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Ovulation Calculator
Estimate ovulation and a possible fertile window for a regular cycle. Change the inputs below and the estimate updates immediately.
Women’s health · calendar estimates
Cycle timing varies. These dates cannot confirm ovulation and must not be used to prevent pregnancy.
Your calculator inputs stay in this page and are not saved or sent to us.
Input
Your numbers
Calculated result
Estimated ovulation
September 28, 2026
Possible fertile window
September 23, 2026 – September 29, 2026
Actual fertile days may fall outside these dates. Not for contraception.
Predicted next period
October 12, 2026
Estimate only. Verify important decisions with an appropriate professional or official source.
How to use it
A quick estimate, without the spreadsheet.
Inputs and units
Enter the first day of your last period and your usual cycle length (21–45 whole days). This is intended for a regular cycle and does not use symptoms, temperature measurements or ovulation tests.
Formula
Next period = last period + cycle length. Estimated ovulation = next period − 14 days. The possible fertile window is five days before through one day after that estimated ovulation date.
Worked example
With a January 1, 2026 period and a 28-day cycle, the next period is January 29, estimated ovulation is January 15, and the possible fertile window is January 10–16.
Assumptions and limitations
Calendar estimates assume a regular cycle. Irregular cycles, breastfeeding, hormonal contraception and conditions affecting ovulation can make predictions unreliable. This calculator cannot confirm ovulation or pregnancy and must not be used to prevent pregnancy. The 14-day assumption is approximate: ovulation timing varies even in regular cycles. Actual fertile days can fall outside this window.
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