Ovulation & Fertile Window Calculator
By Santaji GadepatilinUpdated October 27, 2026
Thursday, October 22, 2026
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calculatedThursday, November 5, 2026
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How to use this calculator
Choose the first day of your last period and your average cycle length. Your estimated ovulation date, fertile window and next period are shown.
How it works
Next period = LMP + cycle length Ovulation ≈ next period − 14 days
Cycles vary month to month, so these dates are estimates. Ovulation tests or tracking basal body temperature can give a more personal picture. Speak to a doctor if your cycles are very irregular.
Frequently asked questions
When does ovulation happen?
About 14 days before the next period starts — day 14 of a 28-day cycle, but later in longer cycles.
What is the fertile window?
About six days — the five days before ovulation and the day of ovulation (plus a day after as a margin), because sperm can survive several days.
Can I use this for contraception?
No. Cycle-based estimates are not reliable enough to prevent pregnancy.