Pregnancy Due Date Calculator

This calculator applies Naegele's rule — adding 280 days, or 40 weeks, to the first day of your last menstrual period — to estimate a due date. It's a widely used starting estimate, not a precise prediction: it assumes a regular 28-day cycle with ovulation around day 14, and only a small share of babies are actually born on their exact calculated due date, with most arriving within a couple of weeks either side of it.

Expecting parents typically use a number like this early on, before an ultrasound gives a more individually accurate estimate based on measuring the fetus directly rather than assuming a textbook cycle. It's a useful placeholder for planning purposes in the meantime.

This is general pregnancy information, not medical advice or a diagnosis — always confirm dates and follow guidance from your own healthcare provider. Nothing you enter here, including your last period date, is stored or sent anywhere; it stays in your browser.

Naegele's rule: due date = first day of last menstrual period + 280 days. The 280-day figure comes from assuming a 28-day cycle with ovulation and conception around day 14, then adding a standard 266-day gestation period from conception — in other words, it's built on cycle averages, not your specific cycle.

That's exactly why an early ultrasound is generally considered more accurate than LMP-based dating: it measures the fetus directly and isn't thrown off by an irregular cycle, uncertain ovulation timing, or an imprecisely remembered last period date the way Naegele's rule can be.