How to Calculate Your Due Date (Free Pregnancy Tool)

One of the first questions after a positive test is when your baby is due. While only about 4 percent of babies arrive on their exact due date, a good estimate helps you plan appointments, leave and milestones. This guide explains how due dates are estimated with Naegele's rule and walks you through the Due Date Calculator, which gives you your estimated due date, conception date, current trimester and how many weeks pregnant you are. It is free and runs privately in your browser. It is not medical advice.

How a due date is estimated

Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception, because the LMP date is usually known while the exact conception date often is not. The standard method is Naegele's rule:

Due date = first day of last period + 280 days (40 weeks)

Equivalently, take the first day of your last period, add one year, subtract three months, and add seven days. This assumes a regular 28-day cycle with ovulation around day 14. Conception is estimated about two weeks after the LMP, which is why a pregnancy is counted as 40 weeks even though the baby develops for roughly 38.

How to use the Due Date Calculator

Getting your estimate takes seconds:

1. Open the Due Date Calculator.
2. Enter the first day of your last menstrual period.
3. If you know it differs from 28 days, you can factor in your usual cycle length.
4. Read your estimated due date, approximate conception date, current trimester and how many weeks and days pregnant you are.

The tool updates instantly, so you can see your progress at a glance. Pair it with the Age Calculator later to track your baby's age after birth.

A worked example

Suppose the first day of your last period was 1 January 2026.

• Add 280 days, or add a year, subtract three months and add seven days.
• Estimated due date: 8 October 2026.
• Estimated conception: around 15 January 2026 (about two weeks after the LMP).
• If today is 1 April 2026, you are about 13 weeks pregnant and entering the second trimester.

If your cycle is longer than 28 days, ovulation happens later, so the due date shifts a little later as well.

What the trimesters mean

Pregnancy is divided into three trimesters, each with its own milestones:

The first trimester covers early development, the second is often the most comfortable, and the third leads up to birth. Most pregnancies last between 37 and 42 weeks, so anywhere in that window is considered full term.

Why your estimate may change

An LMP-based due date is a starting point, not a guarantee. It can be adjusted by your healthcare provider because:

• Cycles vary. If yours is shorter or longer than 28 days, ovulation and conception shift.
• Early ultrasound is more accurate. A first-trimester dating scan measures the baby and can move the date by several days.
• The LMP date may be uncertain. Irregular periods make the first day harder to pin down.

Always confirm your due date with your doctor or midwife, who can use ultrasound for a more precise estimate.

Private, free and not medical advice

The Due Date Calculator runs entirely in your browser. The date you enter is never uploaded or stored, so your information stays private. There is no sign-up. This tool gives an estimate using a standard formula and is not a substitute for professional care. For anything about your health or your pregnancy, talk to a qualified provider. Find more handy tools in calculators or browse all tools.