Conception Date Calculator
Enter your due date, or the first day of your last period and your cycle length. The calculator gives the most likely conception date and the days of intercourse that could have led to it.
Free, no sign-up. What you enter stays in your browser: it is not saved or sent to HerFound. Reviewed .
Work it out from
A due date from a scan gives a better estimate than one from period dates.
Enter a due date to see the most likely conception date.
This is general information, not medical advice. It cannot take your own health history into account. For anything that worries you, speak to a doctor, midwife or pharmacist.
How it works
From a due date: most likely conception date = due date minus 266 days (38 weeks). A due date is set at 280 days after the first day of the last period, and conception is assumed to happen 14 days into that count.
From a last period: most likely conception date = first day of the last period + cycle length minus 14 days. This is the estimated day of ovulation. In a 28-day cycle it is day 14; in a 32-day cycle it is day 18.
The range covers the 5 days before that date as well as the date itself. Sperm can survive for up to about 5 days, and the egg lives for less than a day after ovulation. So intercourse on any of those 6 days could have led to the pregnancy.
What it cannot tell you
- It cannot establish paternity. If intercourse with more than one partner fell inside or near the range, dates alone cannot tell you who the father is. Only a DNA test can.
- The date is an estimate. Ovulation often does not fall exactly 14 days before the next period, and a due date from a scan can itself be several days out. The true date can be a week or more either side.
- It is less reliable with irregular cycles, soon after stopping hormonal contraception, or when you are unsure of the date of your last period.
- It does not apply to IVF. With IVF the fertilisation date is known, so use the IVF due date calculator.
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Common questions
To work out a conception date from a due date, subtract 266 days (38 weeks). A due date of 8 October gives a likely conception date of 15 January. This is an estimate, and the real date may be several days earlier or later.
A conception date calculator gives an estimate, not an exact day. It assumes ovulation happened 14 days before the next period was due, which is not true for everyone. Expect the true conception date to fall within about a week either side, and sometimes more.
No, a conception date cannot tell you who the father is. The estimate has a margin of a week or more, and sperm can survive for up to about 5 days, so the possible days often overlap. A DNA paternity test is the only reliable way to know.
The conception date is not always the day you had sex. Sperm can survive in the body for up to about 5 days, so conception can happen several days after intercourse. That is why this calculator shows a range of days and not one date.
Conception usually happens about 14 days before the next period is due. In a 28-day cycle that is around day 14, counting the first day of the period as day 1. In a longer cycle it is later: around day 21 in a 35-day cycle.