Due Date and Conception Calculator

How to use the due date and conception calculator

Enter the first day of the last menstrual period and an average cycle length, and the tool returns an estimated due date, an estimated conception date, the completed weeks and days of gestation as of today, and the four term windows resolved into calendar dates. The arithmetic underneath is Naegele's rule, published in the early nineteenth century and still the standard hand calculation: 280 days, or 40 weeks, from the first day of the last period.

From a last period beginning 1 January 2026 with a 28-day cycle, that lands on 8 October 2026, a Thursday, with conception estimated at 15 January 2026. Nothing on this page is a diagnosis and none of it replaces the estimate your midwife or doctor records.

The cycle-length field matters more than it looks, because Naegele assumes a 28-day cycle ovulating on day 14 and most people do not have one. The adjustment is one day per day: the tool adds the difference between your cycle and 28 to the 280.

A 35-day cycle therefore pushes the estimate seven days later, to 15 October 2026 on the same last period, and a 24-day cycle pulls it four days earlier, to 4 October 2026.

The conception estimate moves with it for a reason worth knowing: the luteal phase, the stretch between ovulation and the next period, holds steady at about 14 days, so a longer cycle means a longer wait before ovulation rather than a longer wait afterwards. Whatever the cycle length, the gap from estimated conception to due date stays fixed at 266 days.

A dating ultrasound beats any calculation from a remembered date, and the reason is simple: the scan measures the pregnancy while the arithmetic infers it. ACOG Committee Opinion 700, Methods for Estimating the Due Date, treats a first-trimester crown-rump length measurement as the most accurate method available and sets out when the scan should replace the period-based estimate.

Before 9 weeks a discrepancy of more than 5 days is enough to redate; from 9 to 16 weeks the tolerance is 7 days, and it widens as the pregnancy advances, because ultrasound dating itself becomes less precise later on.

Working backwards to the conception date

A due date and a conception date are one calculation read in opposite directions. The due date is the last period plus 280 days. Conception is that same last period plus however far into the cycle ovulation falls, which this page takes as the cycle length minus fourteen days.

That fourteen is the part worth understanding. The luteal phase — ovulation to the next period — is the stable half of a cycle, at roughly fourteen days. The first half is what varies between people. So a longer cycle pushes ovulation later while leaving the gap to the next period where it was.

One consequence surprises almost everybody. Gestational age is counted from the last period, roughly two weeks before conception could have happened. At six weeks pregnant the embryo is about four weeks old. The convention exists because a last period is a date somebody can name and a day of conception usually is not.

From one word to four term bands

The caveat that belongs on every result: the great majority of babies do not arrive on the estimated date. That is not a failure of the arithmetic, it is what an estimate of this kind is. The due date is the midpoint of a distribution roughly two weeks wide, and arriving a week either side of it is entirely ordinary.

This is also why ACOG stopped using the single word term in 2013. Committee Opinion 579, Definition of Term Pregnancy, replaced it with four bands, because outcomes differ measurably across the weeks that the old single label lumped together: early term is 37w0d to 38w6d, full term is 39w0d to 40w6d, late term is 41w0d to 41w6d, and post term is 42w0d onward.

The same example resolved into the four ACOG bands, still from a 1 January 2026 last period on a 28-day cycle.

Laid out as dates rather than week numbers, the shape of the answer is obvious: full term alone is 14 days wide, and early, full and late term together cover 35 days.

The formula, worked line by line

Naegele's rule is one addition. Take the first day of the last menstrual period and add 280 days, which is 40 weeks.

The rule is named for Franz Karl Naegele, the German obstetrician who popularised it in the early nineteenth century, and the hand version taught in textbooks phrases it differently to make it easier without a calendar: add one year, subtract three months, add seven days. Both forms give the same answer to within a day, and both assume a 28-day cycle with ovulation on day 14.

This tool adds one refinement to the textbook rule, which is to correct for a cycle that is not 28 days. The correction is one day per day, added to the 280. It works because the second half of the cycle, from ovulation to the next period, is the stable half: a longer cycle almost always means a longer first half, so ovulation and therefore conception happen later, and the due date moves later with them.

due date   = last period + 280 + (cycle length - 28)
conception = last period + (cycle length - 14)
conception to due date = 266 days, at any cycle length
gestation  = days since the last period, read as weeks + days
ACOG bands: 37w0d early | 39w0d full | 41w0d late | 42w0d post

Hold the number honestly. The overwhelming majority of babies do not arrive on the estimated date, and a birth a week either side of the estimate is unremarkable.

The arithmetic also rests on assumptions that may not hold: that the remembered date of the last period is accurate, that the cycle length entered is typical rather than the one cycle that stood out, and that ovulation followed the usual pattern. Irregular cycles, recent hormonal contraception, breastfeeding and bleeding early in pregnancy all weaken the count. That is why a first-trimester ultrasound is the standard: it measures the pregnancy directly rather than inferring it.

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