A due date calculator gives expectant parents a clear estimate of when their baby will arrive, and it is usually one of the first things people search for after a positive pregnancy test.
Most pregnancies last around 40 weeks from the first day of the last menstrual period. A due date calculator works backwards from this standard to give an estimated delivery date.
How a due date is calculated
The most common method is Naegele’s rule: take the first day of your last menstrual period, add one year, subtract three months and add seven days. The result is your estimated due date. Most online due date calculators use this formula automatically.
Conception date method
If you know the exact date of conception, add 266 days. This works well for those tracking ovulation or using fertility treatment, where conception timing is precise.
Ultrasound dating
A dating scan in the first trimester is the most accurate way to confirm a due date, particularly for women with irregular cycles. Crown-rump length measurements between weeks 8 and 13 are highly reliable.
Only an estimate
Only around 5% of babies arrive on their actual due date. Most are born within two weeks either side, which is considered full term.
Further reading: moomie
