Due Date Calculator by Last Period & Cycle Length

How the last-period method works

A pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. The standard estimate of the due date is 280 days, or 40 weeks, after that day.^1,2 This is often taught as Naegele’s rule: add one year, subtract three months, add seven days. We count the actual days instead, which avoids small errors around months of different lengths and leap years.³

Worked example: 28-day cycle

March 2, 2026First day of last period
+280 days40 weeks
=Monday, December 7, 2026Estimated due date

Adjusting for your cycle length

The 280-day rule assumes ovulation around day 14 of a 28-day cycle. If your cycles are regularly longer or shorter, the usual adjustment assumes ovulation shifted by the same amount, and moves your due date by the difference between your cycle and 28 days.^3,4

Worked example: 32-day cycle

March 2, 2026First day of last period
+280 days40 weeks
+4 days32-day cycle is 4 days longer than 28
=Friday, December 11, 2026Estimated due date

The adjusted date is a calculation anchor. Behind the scenes we also work out an “equivalent” period date, 280 days before your due date, so every week and milestone lines up. It is not your actual period date.

Irregular or unknown cycles

Normal cycles in adults run from about 24 to 38 days.⁵ If yours vary a lot from month to month, there is no single number that tells you when you ovulated, so we don’t invent an adjustment. Choose “Not sure or irregular” and the calculator assumes 28 days.

If your cycles are irregular or you are unsure of the date, speak to your midwife or doctor. An early ultrasound is the more reliable way to date your pregnancy.⁴¹

When a scan changes the date

Most people have an ultrasound in the first or early second trimester. If the scan’s estimate differs from your period-based date by more than set amounts, which depend on how far along you are, your care team will usually use the scan date. Up to 13 weeks 6 days, a crown-rump length measurement is the most accurate way to date a pregnancy.¹ You can compare both dates with our ultrasound due date calculator, and read why scan and period dates differ.

Questions people ask

Should I count from the first or last day of my period?

From the first day: the day your last period started. Then adjust the cycle length if your cycles are usually shorter or longer than 28 days.⁴

What if I don’t remember the date?

Use your best estimate and treat the result as rough, and speak to your midwife or doctor. A first-trimester ultrasound can estimate how far along you are more accurately than an uncertain period date.^1,4

Why does a longer cycle push my due date later?

The standard 280 days assumes ovulation around day 14 of a 28-day cycle. In a regularly longer cycle, ovulation is assumed to happen later by the same number of days, so the estimated due date moves later too.^3,4

Is the cycle-adjusted date more accurate?

Only if your cycles are regular and you know their length. It is still an estimate of when ovulation happened, not a measurement, which is why an early scan is used to confirm the date.^1,3

Calculate your due date from your last period