Conception Calculator

How this conception calculator works

The conception calculator estimates the days you are most likely to conceive in a cycle, along with the due date if you do. Enter the first day of your last period and your average cycle length. The calculator finds your next period, then counts back to the days when ovulation is most likely.

Next period = first day of last period + cycle length
Ovulation window = 16 to 12 days before the next period
Conception dates = ovulation window + 1 day
Due date = first day of last period + 280 days + (cycle length − 28)

For a 28-day cycle that started on October 1, the most likely conception dates are October 13 to 18, and the due date if you conceive is July 8 of the following year.

What the conception date calculator shows

• Most likely conception dates. The ovulation window plus one more day, since an egg can be fertilized for up to about a day after it is released.
• Ovulation window. The calculator's estimate: two days either side of 14 days before the next period, the usual timing of ovulation. Ovulation can happen anywhere from about 7 to 19 days before a period, so your actual ovulation day may fall outside this window.
• Best days to have sex. Your fertile window, from three days before the ovulation window through its last day. Sperm can survive several days, so sex before ovulation can still lead to pregnancy.
• Earliest pregnancy test. Five days before your next period is due. Many tests are more accurate once your period is actually late.
• Due date if you conceive. The standard 280 days from your last period, adjusted by how much longer or shorter your cycle is than 28 days.
• Next 6 cycles. Conception dates and the matching due date for each of the next six cycles.

How to calculate conception date

Conception happens when a sperm fertilizes an egg, which can only happen around ovulation. Pregnancy is possible from sex in the few days before ovulation and on the day itself, because sperm can live for 3 to 5 days while an egg lives for only 12 to 24 hours. That is why conception is dated to ovulation, not to the day you had sex.

Pregnancy weeks are counted from the first day of the last period, about two weeks before conception in a 28-day cycle. A pregnancy conception calculator that starts from your period therefore places conception at roughly week 2 of pregnancy and the due date about 38 weeks (266 days) after conception.

Frequently asked questions

When did I conceive?

To answer "when did I get pregnant," count back from the period you missed. If your cycles are regular, you most likely conceived 11 to 16 days before the date your next period was due. Enter the first day of your last period above to see the exact dates. An early ultrasound gives a more precise date.

Can I use a conception calculator by due date?

Yes. If you already know your due date, subtract 266 days (38 weeks) to estimate the conception date. For a due date of July 8, that is around October 15 of the year before. A due date from an ultrasound gives a better estimate than one based only on your period.

How accurate is the date of conception?

It is an estimate. Ovulation can move by several days from cycle to cycle, and sperm can survive for days before fertilizing an egg, so the exact day is rarely known without fertility tracking or IVF records.

Can I use this calculator to avoid pregnancy?

No. Calendar predictions are not reliable birth control because ovulation timing varies and sperm can live for several days.