Pregnancy Due Date Calculator — Free Online Tool
Enter the first day of your last menstrual period (LMP) to estimate your due date, likely conception date, current week of pregnancy, and trimester milestones.
What is the Pregnancy Due Date Calculator?
Pregnancy is dated from the first day of the last menstrual period — not from conception — because the LMP is a date most people know. The standard estimate adds 280 days (40 weeks) via Naegele's rule. Only about 1 in 20 babies arrives exactly on the due date; most come within two weeks on either side, which is why doctors call it an “estimated” date.
Dating assumes a textbook 28-day cycle with ovulation on day 14. If your cycles are longer, shorter, or irregular, an early ultrasound gives a more accurate date — and your provider will use that instead. Use this calculator for planning and curiosity; confirm everything at your appointments.
How to use this calculator
- Pick the first day of your last period.
- Click Calculate Due Date.
- See your estimated due date, conception window, current week, and trimester.
Formula
Frequently asked questions
How accurate is a due date from LMP?
It's an estimate — about 5% of babies arrive on the date. An early ultrasound (before 13 weeks) dates pregnancy more precisely.
Why is pregnancy counted from the last period?
Because that date is known, while conception usually isn't. The two “extra” weeks are simply convention.
What are the trimester date ranges?
Roughly: weeks 1–13 (first), 14–27 (second), 28–birth (third).
My cycles are irregular — is this still useful?
Less so. Tell your provider; they'll likely date the pregnancy by early ultrasound instead.