Due Date Calculator
Enter the first day of your last period to get an estimated due date. It is worked out as 280 days (40 weeks) from that day, adjusted if your cycle is longer or shorter than 28 days.
Free, no sign-up. What you enter stays in your browser: it is not saved or sent to HerFound. Reviewed .
From the first day of one period to the first day of the next. Leave it at 28 if you are not sure.
Enter the first day of your last period to see your estimated due 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
The calculator uses Naegele's rule, the method most clinics start with: estimated due date = first day of your last period + 280 days. That is 40 weeks, counted from the period, not from conception.
The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is usually a different length, the calculator shifts the date by the difference. A 32-day cycle moves the due date 4 days later; a 25-day cycle moves it 3 days earlier.
How far along you are is counted the same way: days since the first day of your last period, with the same cycle adjustment. The second trimester starts at 14 weeks and the third at 28 weeks.
What it cannot tell you
- It is an estimate. Only about 1 in 20 babies arrive on their due date; most are born in the two weeks either side.
- A dating ultrasound in the first trimester is more accurate than any calculation from period dates. If your clinic gives you a different date, use theirs.
- It is less reliable if your cycles are irregular, if you recently stopped hormonal contraception, or if you are not sure of the date of your last period.
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Common questions
A due date is calculated by adding 280 days (40 weeks) to the first day of your last period. This is called Naegele's rule. If your cycle is longer or shorter than 28 days, the date is moved later or earlier by the difference.
A due date from your last period is an estimate, usually within a week or two. Only about 5% of babies are born on the date itself. A first-trimester ultrasound is more accurate, and clinics normally go by the ultrasound date when the two disagree.
Pregnancy is counted from the first day of the last period, which is about two weeks before conception. That is why a pregnancy is described as 40 weeks long even though the baby develops for about 38 weeks.
If your cycle is not 28 days, add or subtract the difference. With a 35-day cycle you probably ovulated about a week later than the standard rule assumes, so the due date is about 7 days later. This calculator makes that adjustment for cycles from 21 to 45 days.