method counts back from the next period.
The fertile window of six days comes from a study of 221 women trying to conceive, published by Wilcox, Weinberg and Baird in The New England Journal of Medicine in 1995. Almost all pregnancies came from intercourse in the 5 days before ovulation or on the day itself. If you have tracked your own luteal phase with tests, you can change the 14 days under the advanced option.
How to confirm ovulation
A calendar only predicts. These signs show what your body is actually doing:
- Ovulation test strips detect the rise in luteinising hormone (LH) in urine, usually a day or so before ovulation.
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the fertile days.
- Basal body temperature rises slightly after ovulation. It confirms ovulation has happened but only after the event.
Using a test or mucus changes alongside the calculator gives a better idea of your fertile days than dates alone.
Limits of an ovulation calculator
Date-based estimates are least reliable when cycles are irregular, after stopping hormonal contraception, while breastfeeding, and in conditions that affect ovulation. They must not be used as contraception.
The NHS advises seeing a doctor if you have been trying for a baby for a year without success, and ACOG advises seeing one after 6 months if you are 35 or older. See a doctor sooner if your periods are irregular or absent.
This calculator gives an estimate from a published formula. It does not diagnose any condition and is not a substitute for a doctor. Talk to a doctor before making decisions about your health, diet or treatment.
Ovulation questions
How do I calculate my fertile days?
Take 14 away from your cycle length to find your likely ovulation day, then count back 5 days. For a 28-day cycle, ovulation is around day 14 and the fertile days are days 9 to 14. For a 30-day cycle, they are days 11 to 16. Day 1 is the first day of your period.
Can I get pregnant outside the fertile window?
It is unlikely but possible, because ovulation can come earlier or later than the calendar predicts. Sperm can survive for up to about 5 days, so intercourse a few days before an early ovulation can still lead to pregnancy. This is why calendar dates are not reliable contraception.
Can an ovulation calculator help me have a baby boy?
No. Some websites claim that timing intercourse close to ovulation gives a boy, but a 1995 study in The New England Journal of Medicine found that timing did not change the baby's sex. The sex is set by the sperm, not by the day. Beware of any site that promises otherwise.
Does this work with irregular periods?
Only roughly. If your cycle changes by more than a week from month to month, the calculator cannot know which length applies this time. Ovulation test strips or tracking cervical mucus give a better guide. Irregular cycles are also worth mentioning to a doctor, especially if you are trying to conceive.
Is ovulation always on day 14?
No. Day 14 is only typical for a 28-day cycle. Ovulation is usually about 14 days before the next period, so it is later in a long cycle and earlier in a short one. It can also shift a few days between cycles in the same woman.
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Last reviewed 30 September 2026. Results are estimates; official notices always take precedence.