Ovulation & fertile window calculator
Estimate your most fertile days from the first day of your last period and your usual cycle length. Private — it runs in your browser, and nothing is saved.
How it works
Ovulation typically occurs about 14 days before your next period. So we estimate your next period as your last period plus your cycle length, then count back 14 days to find your likely ovulation day. The fertile window is the five days before ovulation through the day of ovulation, because sperm can survive several days in the body.
These are estimates based on your averages, not a diagnosis. If you’re trying to conceive or have irregular cycles, your midwife or doctor can help.
Common questions
When do I ovulate?
Ovulation usually happens about 14 days before your next period starts — not on a fixed “day 14”. So if your cycle is 30 days, you likely ovulate around day 16; if it’s 26 days, around day 12. This calculator uses your cycle length to estimate it.
What is the fertile window?
The fertile window is roughly the five days before ovulation plus the day of ovulation itself, because sperm can survive several days. Conception is most likely in the two to three days leading up to ovulation.
How accurate is this?
It’s an estimate based on averages. Cycle timing varies month to month, especially if your cycles are irregular. Tracking basal body temperature or using ovulation predictor kits gives a more personal signal.
Is my data private?
Yes. Everything is calculated in your browser and nothing is saved or sent anywhere — the same private-by-default approach Fawna takes with all reproductive data.
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