How to calculate your pregnancy due date (Naegele’s rule)

Every due date estimate is calendar math with a documented rule. The Pregnancy Due Date Calculator implements Naegele’s rule and the standard conception/IVF offsets from ACOG Committee Opinion 700.

How it works

1. Method — LMP, conception date, or IVF transfer day.
2. Date — plus cycle length if dating by LMP.
3. Results — due date, weeks + days today, trimester milestones.

The rules

LMP dating: due date = LMP + 280 days, shifted by (cycle − 28) days. Conception dating: + 266 days. IVF: + 261 days from a day-5 transfer, + 263 from day-3. Gestational age counts from LMP, which is why "4 weeks pregnant" happens barely two weeks after conception.

What the estimate means

A due date is the middle of a distribution, not an appointment — roughly 96% of births miss the exact day. Per ACOG CO 700, a first-trimester ultrasound measurement supersedes calendar dating when they disagree. Informational only — confirm with your prenatal provider.

Sources & methodology

Methodology reference: ACOG Committee Opinion 700 — Methods for Estimating the Due Date. Informational only — not medical advice. Confirm dating with your prenatal care provider; first-trimester ultrasound overrides calendar estimates. Everything runs client-side in your browser; nothing is uploaded.

The Due Date Calculator tool is free, runs locally, and never sends your data to a server. More health calculators at maratool.com.