Ovulation calculator
The estimate puts ovulation 14 days before the next expected period. The fertile window shown here is the five days before that day plus the ovulation day itself.
Cycles vary, and an app cannot see a temperature shift or a hormone test. Do not use this as contraception or as a fertility treatment.
Calculator
Required fields are marked with *.
Use a comma or a dot for the decimal, as in 12.5 or 12,5. Type thousands as 1000 or 1 000. A single comma or dot is the decimal mark, so 1,000 means 1.
Results
The result will appear here.
How to use this calculator
- Enter the first day of the last menstrual period and the usual cycle length.
- Optionally enter a date to see whether it falls inside the estimated window.
- Read the ovulation day, the window, and the next period.
Formula
next period = last period + cycle length
ovulation = next period − 14 days
The fertile window runs from five days before ovulation through ovulation day. Sperm can survive for several days; the egg is viable for about a day. Those are averages, not a guarantee.
Worked example
Period on 1 January 2026, 28-day cycle
The next period is 29 January. Ovulation is 14 days earlier, on 15 January.
The window is 10 January through 15 January. 12 January falls inside it.
Ovulation 15 January 2026. Fertile window 10–15 January.
Questions
Can this prevent pregnancy?
No. Calendar estimates miss early and late ovulation. Use a method you have discussed with a clinician if you need contraception.
Why 14 days before the next period?
The luteal phase is often about 14 days. It is not 14 days for everyone, which is why the date is an estimate.
What cycle lengths are accepted?
21 to 45 days. Outside that range the 14-day assumption is a poor fit, so the page asks you to stop.
Sources
Last reviewed November 12, 2026