Ovulation Calculator
Last updated: October 15, 2026
Estimate a fertile window and ovulation date from cycle information.
Estimated ovulation date
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- Estimated fertile window: —
- Estimated next period: —
On this page
Quick summary: Estimate a fertile window and ovulation date from cycle information.
Read the method, example and limitations
Updated reference sections in this guide are in English.
Guide language: English. Updated October 15, 2026.
What this calculator estimates
The Ovulation Calculator estimates a likely fertile window from menstrual-cycle dates and cycle-length assumptions. It is a general educational estimate rather than a diagnostic or contraceptive tool.
Inputs and units
Enter First day of last period, Average cycle length (days). Use the units shown. Required fields cannot be blank. Use zero only where that field and the model allow it. Select Calculate after changing the values.
Method and assumptions
Estimated next period = first day of the last period + entered cycle length. The displayed ovulation estimate is 14 days before that next period. The illustrative fertile interval runs from five days before to one day after that estimate.
Worked example
A period beginning January 1, 2026 with a 28-day cycle gives next period January 29 and estimated ovulation January 15. The illustrative interval is January 10 through January 16.
Limitations
Actual ovulation timing varies. The 14-day assumption is not a biological guarantee, particularly with irregular cycles. This calendar estimate does not establish safe contraceptive days or confirm ovulation. Supported cycle lengths are whole values from 15 to 60 days.