Due Date Calculator

Naegele's rule is one addition

The classical obstetric convention takes the first day of the last menstrual period — the LMP — and adds 280 days. That is the entire calculation, and 280 days is exactly 40 seven-day weeks — the week count and the day count are one measurement expressed two ways. Worked through with the site's engine, an LMP of January 1, 2026 gives an estimated due date of October 8, 2026. Nothing else was supplied, because with the default cycle nothing else is used.

The rule produces one fixed reference date, and every other figure on this page is measured from a second date the engine derives alongside it: the dating start, day zero of gestational-age counting. With the default 28-day cycle the dating start is the LMP itself — January 1, 2026 in the example above. Enter a different cycle length and the two separate, which is the whole mechanism of the adjustment described further down.

The companion guide — how pregnancy due dates are calculated — is the longer read: the same arithmetic taken apart at more length, with the assumptions and the limits given the room they deserve.

Why conception mode adds 266 days instead

The second mode accepts a known conception date and adds 266 days rather than 280. The gap between those two figures is exactly 14 days, and it is not a rival estimate. It is the same estimate measured from a starting line 14 days further along.

The reason is a convention about counting. Gestational age is counted from the LMP, not from conception. Because the 28-day model places ovulation near cycle day 14, a conception date implies an LMP 14 days earlier, and the engine sets day zero there rather than on the conception date itself. So 280 − 14 = 266. The two modes agree whenever the inputs agree: a conception date of January 15, 2026, which is 14 days after the LMP used above, returns October 8, 2026 — the same due date, from the same day zero.

That is also the clearest way to see why a gestational age runs roughly two weeks ahead of the age of the embryo itself. The count starts before conception is assumed to have happened, so the number of weeks reported is larger than the elapsed time since conception by that same convention. The 280-versus-266 difference is that statement written as arithmetic: a labeling convention, consistently applied, rather than a claim that anything begins at the LMP.

The 28-day assumption, and what the cycle-length field does

Naegele's rule carries one hidden input: it assumes a 28-day cycle with ovulation near day 14. The optional cycle-length field carries that assumption forward rather than replacing it — the model places ovulation one luteal phase, 14 days, before the next expected period — so entering a longer cycle moves day zero and every date derived from it later by (cycle length − 28) days, and a shorter one moves them earlier by the same amount. That single offset is applied to both the dating start and the due date.

Same LMP of January 1, 2026, three cycle lengths, three due dates from the same engine:

• 28-day cycle (the default): October 8, 2026
• 32-day cycle: October 12, 2026 — 4 days later
• 24-day cycle: October 4, 2026 — 4 days earlier

The shift is linear and symmetric because the adjustment is a single addition, and that is worth noticing for its own sake: the field does not make the estimate cleverer, it slides the whole estimate sideways. Everything downstream moves with it by the same number of days — the dating start, both trimester boundaries, and the gestational age reported on any given date.

Two things follow that the field cannot do. It cannot help with cycles that vary, because one field takes one length and applies one offset; a cycle that runs short one month and long the next has no single length to enter, and an average of several is not a fact about the cycle in question. And it cannot repair an uncertain LMP: every date downstream is measured from that first date, so an LMP recalled as somewhere in the middle of a month carries its uncertainty into the due date, one day for one day. The field accepts whole numbers from 20 to 45 days and returns an explanation rather than a number outside that range, because past it the offset stops describing anything the rule can represent.

Gestational age and the trimester boundaries as implemented

Once day zero is fixed, gestational age is simply the days elapsed since it, reported as whole weeks plus the leftover days. On April 15, 2026, the default-cycle example above reads 14 weeks 6 days and falls in trimester 2. The 32-day version of that same LMP reads 14 weeks 2 days on the identical calendar day. Neither reading is wrong. They are one date measured from two day zeros 4 days apart, and the gap between them never closes.

The trimester boundaries are drawn from that same day zero. This site implements them at 13w0d for the second trimester and 27w0d for the third — 91 and 189 days respectively. For the default example they fall on April 2, 2026 and July 9, 2026. Other week-based schemes are in circulation, so this page labels the convention it implements rather than presenting it as a boundary in nature; a different trimester shown elsewhere is usually a different convention, not a broken calculation.

Two edge cases are handled by refusing rather than inventing. A reference date earlier than day zero produces no gestational age at all, because there is no elapsed time to report. And past 44 weeks the engine stops reporting a running age — that constant is the reportable ceiling the implementation carries, beyond which it returns nothing instead of an implausible figure. Both behaviours are properties of the code, not statements about pregnancy.

A due date is an estimate

It is an estimate at every point, and clinicians routinely revise dating, commonly after an ultrasound measurement. The two routes to a date start from different information: this page works from a remembered date and an assumed cycle length, while a clinical assessment starts from something else entirely. Two such dates disagreeing is the expected case rather than a sign that either is broken, and the date your care team is working from supersedes anything computed here.

What a page like this one can offer is transparency about the convention it applies: 280 days from the LMP, 266 from a conception date, one offset for cycle length, and two labeled trimester boundaries — all of it visible rather than hidden behind a result. It is not prenatal care, it cannot confirm a pregnancy or say anything about how one is progressing, and it does not replace a conversation with a qualified healthcare provider.

The site's other calculators during pregnancy

BMI is documented as not meaningful during pregnancy, and the adult categories are not intended to be read then — the guide on what BMI does and does not measure sets that limit out in full. The remaining calculators here estimate body composition and energy needs from formulas fitted to general adult populations, and each is documented with its own limitations rather than adapted to pregnancy. The closest relative of this page is the ovulation calculator, which applies the same 14-day convention to a different question. Every figure printed above was produced at build time by the same tested, time-zone-independent engine the calculator runs, from the fixed dates quoted in the text, so the copy and the code cannot drift apart. Nothing you enter into the form is transmitted or stored.

Frequently asked questions

Not medical advice: this calculator provides general educational estimates from a published dating convention. A due date is an estimate; clinicians routinely revise dating, commonly after an ultrasound measurement. This page cannot account for your individual health context, must not be used to diagnose or treat any condition, and does not replace prenatal care — consult a qualified healthcare professional. Dates you enter are processed locally in your browser and never transmitted. See the methodology page.