Pregnancy Due Date Calculator
This calculator gives a general estimate — not a medical diagnosis or a guarantee — of your due date and how far along you are. It updates instantly, and the link in your address bar always reflects your numbers so you can save or share it.
How this calculator works
By default we use Naegele's rule, the standard method taught in obstetrics: due date = first day of your last menstrual period (LMP) + 280 days (40 weeks), adjusted by a few days if your average cycle is longer or shorter than the textbook 28 days. If you'd rather calculate from a known conception date, we add 266 days (38 weeks) instead — both methods estimate the same underlying event and should land within a day or two of each other. This is a general estimate only, not medical advice or a diagnosis; your care provider's dating, often confirmed by ultrasound, is what should guide your care.
Worth knowing: due dates are honest guesses, not appointments. Only around 5% of babies are born on their exact due date — full-term birth normally spans several weeks around it, so treat the date as a useful planning target rather than a countdown.
Worked example
Last menstrual period: May 1, 2026, with the default 28-day cycle. Naegele's rule gives a due date of February 5, 2027 (May 1 + 280 days). Checking against a reference date of August 13, 2026, that's 104 days after the LMP — week 14 of pregnancy, which falls in the 2nd trimester (weeks 14–27).
Frequently asked questions
How accurate is the estimated due date?
It's a helpful planning target, not a prediction you can count on. Only about 5% of babies are actually born on their exact calculated due date — most arrive anywhere from a couple of weeks early to a couple of weeks late, and that's entirely normal. Think of the date as the center of a wide, healthy range rather than a deadline.
Why does average cycle length matter?
Naegele's rule assumes a textbook 28-day cycle with ovulation around day 14. If your cycles run longer or shorter, ovulation (and so conception) tends to shift with them — a longer cycle usually means later ovulation, a shorter one earlier. Entering your average cycle length nudges the estimate to better match when conception likely happened.
What if I don't know my last menstrual period?
Switch the calculation method to 'From conception date' and enter that date instead — for example if you're tracking ovulation or used fertility treatment with a known date. The calculator adds 266 days from conception rather than 280 days from LMP, which lands on essentially the same estimate.
Is this a substitute for advice from my doctor or midwife?
No — please don't treat it as one. This is a general estimate based on a simple, decades-old formula, not medical advice. Always follow the dating your doctor or midwife gives you, which is often confirmed or adjusted using an early ultrasound measurement. It's completely normal for an ultrasound date to differ from this calculator by several days to a week or two — that's not a red flag, it's just a more precise measurement taking over from a date-based estimate.