FitnessCalculator

Due Date Calculator

An estimated due date from your last menstrual period (Naegele's rule, 280 days, with an optional cycle-length adjustment) or from a known conception date (266 days) — plus gestational age today and the trimester boundaries. This is calendar arithmetic, not prenatal care: ultrasound dating by your clinician supersedes it. Note that the other tools on this site (BMI, calories) use formulas for non-pregnant adults and do not apply during pregnancy. Related: the ovulation calculator.

Estimate the due date

Example: LMP January 1, 2026 → estimated due date October 8, 2026.

Count from

Enter a date to see the estimate.

A nineteenth-century rule, applied transparently

Naegele's rule dates to the 1800s and survives because it is simple and roughly right: LMP + 280 days. From an LMP of January 1, 2026 the estimate lands on October 8, 2026, and on April 15, 2026 the pregnancy would be at 14 weeks 6 days (trimester 2) — both computed by the same tested, time-zone-independent engine as the calculator. The rule bakes in a 28-day cycle with ovulation near day 14; the cycle-length field adjusts the calendar by (cycle − 28) days, a refinement that helps with regular-but-longer cycles and does nothing for irregular ones.

What outranks this calculator

Everything clinical. First-trimester ultrasound dating is measurably more accurate than any menstrual-date arithmetic, and care teams re-date pregnancies on that basis routinely. Use this page to orient yourself before an appointment or to sanity-check a date you were given — never to contradict one. Pregnancy also changes nutritional and exercise needs in ways the general-purpose calculators on this site do not model, so treat their outputs as not applicable while pregnant.

Frequently asked questions

What is Naegele's rule?

The classical obstetric convention: estimated due date = first day of the last menstrual period plus 280 days (40 weeks). It assumes a 28-day cycle with ovulation around day 14; the optional cycle-length input shifts the estimate by (your cycle length − 28) days to soften that assumption.

How does the conception-date mode differ?

It adds 266 days (38 weeks) to the conception date instead — the same arithmetic minus the assumed two weeks between the LMP and ovulation. Gestational age is still counted from the LMP convention, which is why the calculator dates it from 14 days before conception.

How likely is birth on the estimated date?

Only around 4–5% of babies arrive on the due date itself; most are born within a window of roughly two weeks either side. The date is a planning anchor for care schedules, not a prediction of the birthday.

Why does my clinic give a different due date?

First-trimester ultrasound measures the embryo directly and is more accurate than any calendar rule, so clinicians re-date pregnancies when ultrasound and LMP disagree beyond set margins. Whenever you have an ultrasound-based date, that one wins — full stop.

Where do the trimester boundaries come from?

This page uses a common week-based convention: second trimester from 13w0d and third from 27w0d. Other schemes exist (for example 14w0d and 28w0d), which is why the boundaries are labeled with the convention rather than presented as facts of biology.

Not medical advice: this is a calendar estimate from a historical convention. It is not prenatal care, cannot confirm a pregnancy or its health, and is superseded by ultrasound dating. Discuss all pregnancy dating and care decisions with a qualified healthcare provider. Data is processed locally in your browser and never transmitted. See the methodology page.