What is Naegele's rule?

Naegele's rule is the standard method for estimating a pregnancy due date. It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The rule assumes a 28-day cycle with ovulation on day 14.

Should I use last menstrual period or conception date?

Use last menstrual period if you know that date, as it is the standard method used by healthcare providers. Use conception date only if you know it precisely, for example from IVF or closely tracked ovulation.

What does gestational age mean?

Gestational age is counted from the first day of the last menstrual period, not from conception. At 4 weeks gestational age, actual conception typically occurred around 2 weeks earlier. This is the standard measure used in clinical settings.

How accurate is this due date estimate?

The result is an estimate based on Naegele's rule and the dates you enter. Only around 5% of babies are born on their exact due date. Confirm your dates with a healthcare provider, who may use ultrasound measurements for a more precise estimate.

Due date calculator

Due date calculator is a health tool that estimates a pregnancy due date using Naegele's rule. It accepts either the last menstrual period or a known conception date, reports gestational age in weeks and days, and identifies the current trimester. The tool runs in your browser.

Calculate from

First day of last period

Dedicated pages

Pregnancy week and trimester calculators.

Pregnancy due date calculator

Calculate your estimated due date from your last menstrual period or conception date. Includes gestational age, trimester, and weeks remaining. Free, in-browser.

Pregnancy week calculator

Find out how many weeks pregnant you are from your last period or conception date. Shows gestational age in weeks and days, trimester, and due date. Free, in-browser.

Pregnancy trimester calculator

Find out which trimester you are in and how far along you are. Enter your last period or conception date. Free, runs in your browser.

How to calculate your due date

Choose your calculation method: last menstrual period or conception date.
Select the date using the date picker.
Your estimated due date, gestational age, and trimester appear instantly.
Confirm the result with your healthcare provider.

Common uses

Planning ahead for appointments, leave, and preparations around your estimated due date, then work out the baby's exact age from the birth date with an age calculator
Sharing a quick gestational age and trimester estimate with a midwife or partner before a formal appointment
Calculating the number of days or weeks until the due date alongside a date calculator

Technical specification

Algorithm or formula: Naegele's rule. From last menstrual period: due date = LMP + 280 days. From conception date: due date = conception + 266 days. Gestational age = days since LMP, trimester boundaries at 13 and 27 weeks.
Browser API or library: Native JavaScript Date. No external library.
Input limits: Any date the Date object accepts. Negative gestational ages and dates more than 42 weeks past LMP fall outside typical clinical use.
Output: Estimated due date, gestational age (weeks and days), trimester, and days or weeks remaining.
Known limitations: Naegele's rule assumes a 28-day cycle with ovulation on day 14. Cycle-length adjustment is not exposed. Ultrasound dating is more accurate after the first trimester.

Frequently asked questions

What is Naegele's rule?: Naegele's rule is the standard method for estimating a pregnancy due date. It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The rule assumes a 28-day cycle with ovulation on day 14.
Should I use last menstrual period or conception date?: Use last menstrual period if you know that date, as it is the standard method used by healthcare providers. Use conception date only if you know it precisely, for example from IVF or closely tracked ovulation.
What does gestational age mean?: Gestational age is counted from the first day of the last menstrual period, not from conception. At 4 weeks gestational age, actual conception typically occurred around 2 weeks earlier. This is the standard measure used in clinical settings.
How accurate is this due date estimate?: The result is an estimate based on Naegele's rule and the dates you enter. Only around 5% of babies are born on their exact due date. Confirm your dates with a healthcare provider, who may use ultrasound measurements for a more precise estimate.

Reviewed and tested May 25, 2026.