🤰 Pregnancy Due Date Calculator
The classic estimate adds 280 days (40 weeks) to the first day of your last period — adjusted for your cycle length. Enter your values below — results update instantly, entirely on your device.
Fill in the fields above — results appear instantly.
How the pregnancy due date calculator works
Naegele's rule: due date = first day of last menstrual period + 280 days (40 weeks). For cycles other than 28 days, add or subtract the difference (a 30-day cycle adds 2 days). Gestational age is counted from that first day, so "weeks pregnant" starts about two weeks before conception.
Example: Last period starting Jan 1 with a 28-day cycle gives a due date of about October 8.
This is an estimate — only about 4% of babies arrive exactly on the due date, and a full-term birth is anything from 37 to 42 weeks. An early-pregnancy ultrasound gives a more accurate date and takes precedence over the LMP calculation. This tool is informational, not medical advice — and because it runs entirely in your browser, the date you enter is never uploaded anywhere.
Frequently asked questions
How is a due date calculated?
By Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. Gestational age is measured from that day, which is why you're counted as "2 weeks pregnant" around conception — the clock starts before the egg is fertilised.
How accurate is a due date?
It's an estimate. Only around 4% of births land on the exact due date; most happen within a week either side, and 37–42 weeks is all considered full term. A first-trimester ultrasound dates the pregnancy more precisely and is used in preference to the last-period calculation when they differ.
What if my cycle isn't 28 days?
The tool adjusts: a longer cycle pushes the due date later and a shorter one earlier, because ovulation (and so conception) shifts. Enter your average cycle length for a better estimate.
Is the date I enter kept private?
Yes — the calculation runs entirely in your browser and the page works offline. Nothing you type is sent to any server, which for something this personal is the point.
Is this pregnancy due date calculator accurate and private?
Yes. It uses the standard published formula, shows its working under every result, and computes locally in your browser — your inputs are never sent to a server, and the page works offline.