Ovulation Calculator
Predict ovulation date and fertile window for the next few cycles.
Written by Anil Gautam · Updated
How does the ovulation calculator predict your fertile days?
It uses the calendar method. Ovulation is placed 14 days before your next expected period, which is (cycle length − 14) days after the first day of your last period. Because those 14 days are held fixed, a longer cycle moves ovulation later. The fertile window runs from 5 days before ovulation to 1 day after it, 7 days in all, since sperm can survive for up to about five days while an egg lasts around a day after release.
Enter 1 October 2026 with a 28-day cycle and the tool shows likely ovulation on 15 Oct 2026, a fertile window from 10 Oct to 16 Oct, and the next period on 29 Oct. With a 32-day cycle, ovulation moves to 19 Oct, the window to 14 to 20 Oct, and the next period to 2 Nov.
What if your cycles are irregular?
Later cycles repeat the same length, so any error in your average grows with each one. The third predicted ovulation sits three cycle lengths after your last period, minus 14 days, so an average that is one day off moves it by three days. If your cycles range from 26 to 32 days, run the tool twice: from a 1 October start, 26 days gives a window of 8 to 14 Oct and 32 days gives 14 to 20 Oct, so the days to watch run from 8 to 20 October.
Cycle length is the number of days from the first day of one period to the first day of the next, and an average of several recent cycles gives a steadier figure than a single one. The cycle slider covers 20 to 40 days and the second slider shows 1 to 6 cycles. Cycle 1 always starts on the date you enter, so if your last period began weeks ago, cycle 1 may already be over and cycle 2 holds the next window.
How reliable are the predicted dates?
The 14-day figure is an average. People differ in how long that phase lasts, and stress, illness, breastfeeding or conditions such as PCOS can delay ovulation or stop it for a cycle. Because the dates are estimates, they are not reliable as a way to avoid pregnancy. If your cycles often fall outside 21 to 35 days, a doctor can look into why. When a period is late and you want to date a possible pregnancy, the Pregnancy Due Date Calculator starts from the same first-day-of-period date.