Due Date Calculator

The standard formula adds 280 days (40 weeks) to the first day of the last menstrual period (LMP), assuming a 28-day cycle. This calculator returns both the estimated due date and the current week of pregnancy.

When to use this calculator

You know your LMP and want a quick estimate of when your baby is due.
You're tracking weeks-of-pregnancy alongside the EDD.
You need a planning date for maternity leave, appointments, or family.

How it works

Uses Naegele's rule: take the first day of your last menstrual period, add 7 days, then add 9 months — equivalent to LMP + 280 days. This assumes a regular 28-day cycle with ovulation on day 14. For longer or shorter cycles, the actual due date shifts by the difference from a 28-day cycle.

Real-world examples

LMP 1 January in a 28-day cycle → estimated due date 8 October.
LMP 15 March in a 32-day cycle → due date roughly 4 days later than the basic formula gives.
Maternity leave planning: a 35-week pregnancy point lands 5 weeks before the EDD.

Limitations

Assumes a regular 28-day cycle — irregular cycles shift the estimate.
Only about 4% of babies are born exactly on the EDD; most arrive within ±2 weeks.
First-trimester ultrasound dating is more accurate than LMP-based dating.
Not a substitute for medical assessment or a clinical EDD from a healthcare provider.

Frequently asked questions

How is the due date calculated?

By Naegele's rule: first day of last period + 280 days. That equals adding 7 days then 9 months to the LMP.

How accurate is the estimated due date?

Within roughly two weeks. About 80% of babies arrive between 38 and 42 weeks; only around 4% are born exactly on the EDD.

What if my cycle isn't 28 days?

Adjust by the difference. A 32-day cycle pushes ovulation about 4 days later, so add 4 days to the basic estimate. Short cycles shift the date earlier.

Is LMP dating more accurate than ultrasound?

No. A first-trimester ultrasound is the gold standard. LMP-based estimates are useful when ultrasound dating is not yet available.

Should I plan delivery on this exact date?

Treat it as a target window, not a date. Use the EDD for appointments and prep, but be ready for delivery anywhere in the 38–42 week range.

Results

How to Use the Duedate Calculator