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CalculatorBuddy

Conception Calculator

Estimate your most probable conception days, fertile windows, 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 fertile window

October 26, 2026 – 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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Keep a baseline, then change an assumption to see its effect side by side.

Important dates for the next 6 cycles

Period startConception windowDue date
October 17, 2026October 26, 2026 – October 31, 2026July 24, 2027
November 14, 2026November 23, 2026 – November 28, 2026August 21, 2027
December 12, 2026December 21, 2026 – December 26, 2026September 18, 2027
January 9, 2027January 18, 2027 – January 23, 2027October 16, 2027
February 6, 2027February 15, 2027 – February 20, 2027November 13, 2027
March 6, 2027March 15, 2027 – March 20, 2027December 11, 2027
Estimated fertile windowOctober 26, 2026 – October 31, 2026

Plan upcoming cycles from the first day of the last period and average cycle length. The calculator shows a calendar-based fertile window, estimated ovulation range, expected period, test date, and possible due date.

Examples

Last period September 8, 2026; 28-day cycle

The next expected period is October 6 and estimated ovulation is September 22. The fertile window is September 17–22, testing is suggested on or after October 6, and the possible due date is June 15, 2027.

Formula

nextPeriod         = lastPeriod + cycleLength
estimatedOvulation = nextPeriod − 14
fertileWindow      = estimatedOvulation − 5 … estimatedOvulation
ovulationRange     = estimatedOvulation − 2 … estimatedOvulation + 2
testOnOrAfter      = nextPeriod
possibleDueDate    = estimatedOvulation + 266

This model assumes a regular cycle and a 14-day luteal phase. It is a planning aid, not confirmation of ovulation, conception, paternity, or pregnancy.

Sources

Frequently asked questions

What dates does this calculator show?
It estimates ovulation as 14 days before the next expected period and shows the six-day fertile interval ending on that date. It cannot identify the actual date of fertilization.
Why is the pregnancy test date the expected period date?
Home pregnancy tests are generally more accurate after a missed period. Some products permit earlier use, but an early negative may be unreliable.
Can this identify paternity or prevent pregnancy?
No. Calendar dates cannot establish paternity, confirm conception, or provide reliable contraception.

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