Ovulation calculator
See your likely fertile days for the next three cycles, from two numbers you already know.
Updated October 13, 2026 · sources checked October 13, 2026
In short
Ovulation usually happens 10 to 16 days before your next period, according to the NHS. The fertile window is the six days that end on ovulation day: in a landmark study, conception happened only when sex took place in those six days. The calculator places ovulation 14 days before your next expected period.
How the estimate is made
The calculator adds your cycle length to the first day of your last period to predict your next period, then counts back 14 days to estimate ovulation. The fertile window it shows is the five days before ovulation plus ovulation day itself.
That window comes from a study of 221 women trying to conceive (Wilcox and colleagues, 1995): conception occurred only when intercourse happened in a six-day period ending on the day of ovulation, with the chance rising from about 10 % five days before ovulation to about 33 % on the day itself.
| Cycle length | Estimated ovulation |
|---|---|
| 26 days | Around day 12 |
| 28 days | Around day 14 |
| 30 days | Around day 16 |
| 32 days | Around day 18 |
Is it normal?
Cycles that are a little longer or shorter than 28 days are normal: the NHS says regular cycles from 21 to 35 days are within the usual range. Enter your own cycle length for a closer estimate.
After a positive test
Use the due date calculator with the first day of your last period.
Questions parents ask
Can I use this to avoid pregnancy?
No. Cycles vary, and ovulation can come earlier or later than predicted. This calculator is not a method of contraception.
My cycles are irregular. Will it work?
It assumes a regular cycle, so it is less reliable when cycles vary. The NHS notes that regular cycles from 21 to 35 days are normal; if yours are irregular, talk to your doctor or a fertility nurse.
Sources
- NHS — Fertility in the menstrual cycle checked October 13, 2026
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-21 (PMID 7477165) checked October 13, 2026
Mamavela explains; it does not replace your midwife, doctor or pediatrician, who know your situation. If something worries you, call them. In an emergency, call 911 (US), 999 (UK) or 112.