How the fertile window works
Ovulation usually happens about 14 days before your next period starts. Sperm can survive up to 5 days and the egg about 12–24 hours, so the fertile window is roughly the 5 days before ovulation plus the day of ovulation. This calculator uses those standard estimates.
Not birth control. Cycles vary month to month, so calendar estimates are not a reliable way to prevent pregnancy. Ovulation test kits, basal body temperature and cervical mucus tracking can make timing more accurate.
Irregular cycles, very long or short cycles, or trying for over 12 months (6 months if 35 or older) are good reasons to see a doctor.
Sources & review
Checked against these sources on November 10, 2026. Rules and rates can change — always confirm with the official source.
Ovulation Calculator FAQs
When should I take a pregnancy test?
Most home tests are most reliable from the first day of your missed period.
My cycle is irregular — will this work?
Use your average cycle length, but expect the estimate to be less accurate. Ovulation tests may help.
Is my information stored?
No. Nothing is saved or sent anywhere.