Ovulation Calculator
Your likely ovulation day and fertile window, for this cycle and the next ones.
Good to know. Sperm can live up to five days, an egg about one. The fertile window is mostly the days before ovulation.
Not medical advice. Only your midwife or doctor can date a pregnancy.
How it works
In plain words
The time from ovulation to the next period is fairly constant at about 14 days, so ovulation is expected 14 days before the next period is due. The fertile window is the five days before ovulation and the day itself.
The formula
ovulation = first day of last period + cycle length − 14 fertile window = ovulation − 5 days to ovulation next period = last period + cycle length
Worked example
With a period starting on 1 March 2026 and a 28-day cycle, ovulation is likely around 1 March + 14 days: Sunday, March 15, 2026. The fertile window runs from 10 to 15 March.
Sources
- Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation, New England Journal of Medicine, 1995 The six-day fertile window ending on the day of ovulation.
- ACOG Committee Opinion 700, Methods for Estimating the Due Date, 2017 The due date from the first day of the last period.
Sources checked September 30, 2026. 3 automated tests check its answers on every change.
About the ovulation calculator
Pregnancy is only possible on a few days each cycle: the fertile window. An egg survives about a day after ovulation, but sperm can survive up to five days, so the fertile days are the five days before ovulation and the day of ovulation itself. The two or three days just before ovulation are the most fertile.
Ovulation is estimated from your cycle length. The second half of the cycle, from ovulation to the next period, is fairly steady at about 14 days. The first half is what varies, so in a 28-day cycle ovulation is around day 14, in a 32-day cycle around day 18, and in a 25-day cycle around day 11.
If your cycles are irregular, the estimate is less reliable. Ovulation tests, which detect the hormone surge a day or two before ovulation, and tracking basal body temperature or cervical mucus can pinpoint it more closely. The table shows the next three cycles, assuming your cycle length stays the same.
This calculator is not a method of contraception: cycles shift with stress, illness and travel, and even a few days’ error matters. If you have been trying to conceive for a year (or six months if you are 35 or older), or your cycles are very irregular, see a doctor.
Questions people ask
When do I ovulate?
Usually about 14 days before your next period. In a 28-day cycle, that is around day 14, counting the first day of your period as day 1.
How many days am I fertile?
About six: the five days before ovulation and the day of ovulation. The most fertile are the two or three days just before it.
Can I use this to avoid pregnancy?
No. Cycles vary too much for a calendar estimate to be a reliable method of contraception.