Pregnancy Due Date Calculator

Enter the first day of your last period and your cycle length to estimate your due date — about 280 days, or 40 weeks, from your last period. It is an estimate, not a deadline: only about 1 in 20 babies arrive on the exact day.

Free tool Tools Updated June 2026

How the due date is calculated

The standard method, the Naegele rule, counts 280 days (40 weeks) from the first day of your last menstrual period, assuming a 28-day cycle. If your cycle is longer or shorter, this tool adjusts the date by the difference, since ovulation shifts with cycle length.

Your provider may revise the date after an early ultrasound, which measures the baby and is the most accurate way to confirm gestational age, especially if your cycles are irregular or you are unsure of your last period.

Related questions

How accurate is a due date?
It is an estimate. Only about 1 in 20 babies are born on their due date, and a normal, full-term birth can happen anywhere from 37 to 42 weeks. An ultrasound in the first trimester gives the most accurate dating.
What if I do not know my last period date?
Use your best estimate and treat the result as rough, then ask your provider. An early ultrasound can date the pregnancy without a known last-period date and is the most reliable method.
Can the due date change?
Yes. Providers often adjust the date after a first-trimester ultrasound if it differs from the last-period estimate by more than about a week. After that, the due date usually stays fixed for the rest of the pregnancy.

Sources & further reading

  1. ACOG — Methods for Estimating the Due Date
  2. Cleveland Clinic — Fetal Development: Stages of Growth

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This tool gives general estimates for education only and is not medical advice or a diagnosis. Due dates are estimates, and an early ultrasound is more accurate than a last-period date. For your own care, contact your healthcare provider.