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Ovulation Calculator

Estimate your ovulation date, fertile window, next period, and due date from the first day of your last period and your average cycle length.

Maintained by CalculatorBuddy TeamEditorial methodology

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ExampleSample values — edit any field to see your result.

Leave blank to use today's date.

Days from the start of one period to the next (default 28).

Results update as you type.

Estimated ovulation date

October 31, 2026

Estimated result

Estimated ovulation range
October 29, 2026 – November 2, 2026
Estimated fertile window
October 26, 2026 – October 31, 2026
Test on or after
November 14, 2026
Next period start
November 14, 2026
Due date if pregnant
July 24, 2027

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Compare scenarios

Keep a baseline, then change an assumption to see its effect side by side.

Important dates for the next 6 cycles

Period startOvulation windowDue date
October 17, 2026October 29, 2026 – November 2, 2026July 24, 2027
November 14, 2026November 26, 2026 – November 30, 2026August 21, 2027
December 12, 2026December 24, 2026 – December 28, 2026September 18, 2027
January 9, 2027January 21, 2027 – January 25, 2027October 16, 2027
February 6, 2027February 18, 2027 – February 22, 2027November 13, 2027
March 6, 2027March 18, 2027 – March 22, 2027December 11, 2027
Estimated ovulation dateOctober 31, 2026

Estimate ovulation, the fertile window, the next expected period, and a possible due date from the first day of the last period and average cycle length. These are calendar planning estimates for people with reasonably regular cycles.

Examples

Last period September 8, 2026; 28-day cycle

The next expected period is October 6. Estimated ovulation is September 22, the calendar ovulation range is September 20–24, and the fertile window is September 17–22. Test on or after October 6. A possible due date is June 15, 2027.

Formula

For last-period start D and average cycle length C:

nextPeriod         = D + C
estimatedOvulation = nextPeriod − 14
ovulationRange     = estimatedOvulation − 2 … estimatedOvulation + 2
fertileWindow      = estimatedOvulation − 5 … estimatedOvulation
testOnOrAfter      = nextPeriod
possibleDueDate    = estimatedOvulation + 266

The five-day ovulation range shows uncertainty in a simple calendar model. It does not mean an egg remains available throughout that range. ASRM defines the fertile window as the six days ending on ovulation.

Limits and reliable use

Cycle length is an average, while ovulation can move between cycles. Ovulation tests, cervical mucus tracking, and clinician evaluation provide different evidence than this calendar projection. Seek clinical advice for irregular cycles, pregnancy concerns, or difficulty conceiving.

Sources

Frequently asked questions

How is the ovulation date estimated?
The calculator places ovulation 14 days before the next expected period: last-period date + average cycle length − 14. This is a calendar estimate, not a measurement of ovulation, and real cycles can vary.
What is the fertile window?
ASRM defines the fertile window as the six-day interval ending on ovulation. The calculator therefore shows five days before the estimated ovulation date through that date. The separate five-day ovulation range illustrates uncertainty in the calendar estimate.
When should I take a pregnancy test?
The result marks the expected next-period date. The US Office on Women's Health says most home tests are more accurate after a missed period and that testing earlier makes a false negative more likely.
Can I use this as birth control?
No. Calendar estimates cannot confirm ovulation and are not reliable contraception. Use a contraceptive method and professional guidance if avoiding pregnancy.

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