Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your ovulation date and fertile window.
Last updated: September 5, 2026
Frequently asked questions
How is ovulation date calculated from cycle length?
Ovulation typically occurs 14 days before your next period starts, regardless of how long your total cycle is - so it's calculated as (cycle length - 14) days after the first day of your last period.
Why does ovulation timing vary if the luteal phase is fixed at 14 days?
The luteal phase (after ovulation, before the next period) is relatively consistent at about 14 days, but the follicular phase (before ovulation) varies more between cycles and between people - so a longer or shorter overall cycle mostly reflects a longer or shorter follicular phase, not a different luteal length.
What is the fertile window?
The days when pregnancy is possible from intercourse - typically the 5 days before ovulation plus the day of ovulation itself, since sperm can survive several days while the egg is viable for about 24 hours after release.
Does this work for irregular cycles?
It's less reliable for irregular cycles, since the calculation assumes a consistent cycle length. Tracking actual ovulation signs (basal body temperature, cervical mucus, or an ovulation predictor kit) gives more accurate results if your cycles vary significantly month to month.
What's an example ovulation calculation?
If your last period started on the 1st and your average cycle is 30 days, ovulation is estimated around day 16 (30 - 14), meaning roughly the 16th of that cycle.
How is the fertile window calculated from the ovulation date?
It's estimated as the 5 days leading up to ovulation plus the ovulation day itself - for an ovulation date of the 16th, the fertile window would run roughly from the 11th through the 16th.
Is 'ovulation calculator' the same thing as a 'fertility calculator'?
They usually refer to the same underlying calculation - estimating ovulation timing and the surrounding fertile window from cycle data - though some fertility calculators also factor in additional inputs like basal body temperature or hormone test results for a more personalized estimate.