How accurate is a pregnancy due date calculator?

Due date calculators using Naegele's rule provide an estimate based on your last period. Only about 5% of babies are born exactly on their due date, so it's best treated as a planning window.

Pregnancy due date calculator

Estimate your due date from your last period using Naegele's rule.

First day of last period

Average cycle length (days)

Estimated due date

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Est. conception

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Weeks pregnant

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Trimester

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Only about 5% of babies arrive exactly on their due date — most arrive within two weeks either side. Think of this as a planning window, not a deadline.

How Naegele's rule works

The rule adds 280 days (40 weeks) to the first day of your last period, assuming a fairly typical 28-day cycle. If your cycles run longer or shorter, the adjustment for cycle length shifts the estimate closer to your actual conception date. Also want to estimate your fertile window? Try the ovulation calculator.

This is an estimate. Your care provider's dating ultrasound is the most reliable way to confirm gestational age.

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