Pregnancy Due Date Calculator

Enter the first day of your last menstrual period (LMP) to estimate your due date by Naegele's rule, plus how many weeks along you are now and your trimester.

Enter your last period date and press Calculate due date.

Naegele's rule

The standard estimate of a due date — the estimated date of delivery (EDD) — counts 280 days (40 weeks) from the first day of the last menstrual period:

EDD = LMP + 280 days (40 weeks)

The classic shortcut is "add one year, subtract three months, add seven days." If your cycle is longer or shorter than the standard 28 days, the date shifts by the difference, because ovulation (and conception) happens later or earlier in a longer or shorter cycle.

Worked example

Last period started January 1, with a 28-day cycle:

Add 280 days: Jan 1 + 280 days → October 8.
Shortcut: +1 year, −3 months, +7 days = Oct 8.
30-day cycle: shifts the date two days later, to Oct 10.

Why it's only an estimate

Only about 1 in 20 babies arrives on the exact due date; most births happen within two weeks either side. The LMP method assumes regular cycles and known dates. An early ultrasound dates a pregnancy more accurately and is what clinicians rely on. This tool is for general information.

Note: this is an educational estimate, not medical advice. Your clinician's dating — usually confirmed by ultrasound — is the figure to follow.

Frequently asked questions

How is a due date calculated?

By Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. A January 1 LMP gives an estimated due date around October 8 for a standard 28-day cycle.

How accurate is a due date?

It's an estimate. Only about 5% of babies arrive on the exact date; most come within two weeks either side. An early ultrasound dates a pregnancy more precisely than the LMP method.

Does cycle length change the due date?

Yes. A cycle longer than 28 days pushes ovulation and the due date later by the difference; a shorter cycle moves it earlier. This tool adjusts the estimate for your cycle length.

What if I don't know my last period date?

Then LMP dating can't be used reliably. An ultrasound measurement of the baby can establish gestational age and a due date — ask your clinician.

MB
Mustafa Bilgic · Editor, Calcool
Naegele's rule (LMP + 280 days) is the standard clinical estimate. For pregnancy health information see the American College of Obstetricians and Gynecologists (ACOG). This is an estimate, not medical advice.

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