Ovulation Calculator

If you're trying to conceive, the fertile window is the handful of days each cycle when intercourse is most likely to lead to pregnancy. It spans roughly the five days before ovulation plus the day of ovulation itself, because sperm can survive several days in the reproductive tract while the egg is viable for about a day.

We estimate ovulation as about 14 days before your next expected period — the length of the luteal phase, which is fairly consistent even when cycle length varies. From that, we mark the fertile window around it.

This is genuinely an estimate. Ovulation shifts from cycle to cycle, and stress, illness, and other factors move it around. Use it as a planning aid, and consider ovulation predictor kits or tracking basal body temperature if you want more precise timing.

Formula. Estimated ovulation = (first day of your last period + your cycle length) − 14 days. The fertile window is then ovulation − 5 days to ovulation + 1 day.

Why 14 days. The luteal phase — from ovulation to the start of your next period — is relatively constant at around 14 days across different cycle lengths, so counting back from the expected next period is more reliable than counting forward from the last one.

Confidence. We label this estimate-only, and deliberately so: individual ovulation timing varies substantially, so this is a rough guide for planning, never a prediction and never a form of contraception.