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Pregnancy Due Date Calculator

Estimate your due date and gestational age from your last period.

Most cycles run 20 to 45 days. The default is 28.

Your estimate

Enter the first day of your last period to see your estimated due date.

This is an estimate for general information only, not medical advice. Due dates vary, and only a healthcare provider can confirm your dates and monitor your pregnancy.

Worked examples

LMP 30 March 2025, 28-day cycle
Estimated due date is Sunday, January 4, 2026 (280 days after the last period), with estimated conception around April 13, 2025.
LMP 1 June 2025, 32-day cycle
A longer cycle adds 4 days, so the estimated due date moves to Tuesday, March 11, 2026 instead of March 7.

How your due date is estimated

Pregnancy is measured from the first day of your last menstrual period (LMP), not from the day you conceived. A full-term pregnancy is counted as 40 weeks, which is 280 days, from that LMP date. This calculator starts there and then makes one adjustment for your cycle length:

  • Estimated due date = LMP + 280 days + (cycle length − 28 days)
  • Estimated conception = LMP + (cycle length − 14 days)
  • Gestational age = whole weeks and days from LMP to today

So a last period starting on 30 March 2025 with a standard 28-day cycle gives a due date of 4 January 2026, exactly 280 days later.

Why cycle length matters

The classic formula behind due dates is Naegele’s rule: add a year to the LMP, subtract three months, and add seven days. That comes out to about 280 days, and it quietly assumes a 28-day cycle with ovulation around day 14. Real cycles vary. If yours runs longer than 28 days, you probably ovulate later, so the due date shifts later by the difference. A 32-day cycle adds roughly four days; a 24-day cycle takes about four days off. This tool makes that adjustment for you.

The three trimesters

Pregnancy is commonly split into three trimesters, measured in gestational weeks:

  • First trimester: weeks 0 to 13.
  • Second trimester: weeks 14 to 27.
  • Third trimester: weeks 28 and beyond, up to birth.

The calculator shows your current gestational age (for example, “12 weeks, 3 days”) and which trimester that falls in, based on today’s date compared with your LMP.

Accuracy and a note on medical advice

Estimated due dates are exactly that: estimates. Only about one baby in twenty arrives on the predicted day, and most healthy births happen in the weeks around it. LMP dating assumes you recall the date correctly and ovulate on a regular schedule, so irregular cycles or an uncertain last period can shift the true date. An early ultrasound is generally the most reliable way to confirm dating.

Please use this calculator for general information and planning only. It is not a diagnosis and it is not a substitute for professional care. For anything that affects your health or your pregnancy, talk with a qualified healthcare provider who can review your full history and confirm your dates.

Frequently asked questions

How is my due date calculated?
This calculator uses the last menstrual period (LMP) method. It adds 280 days (40 weeks) to the first day of your last period, then adjusts for your cycle length. For a standard 28-day cycle, the estimate is LMP plus 280 days. The result is an estimate, and your healthcare provider may refine it with an ultrasound.
What is Naegele's rule?
Naegele's rule is the classic formula for estimating a due date: take the first day of the last period, add one year, subtract three months, and add seven days. That works out to about 280 days. This tool applies the same 280-day rule and then adjusts for cycles that are longer or shorter than 28 days.
How does cycle length change my due date?
Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer, you likely ovulate later, so the due date shifts later by the same number of days (cycle length minus 28). A 32-day cycle pushes the estimate about 4 days later, while a 24-day cycle pulls it about 4 days earlier. It is still an estimate.
How accurate is dating from the last period?
LMP dating is a reasonable starting point but it assumes you remember the date accurately and that you ovulate on a predictable schedule. Irregular cycles, late or early ovulation, and uncertain LMP dates all reduce accuracy. Only about 1 in 20 babies arrives on the exact estimated date. An early ultrasound is usually the most accurate way to date a pregnancy, so treat this figure as a guide and confirm it with your provider.
What does gestational age mean?
Gestational age is how far along the pregnancy is, measured in weeks and days from the first day of your last period, not from conception. Because it counts from LMP, gestational age is roughly two weeks ahead of the actual age of the embryo. Providers use gestational age to track milestones and schedule appointments.
What is the difference between the conception date and the LMP?
The last menstrual period (LMP) is the first day of your last period, which happens before you conceive. Conception usually occurs around ovulation, roughly two weeks later in a 28-day cycle. That is why this calculator estimates conception as the LMP plus your cycle length minus 14 days. The conception date is an estimate too, since exact ovulation timing varies.

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Last updated: 2026-07-01