Pregnancy due date calculator
Calculate your due date from the first day of your last period and your cycle length. This baby due date calculator runs entirely in your browser — nothing is saved or sent.
The first day of bleeding, not the last.
First day of one period to the next. Default 28.
How to use it
How a due date is calculated
The standard method is Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. Because that assumes a 28-day cycle with ovulation on day 14, this calculator adjusts for your actual cycle length — adding the difference from 28. So a 30-day cycle pushes the date two days later. The reasoning is explained in full on Naegele’s rule and your due date and the how it’s calculated page.
Worked example
If your last period began on 1 January with a 28-day cycle, your estimated due date is 8 October the same year (1 January + 280 days). With a 32-day cycle, it moves to 12 October, four days later.
Accuracy and limitations
A due date is a reference point, not a deadline. Births naturally spread across roughly a month around it, and a due date based on the last period is only as good as the date and cycle length you enter. If you’re unsure of your last period, or your cycles are irregular, an early ultrasound is the most accurate way to date the pregnancy — see how accurate is your due date.
Other dating methods
Know a different date? The full OB wheel calculator also dates from a known due date, an ultrasound, a conception date, or IVF timing, and shows every milestone. This page keeps things simple for the most common case: dating from your last period.
Frequently asked questions
How is a pregnancy due date calculated?
The usual method adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for cycle length. This is Naegele’s rule. An early ultrasound can refine it.
How accurate is a due date calculator?
It’s an estimate. Most babies arrive within about two weeks either side of the date, and only a small share are born exactly on it. A first-trimester ultrasound is the most accurate way to date a pregnancy.
Does cycle length change my due date?
Yes. A longer cycle means later ovulation, pushing the due date back; a shorter cycle pulls it forward. The adjustment is your cycle length minus 28 days. If you don’t know it, 28 is the default.
What if I know my conception or IVF date instead?
Use the full OB wheel, which dates from conception (+266 days) or IVF transfer/retrieval precisely. This page focuses on the most common last-period method.
Why might my due date change at a scan?
If an ultrasound measurement disagrees with your last-period estimate beyond a set threshold, clinicians usually re-date to the scan because early ultrasound is more reliable.
Last meaningfully updated: 2026-08-25.