Conception Date Calculator
Enter your due date, or the first day of your last period and your cycle length. You get an estimated conception date and a likely window.
This is an estimate for general information and is not medical advice. Only a dating scan can give a firmer date, and your midwife will use it.
Worked examples
From a due date
Due date 1 March 2027
Conception about 8 June 2026
266 days earlier.
From a last period
Last period began 1 January 2027, 28 day cycle
Conception about 15 January 2027, due 8 October 2027
Ovulation on day 15.
About Conception Date Calculator
A conception date calculator works backwards from the information you have to estimate when an egg was fertilised. If you know your due date, it subtracts 266 days, which is 38 weeks. If you know the first day of your last period, it adds your cycle length minus 14 days, which is when ovulation normally happens. Both ways give the same sort of answer.
Pregnancy is counted from the first day of the last period, about two weeks before conception, which is why a 40 week pregnancy is only about 38 weeks from conception. Sperm can live for up to five days and an egg for about a day, so the page also shows a window from five days before the estimated date to one day after. It is a guide and an estimate, not medical advice.
How to use Conception Date Calculator
- Choose whether you know your due date or your last period.
- Enter the due date, or the first day of your last period and your usual cycle length.
- Read the estimated conception date and the window for sex.
- Compare with your dating scan date if you have one.
Common uses
- Working out when you probably conceived
- Checking dates against a dating scan
- Satisfying curiosity about a due date
- Writing a baby book or journal
Frequently asked questions
How is the conception date worked out?
From a due date, it is 266 days earlier. From your last period, it is your cycle length minus 14 days after the first day of the period, which is the usual day of ovulation. A 28 day cycle gives day 14.
How accurate is the estimate?
It is a guide. Ovulation does not happen on the same day in every cycle, and babies are born over a range from 37 to 42 weeks. Only about 1 baby in 20 is born on the due date.
Why is there a window of several days?
Sperm can survive for up to five days in the body, while the egg lives for about a day. So sex in the five days before ovulation, or on the day, can lead to pregnancy.
Does a dating scan change the date?
Yes. Your midwife or sonographer may adjust your due date after the scan. If you have a scan date, use that due date here for a better estimate.
Is this medical advice?
No. It is an estimate and not medical advice. Your midwife or doctor can give you advice for your own situation.