Only about 1 in 20 babies actually arrive on their estimated due date. What you’re really getting is a statistical midpoint, not a prediction of a specific day.
Free pregnancy tool
Due Date Calculator
Estimate your due date from your last period, a known conception date, or an IVF transfer date.
Runs entirely in your browser. Nothing you enter is sent anywhere. Want the full week-by-week picture? Try the Pregnancy Calculator.
Why three different starting points
Naegele's rule (the standard "last period plus 280 days" method, first described in the early 1800s and still the basis for most due date estimates) assumes a 28-day cycle with ovulation on day 14. Enter your real average cycle length and this calculator adjusts for it, since a longer or shorter cycle shifts ovulation, and with it the real conception date, even when the last-period date is identical. If you know your actual conception date, that skips the cycle-length guesswork entirely: due date is simply 266 days later. IVF transfers are the most precise of the three, since the embryo's age is known exactly at transfer, so the calculation works backward from a known developmental stage rather than an estimate.
Due date calculator FAQ
How accurate is a due date estimate?
Only about 1 in 20 babies actually arrive on the estimated due date itself. It is a statistical midpoint, most babies are born within about two weeks either side of it, not a prediction of a specific day.
What if I do not know my last period date exactly?
Use the conception-date or IVF-transfer mode instead if you know either of those more precisely, they skip the last-period estimate entirely. Otherwise, your best estimate of the date, combined with your average cycle length, is still more useful than leaving it blank.
How does cycle length affect my due date?
The standard calculation assumes a 28-day cycle with ovulation on day 14. A longer cycle means later ovulation and a later real conception date, even with an identical last-period date, so this calculator shifts the estimate by the difference between your actual cycle length and 28 days.
What is Naegele's rule?
The standard method of adding 280 days (40 weeks) to the first day of the last menstrual period to estimate a due date. It dates back to the early 1800s and, cycle-length adjustments aside, is still the basis most due date calculations use today.
Is my data private?
Yes. Every calculation runs in your browser. Nothing you enter is sent to a server, logged, or stored.
Free due date calculator by ANUPRESS
Your actual cycle length changes the math
The standard “add 280 days” rule assumes ovulation on day 14 of a 28-day cycle. A 35-day cycle pushes real ovulation, and with it real conception, about a week later, even with an identical last-period date. This calculator adjusts for that automatically once you enter your own average cycle length, rather than quietly assuming everyone’s cycle is textbook-standard.
Three starting points, three levels of precision
Last period plus cycle length is the most common input but carries the most uncertainty, since it estimates ovulation rather than knowing it. A known conception date skips that estimate entirely. An IVF transfer date is the most precise of the three: the embryo’s exact age at transfer is known, so the calculation works from a confirmed developmental stage rather than a guess about ovulation timing.
What this date is actually useful for
Treat the result as a planning anchor, for maternity leave, appointment scheduling, and knowing roughly which trimester you’re heading into, rather than a fixed appointment on the calendar. For the full week-by-week picture, trimester, milestones and percentage complete, our Pregnancy Calculator builds on this same due date.