CalcKits

All Calculators

Pregnancy Due Date Calculator

By Sofia Andersen · Last updated 15 October 2026

The first question after a positive test is usually: when? This calculator gives you a date to plan around. It is a planning date, not a promise. Only a small share of babies arrive on that exact day, and most arrive within two weeks either side of it.

Enter your numbers

Result

General information only, not medical advice. Confirm your due date with your midwife or doctor, who may adjust it after an ultrasound. If you have pain, bleeding, reduced movement, or any urgent symptoms, get urgent care right away.

Which date do you actually know?

Most people start from the first day of their last menstrual period, often shortened to LMP. Enter that date and the calculator adds 280 days, which is 40 weeks. The calculator offers three starting points: last period, conception date, and ultrasound-adjusted date. The ultrasound option lets you enter a date your care team has already given you, so the trimester milestones line up with the date they are using. All dates show in day-month-year order.

If your cycles are close to 28 days, the standard estimate applies as it stands. If your cycles are regularly longer or shorter, the calculator shifts the date: a 35-day cycle moves the estimate about a week later, a 21-day cycle about a week earlier. If you know your conception date instead, add 266 days, or 38 weeks, to that date.

Why 280 days?

Doctors have used Naegele’s rule for a long time. In its classic form, you take the first day of your last period, add seven days, go back three months, and add a year. It lands on the same week as adding 280 days, and for many dates it lands on the same day. The calculator uses the 280-day count directly, because it is the cleaner of the two.

The rule assumes a regular 28-day cycle with ovulation around day 14. Real cycles vary, which is one reason the date is an estimate. An early ultrasound usually gives a firmer date, and your midwife or doctor will use that one if it differs by more than a few days.

One date, worked through. Say the first day of your last period was 1 January 2026. Add 280 days. January has 31 days, so 30 days remain after the 1st. Counting forward through the year, 280 days from 1 January 2026 lands on 8 October 2026. Naegele’s shortcut agrees here: 1 January plus 7 days is 8 January, back three months is 8 October 2025, plus a year is 8 October 2026. Conception, for a 28-day cycle, sits about two weeks after the period starts, so roughly 15 January 2026. The trimesters then run like this: the first trimester ends around 8 April 2026 (13 weeks and 6 days), the second ends around 15 July 2026 (27 weeks and 6 days), and the third runs from 28 weeks to birth.

The part the calculator can’t do

Think of the due date as the middle of a window, not an appointment. A first baby often arrives a little late. A second or third may come a little earlier. Your midwife or doctor will track growth, check dates against the ultrasound, and talk through what happens if the pregnancy runs past 41 or 42 weeks.

The calculator cannot tell you anything about how the pregnancy is going. That is what antenatal appointments are for: blood pressure, growth, blood tests, and a chance to ask questions. If anything feels off between visits, pain, bleeding, fluid leaking, a headache that will not lift, swelling that comes on fast, or less movement than usual, do not wait for the next appointment. Get checked the same day.

Frequently asked questions

How accurate is a due date from my last period?

It is a good estimate when cycles are regular and the first day of the last period is certain. An early ultrasound often refines it, and your care team will use the scan date if it differs by more than a few days.

What if my cycles are irregular?

Then a period-based date is shakier, because ovulation may not have fallen on day 14. An early ultrasound is the most reliable way to date the pregnancy, so bring your cycle history to your first appointment.

What if I know the conception date?

Add 266 days, or 38 weeks, to the conception date. That is the same endpoint as 280 days from the last period for a textbook 28-day cycle, just counted from the other end.

Does an ultrasound change the due date?

Sometimes, yes. If the scan suggests a date more than a few days away from the period-based estimate, your care team usually switches to the scan date, because early scans measure the baby directly rather than relying on cycle assumptions.

Related calculators

Dates here are estimates and can change after an ultrasound. This page is general information, not medical advice, a diagnosis, or treatment, and it cannot replace a qualified health professional who knows your situation. If you have pain, bleeding, reduced movement, or any urgent symptoms, get urgent care right away.