Ovulation Calculator
Free online ovulation calculator. Enter your last period and cycle length to find your fertile window and ovulation day.
Select the date of your last period
Days
Select the date of your last period
How It Works
- Enter the first day of your last menstrual period (LMP).
- Enter your average cycle length (typically 21-35 days).
- The calculator estimates your next ovulation date.
- Your fertile window (5 days before and the day of ovulation) is highlighted.
- Track over several months for greater accuracy.
Formula
Ovulation day = First day of LMP + cycle length - 14
Fertile window = Ovulation day - 5 days to ovulation dayExample
Input: Last period: January 1 | Cycle length: 28 days
Result: Ovulation: January 15
Fertile window: January 10 - January 15
FAQ
It uses the standard luteal phase model: ovulation occurs approximately 14 days before the next period, so cycle length minus 14 gives the estimated ovulation day.
They assume a regular cycle. Irregular cycles, stress, illness, or medication can shift ovulation. Use a physical ovulation test for confirmation.
No. All calculations run entirely in your browser. No personal information is ever sent to a server.
No. Ovulation timing is only an estimate and must never replace proven contraceptive methods.