Due Date Calculator: How the 40-Week Estimate Works
A due date is 40 weeks from the first day of the last period. Here is why that convention exists and how much to trust it.
What is the Due Date Calculator?
The standard estimate counts 40 weeks, or 280 days, from the first day of the last menstrual period, which is roughly two weeks before conception in a typical cycle. Enter that date and the tool returns the estimated due date. The result is general information only, not medical advice, and only a small share of births land exactly on the estimated date — an ultrasound gives a more reliable estimate.
How to use it
- Enter the first day of your last menstrual period.
- Read the estimated due date.
- Note that the date is an estimate, not a guaranteed delivery day.
Worked example
For a last period starting 2026-01-01, adding 280 days gives an estimated due date of 8 October 2026, which is 40 weeks from that date.
How is a due date calculated?
The standard rule adds 280 days, or 40 weeks, to the first day of the last menstrual period. That counts about two weeks before conception, because ovulation usually happens mid-cycle. For a last period starting 1 January 2026, the estimated due date is 8 October 2026. Ultrasound dating, especially early in pregnancy, is usually more accurate than this date-based estimate.
Frequently asked questions
Why count from the last period instead of conception?
Conception day is often unknown, while the last period is usually remembered, so the 280-day count is the standard convention.
How often are babies born on their due date?
Only a small minority arrive exactly on the date; most arrive within a couple of weeks either side.