How is the due date calculated?

The most common method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14.

How accurate is the estimated due date?

Only about 5% of babies are born exactly on their due date. Most births occur within 2 weeks before or after the estimated date. An ultrasound in the first trimester gives a more accurate estimate.

What are the pregnancy trimesters?

First trimester: weeks 1–12. Second trimester: weeks 13–26. Third trimester: weeks 27–40. Full term is 39–40 weeks; early term is 37–38 weeks.

Due Date Calculator

Name: Due Date Calculator
Author: Miikka

Estimate your baby's due date based on your last menstrual period or conception date. See your pregnancy timeline and key milestones.

By Miikka · CalcBrew · Updated Sep 16, 2026

Calculation method

🩸 First day of last menstrual period

Calculation based on Naegele's rule: LMP + 280 days (40 weeks)

🤰

Enter a date to see the due date

⚠️ This calculator is a guideline. The estimated due date may differ from your doctor's or midwife's assessment. An ultrasound estimate is generally more accurate than a menstrual-based calculation. Always attend regular prenatal appointments.

The due date calculator estimates your baby's birth date and tells you which pregnancy week you are in. Enter the start date of your last period or another known detail to get the due date and how your pregnancy is progressing.

How the calculator works and what it’s for▾

How the due date is estimated

The most common approach is to calculate the due date from the first day of your last period. Pregnancy length is counted forward from that day, and an average full-term pregnancy lasts about 40 weeks.

Pregnancy weeks accumulate from the same starting point, so the calculator shows how far along you are today.

What you enter and what you get

You typically provide the start date of your last period. In return you get an estimated birth date along with your current pregnancy weeks and days.

This gives you an easy-to-grasp timeline for following the stages of your pregnancy.

Who it helps

The tool serves those planning a pregnancy and expecting parents who want a sense of the birth timing and how the pregnancy is unfolding.

It also helps anticipate clinic visits and other key moments during pregnancy.

Good to know

The due date is an estimate, and only a small share of babies arrive exactly on that day. Cycle length and other individual factors affect the accuracy.

The most precise estimate comes from a healthcare ultrasound; use the calculator as supportive guidance.

🔄 Reviewed June 2026

Frequently asked questions

Cite this calculator

APACalcBrew. (2026). Due Date Calculator. https://calcbrew.com/due-date-calculator

MLA“Due Date Calculator.” CalcBrew, 2026, calcbrew.com/due-date-calculator.

Link to https://calcbrew.com/due-date-calculator. Results are estimates; cite the underlying formula or data source for academic work.

More in Health & Nutrition · All calculators