How is the conception date calculated?

Conception usually happens around ovulation — about 14 days before your next period. From your last period and cycle length we estimate that ovulation day. If you enter your due date instead, we count back 266 days, the average length of pregnancy from conception.

Can a conception calculator tell me exactly when I conceived?

Not exactly. Sperm can live for several days and ovulation timing varies, so we show a likely conception window rather than a single guaranteed date.

Why is conception about two weeks after my last period?

Pregnancy is dated from the first day of your last period for consistency, but you are not actually pregnant until ovulation and conception, which happen roughly two weeks later in an average cycle.

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Conception Calculator 🌱

Work out roughly when you conceived — from your last period or your due date.

First day of your last period

Average cycle length (days)

Conception usually happens around ovulation, about 14 days before your next period. Exact timing varies, so treat this as an estimate, not a precise date. This tool is not medical advice.

How conception dating works

Conception happens when sperm fertilises the egg, around the time of ovulation. In an average 28-day cycle that is about two weeks after the first day of your last period. Because pregnancy is officially dated from your last period, your baby’s gestational age is always about two weeks ahead of the actual conception date.

If you already know your due date, the calculator counts back 266 days (38 weeks) to estimate when conception happened.

Related tools

Trying to conceive? The ovulation calculator finds your fertile window. Already pregnant? Use the due date calculator and follow our week-by-week guide.

Conception calculator FAQs

How is the conception date calculated?: Conception usually happens around ovulation — about 14 days before your next period. From your last period and cycle length we estimate that ovulation day. If you enter your due date instead, we count back 266 days, the average length of pregnancy from conception.
Can a conception calculator tell me exactly when I conceived?: Not exactly. Sperm can live for several days and ovulation timing varies, so we show a likely conception window rather than a single guaranteed date.
Why is conception about two weeks after my last period?: Pregnancy is dated from the first day of your last period for consistency, but you are not actually pregnant until ovulation and conception, which happen roughly two weeks later in an average cycle.