Ovulation — the release of an egg from the ovary — only happens once per cycle, and it's the small window around it where pregnancy is actually possible. Working out roughly when it falls just takes your last period date and your average cycle length.
Counting Backward From Your Next Period, Not Forward From Your Last One
A common mistake is assuming ovulation always falls on day 14, no matter what. It doesn't — day 14 is only correct for a 28-day cycle. The part of the cycle after ovulation (the luteal phase) is fairly fixed at about 14 days for most people, so the more reliable method counts backward 14 days from the start of your next expected period, whatever your cycle length is.
A Worked Example
With an LMP of 1 May and a 28-day cycle, your next period is expected around 29 May. Counting back 14 days from there puts ovulation at about 15 May.
The Fertile Window
Sperm can survive in the body for up to 5 days, but the egg itself is viable for only about 24 hours after release. That gives a fertile window of roughly 5 days before ovulation through 1 day after it — in the example above, that's about 10 May through 16 May.
Why This Is an Estimate, Not a Guarantee
This calendar method assumes a regular cycle length. Stress, illness, travel, and normal month-to-month variation can all shift ovulation earlier or later, so it's a solid planning estimate rather than a lab-confirmed result — ovulation predictor kits or a doctor's guidance add more precision if you need it.
Enter your last period date and average cycle length into our ovulation calculator below to see your estimated ovulation day, fertile window, and next expected period.