About this ovulation calculator
Estimate your likely ovulation day, fertile window, and next expected period from the first day of your last period and your average cycle length. This is commonly used when trying to conceive or simply to understand your own cycle better. The estimate relies on the observation that ovulation typically occurs about fourteen days before the next period starts, regardless of how long the overall cycle runs, so a longer or shorter cycle shifts the ovulation day earlier or later without changing that fourteen-day gap at the end.
How the ovulation calculator works
Ovulation day is estimated as your cycle length minus fourteen days after the start of your last period. The fertile window covers the five days before ovulation through one day after, reflecting typical sperm and egg survival times. The next period is estimated one full cycle length after the last one.
Ovulation Calculator examples
Last period 2026-01-01, 28-day cycle
Estimated ovulation 2026-01-15 · Fertile window 2026-01-10 to 2026-01-16
Last period 2026-03-01, 30-day cycle
Estimated ovulation 2026-03-17 · Fertile window 2026-03-12 to 2026-03-18 · Next expected period 2026-03-31
Limitations & privacy
This estimate assumes a consistent luteal (post-ovulation) phase of about fourteen days, which varies between individuals. It is not a contraceptive method and not a substitute for a fertility monitor, ovulation test, or medical advice.
Inputs and results stay in this browser. Only tool identifiers are stored for your recently used tools. You can clear that history from the directory.
Ovulation Calculator: frequently asked questions
Is this a reliable form of birth control?
No. This is a general estimate, not a validated contraceptive method. Speak with a healthcare provider about reliable contraception options.
Why does a longer cycle push ovulation later?
The days after ovulation are relatively constant at about fourteen, so a longer cycle means more days occur before ovulation, pushing the estimated day later.
How wide is the fertile window?
This tool estimates a six-day window: five days before ovulation, when sperm can survive to meet an egg, through one day after ovulation.
Updated September 20, 2026