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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.

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.

Most probable conception days

September 21, 2026 – September 26, 2026

Estimated result

Ovulation window
September 21, 2026 – September 25, 2026
Best intercourse days for pregnancy
September 18, 2026 – September 25, 2026
Pregnancy test
October 2, 2026
Next period start
October 7, 2026
Due date if pregnant
June 16, 2027

Important dates for the next 6 cycles

Period startConception windowDue date
September 9, 2026September 21, 2026 – September 26, 2026June 16, 2027
October 7, 2026October 19, 2026 – October 24, 2026July 14, 2027
November 4, 2026November 16, 2026 – November 21, 2026August 11, 2027
December 2, 2026December 14, 2026 – December 19, 2026September 8, 2027
December 30, 2026January 11, 2027 – January 16, 2027October 6, 2027
January 27, 2027February 8, 2027 – February 13, 2027November 3, 2027
Most probable conception daysSeptember 21, 2026 – September 26, 2026

Enter the first day of your last period and your average cycle length to estimate your most probable conception days, ovulation and intercourse windows, next period, a reasonable pregnancy-test date, and the due date if you conceived this cycle. Leave the last-period date blank to use today.

Formula

The luteal phase is treated as 14 days, so every date is derived from the last period start D and cycle length C:

nextPeriod        = D + C
ovulation         = nextPeriod − 14
ovulationWindow   = ovulation − 2  …  ovulation + 2
conceptionWindow  = ovulation − 2  …  ovulation + 3
intercourseWindow = ovulation − 5  …  ovulation + 2
pregnancyTest     = nextPeriod − 5
dueDate           = ovulation + 266   (= D + C + 252)

The six-cycle table repeats the same arithmetic for period starts D + i×C with i from 0 through 5.

These dates are estimates for planning conception, not a form of birth control. Irregular cycles, illness, and stress can all move ovulation.

Typical fertile days on a 28-day cycle

EventCycle dayRelative to ovulation
Intercourse window opensDay 105 days before
Conception / ovulation windows openDay 132 days before
Most probable ovulationDay 15—
Ovulation / intercourse windows closeDay 172 days after
Conception window closesDay 183 days after
Pregnancy testDay 249 days after

Examples

Last period on September 8, 2026 (28-day cycle)

Next period = September 8 + 28 = October 6. Ovulation = October 6 − 14 = September 22, 2026. Most probable conception days are September 20–25, the ovulation window is September 20–24, the intercourse window is September 17–24, a pregnancy test is reasonable on October 1, and the due date if pregnant is June 15, 2027.

Same last period with a 22-day cycle

A shorter cycle moves conception earlier: next period is September 30, so ovulation is September 16. Most probable conception days are September 14–19, and the due date is June 9, 2027.

Frequently asked questions

How are the most probable conception days calculated?
Ovulation is estimated as 14 days before the next period would start. The next period is your last-period date plus your average cycle length, so ovulation = last period + cycle length − 14. The most probable conception days run from two days before that ovulation date through three days after it (six days in total).
What is the difference between conception days, the ovulation window, and the intercourse window?
The ovulation window is the most probable two days on either side of estimated ovulation (five days). Most probable conception days add one extra day after that window. The intercourse window starts five days earlier because sperm can survive that long, and it runs through the end of the ovulation window.
How is this different from the Pregnancy Conception Calculator?
This Conception Calculator looks forward from your last period to estimate when you are most likely to conceive in upcoming cycles. The Pregnancy Conception Calculator works backward from a due date, last period, or ultrasound to estimate when a conception that already happened likely occurred.
How is the due date related to conception?
If conception happens at ovulation, the estimated due date is ovulation plus 266 days (38 weeks). That is the same as last period + cycle length + 252, and it matches Naegele's rule of last period + 280 days on a 28-day cycle.
Can I use this as birth control?
No. Cycle-based estimates are not reliable enough to prevent pregnancy. Cycle length varies, ovulation can shift from month to month, and sperm can survive for several days. Use a method designed for contraception if you are trying to avoid pregnancy.

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