Conception Calculator

There are two common approaches to estimating a conception date. One works backward from a due date (subtracting the 266-day conception-to-birth duration); the other works forward from the last menstrual period using your cycle length. This calculator uses the second approach, since it only needs your average cycle length and luteal phase length as inputs, with no due date required.

Cycle-Length Method: Ovulation (and conception) occurs approximately one luteal-phase length before the next period would have started. The calculator estimates your ovulation day within the cycle as:

Ovulation Day = Cycle Length - Luteal Phase Length

For a standard 28-day cycle with a 14-day luteal phase, this places estimated ovulation on day 14. The fertile window is then mapped as the 5 days before ovulation through the day after ovulation.

If you know your estimated due date instead and want a conception estimate from that direction, the biological rule of thumb is that a standard pregnancy lasts about 266 days from conception (versus the 280-day LMP-based clinical convention), but this calculator does not take a due date as an input, so use the due date calculator alongside this tool if you need both directions.

Because sperm cells can survive in cervical fluid for up to 5 days, intercourse that occurred days before ovulation can lead to conception. The actual fertilization of the egg takes place in the fallopian tube, usually within 24 hours of the egg's release.

Medical professionals date pregnancy from the LMP (gestational age) rather than the conception date because the menstrual start date is a concrete event, whereas ovulation is internal and silent. An early ultrasound dating scan provides the most accurate verification of the conception timeline.

Enter your average menstrual cycle length in days and your luteal phase length (the days between ovulation and your next period, typically 10-16 days).

The calculator subtracts your luteal phase from your cycle length to estimate which day ovulation occurs, then reports the fertile window as the 5 days before ovulation through the day after.

28-day average cycle, 14-day luteal phase (the typical assumption when the luteal phase is not individually tracked).

Estimated ovulation day: 28 - 14 = 14.

Fertile window: day 14 - 5 = 9 through day 14 + 1 = 15, a 7-day window covering the days when unprotected intercourse is most likely to result in conception.

Once you have an estimated conception date, project the delivery timeline with the due date calculator, or track weekly progress with the pregnancy calculator and ovulation calculator. The estimate here starts from the same last-period date and cycle length as the menstrual cycle calculator, which explains the next-period-minus-14-days rule and where it goes wrong. The basal body temperature calculator works from the other direction: a sustained temperature rise confirms, after the fact, that ovulation has happened.