Pregnancy Due Date Calculator

Calculate estimated pregnancy due date free online using the standard 280-day method from last menstrual period. An estimate to discuss with your healthcare provider.

First Day of Last Menstrual Period

Browser only. An estimate to discuss with your healthcare provider, not a medical determination.

How to Use This Tool

Enter the first day of your last periodThe standard reference point for this calculation

See your estimated due dateCalculated using the standard 280-day method

Discuss with your healthcare providerFor accuracy refined with ultrasound dating

A Standard Starting Estimate, Refined by Your Provider

Naegele's rule -- adding 280 days to the first day of the last menstrual period -- is the standard preliminary method many healthcare providers use as a starting estimate before any ultrasound-based refinement. It's a useful, widely understood reference point, not a precise prediction of the actual delivery date.

Why Actual Delivery Dates Vary Naturally

Only a minority of births occur on the exact calculated date -- normal full-term delivery spans a range of roughly several weeks around the estimate, reflecting genuine biological variation even among entirely healthy pregnancies. This is expected and normal, not a sign that the calculation or the pregnancy itself is somehow off-track.

This Estimate Versus Ultrasound Dating

For pregnancies with cycle lengths different from the typical 28-day assumption, or for general accuracy, healthcare providers often refine or replace this calculation with ultrasound-based dating, which tends to be more precise for individual pregnancies than the standard formula alone.

Frequently Asked Questions

Using Naegele's rule, the standard method: due date = first day of last menstrual period + 280 days (40 weeks). This assumes a typical 28-day cycle, which is the basis most healthcare providers initially use before any ultrasound-based adjustment.

Actual delivery dates vary naturally -- only a minority of births occur on the exact calculated date, with a normal range spanning roughly two weeks before or after. Cycle length variation between individuals also affects accuracy, which is why healthcare providers often refine this estimate using ultrasound measurements.

This calculator uses the standard 28-day assumption. If your actual cycle is meaningfully longer or shorter, your healthcare provider can adjust the estimate accordingly, or rely more heavily on ultrasound dating for greater accuracy.

No -- this provides a standard preliminary estimate using the same method many providers use as a starting point, but your actual healthcare provider, especially combined with ultrasound dating, gives the most accurate and medically relevant estimate for your specific pregnancy.

Full-term delivery is generally considered normal anywhere from about 3 weeks before to 2 weeks after the calculated due date, reflecting the natural variation in pregnancy length even among healthy pregnancies.