Ovulation Calculator
Enter the first day of last menstrual period and cycle length to estimate the ovulation day and fertile window.
Input Data
Results
At a glance:Ovulation ~ next period - 14 days = LMP + (cycle - 14) days; fertile window = ovulation - 5 to ovulation + 1 days; next period = LMP + cycle days.
Formula
ovulation = LMP + (cycle - 14) days.
fertile window = ovulation - 5 to ovulation + 1 days.
nextPeriod = LMP + cycle days.
How to Use
- Pick the first day of your last period and average cycle length.
- Get ovulation day, fertile window and next period date.
Case Studies
LMP 2026-08-01, cycle 28
Ovulation ~ 2026-08-15; fertile 2026-08-10 to 2026-08-16; next period 2026-08-29.
FAQ
Accurate with irregular cycles?
This tool assumes regular cycles; for irregular cycles use ovulation test strips or basal body temperature.
Why is the fertile window 6 days?
Sperm survive ~5 days and the egg ~1 day, so the days around ovulation are the conceivable window.
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References
Content reviewed by the Calculatorism editorial team. Results are for reference only; please refer to the relevant authorities for the official figures.