This conception calculator estimates when you most likely conceived, working back from the first day of your last period or from your due date. It also shows the most likely conception window, your estimated due date and how far along you are today.
How conception date is estimated
From your last period: ovulation usually happens about 14 days before your next period, so in a 28-day cycle it falls around day 14. Conception date ≈ first day of last period + (cycle length − 14) days.
From your due date: a due date is set 280 days (40 weeks) after the last period, so conception is about 266 days (38 weeks) before the due date.
Why the answer is a window, not a single day
An egg survives for about 12 to 24 hours after ovulation, but sperm can live in the body for up to five days. So intercourse in the five days before ovulation, or on the day itself, could have led to pregnancy. The calculator shows that six-day window.
Worked example
If your last period started on 1 April 2026 and your cycle is 28 days, ovulation and conception most likely happened around 15 April 2026, within a window of 10 to 16 April. Your estimated due date is 6 January 2027.
How accurate is it?
Cycle length and ovulation day vary from month to month, so the estimate can be off by several days. A dating ultrasound in the first trimester is the most accurate way to date a pregnancy, and doctors usually use it if it differs from the period-based date by more than a few days.
Frequently asked questions
How many days after my period did I conceive?
In a 28-day cycle, conception usually happens around day 14, counted from the first day of your period. In a longer or shorter cycle, ovulation is about 14 days before your next period, so it moves accordingly.
Can a conception calculator tell who the father is?
No. Because the fertile window spans several days and cycles vary, a calculator cannot reliably pinpoint the day of conception. Only a paternity DNA test can confirm paternity.
Is conception the same as the start of pregnancy?
Not in how pregnancy is counted. Doctors count pregnancy weeks from the first day of the last period, about two weeks before conception.
How soon after conception can I take a pregnancy test?
Most home tests are reliable from the first day of a missed period, about two weeks after conception. Some sensitive tests can detect pregnancy a few days earlier.
Can I conceive during my period?
It is unlikely but possible, especially with short cycles, because sperm can survive up to five days and ovulation may come early.
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Last reviewed: October 2026. This calculator is for information only and does not replace advice from a doctor or other qualified health professional.