How to use the Ovulation Calculator
- Enter the first day of your last period.
- Enter your average cycle length in days.
- Read the estimated ovulation date and fertile window.
- Confirm with ovulation tests or basal temperature if timing matters.
How the calculation works
The luteal phase — ovulation to next period — is relatively fixed at about 14 days, while the follicular phase varies. So ovulation is estimated as cycle length minus 14 from the start of the last period: day 14 in a 28-day cycle, day 18 in a 32-day cycle. Counting forward 14 days from the period start is a common error that misses ovulation entirely in longer cycles.
The fertile window runs about five days before ovulation through the day itself, because sperm survive up to five days in fertile cervical mucus while the egg is viable for only 12–24 hours. Intercourse in the two days before ovulation carries the highest conception probability. Cycle variation makes calendar estimates approximate: ovulation predictor kits detect the LH surge 24–36 hours ahead, and basal temperature confirms ovulation only after it happens.
Ovulation day ≈ cycle length − 14 ; fertile window = ovulation − 5 days to ovulationSource: Fixed luteal phase model with fertile window per Wilcox et al. sperm and oocyte viability data.
Worked example
Last period started 4 September, average cycle 31 days.
- Ovulation day = 31 − 14 = day 17 of the cycle.
- Day 1 is 4 September, so day 17 is 20 September.
- Fertile window = 15 to 20 September.
- Peak days are 18 and 19 September.
Ovulation around 20 September with the fertile window opening on the 15th.
Frequently asked questions
Is ovulation always day 14?+
Only in a 28-day cycle. It is better estimated as cycle length minus 14.
How long is the fertile window?+
About six days: five before ovulation plus the day itself, thanks to sperm survival.
Can I ovulate on my period?+
It is unusual but possible with very short cycles, which is why calendar methods are unreliable contraception.
Should I use ovulation tests?+
They are more accurate than calendar estimates because they detect the LH surge directly.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.