Due date calculator
Estimate your baby's due date from the first day of your last menstrual period, and watch a live countdown to the big day. This uses the same 40-week rule clinicians start with — but a due date is an estimate, not an appointment, and we explain why below.
An estimate for general information only — see the note below.
How to use this calculator
Enter the first day of your last menstrual period (LMP) and your typical cycle length. Most calculators assume a 28-day cycle, but if yours runs longer or shorter, adjusting it gives a more personal estimate. The result is your estimated due date plus a running countdown of how many days remain.
How the due date is estimated
The classic method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last period:
This assumes ovulation around day 14 of a 28-day cycle. If your cycle is longer or shorter, ovulation shifts, so this calculator adjusts by adding the difference between your cycle length and 28 days. A 32-day cycle, for instance, pushes the estimate about four days later than the textbook rule would.
A worked example
If your last period began on January 1 and you have a standard 28-day cycle, adding 280 days lands you on roughly October 8 of the same year. With a 32-day cycle, the calculator adds four more days, giving about October 12. Either way, it's a target window, not a guarantee.
What a due date really means
Only about one in twenty babies actually arrives on the estimated due date. A full-term birth is considered anything from 37 to 42 weeks, and first pregnancies often run a little past the estimate. Early ultrasound dating is generally more accurate than LMP-based dating, especially if your cycles are irregular, so your provider may revise the date after a scan. Use this number to plan and anticipate — but hold it loosely.
Frequently asked questions
What if I don't know my last period date?
LMP-based dating won't work without it, but an early ultrasound can estimate gestational age directly from the baby's measurements. If you're unsure of your dates, your provider will typically rely on a dating scan instead.
Why does cycle length matter?
The 280-day rule assumes you ovulate around day 14. If your cycle is longer, you likely ovulate later, so conception happened later and the due date moves out. Adjusting the cycle length nudges the estimate to better match your body.
How accurate is this estimate?
It's a reasonable starting point, but only a small share of babies arrive exactly on the estimated date. Births anywhere from three weeks before to two weeks after are considered normal-term. Early ultrasound dating is more precise than the LMP method.