Toolrack

Due Date Calculator

Works offlineNothing is uploadedFree, no sign-up
DUE DATE—
GESTATIONAL AGE—
TRIMESTER—
DAYS TO GO—

An estimate by the standard rule — your clinician's ultrasound dating takes precedence over any calculator.

Computed in this tab — nothing you enter leaves it.

The due date by Naegele's rule — last period plus 280 days, corrected for your cycle length, or conception plus 266 when that date is the certain one. With it: gestational age the way clinics write it, the trimester map, and the countdown. Arithmetic on your device, shared with no one.

A rule from 1812, still on every chart

Naegele's rule is the oldest formula in daily clinical use: add seven days to the last period, count forward nine months. It survives because it needs only a date a patient can actually supply, and because 280 days is genuinely the median of human gestation counted that way.

The trimester boundaries at 14 and 28 weeks are conventions too, but load-bearing ones: screening tests, travel advice and workplace protections all key off them, which is why the map is printed and not just the final date.

Frequently asked questions

How accurate is a calculated due date?

It is a statistical anchor, not an appointment: only about one baby in twenty arrives on the exact date, and the great majority arrive within two weeks either side. Clinics still need the anchor — every screening window and milestone is scheduled against it.

Why does cycle length shift the date?

Naegele's 280 days assume ovulation on day 14 of a 28-day cycle. A 35-day cycle typically ovulates a week later, so the whole clock starts a week later — the correction adds exactly the difference. Irregular cycles are the case where ultrasound dating matters most.

Which date wins — this rule or the ultrasound?

The first-trimester ultrasound, always: embryo size in weeks 8–13 dates a pregnancy to within days, before individual growth differences blur it. The rule of thumb clinics use: if scan and period dates differ by more than about 5 days early on, the scan becomes the official date.

Why do weeks count from the period, not conception?

Because the period is the date women reliably know. The convention makes 'weeks pregnant' start two weeks before conception — odd but universal, and every chart, screening window and clinic letter uses it. This page follows the convention for exactly that reason.

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