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Due Date Calculator

An estimated due date is a landmark, not a deadline: 40 weeks from the first day of the last menstrual period (LMP), when conception happened about two weeks later. Only one baby in twenty arrives on it, and any birth from 37 to 42 weeks is full term. This page estimates the date from your LMP and cycle length, shows how far along you are today, and counts the days remaining — a guide, not a medical prediction.

Enter values

Day of the first day of your last period (1–31)

Month of your last period (1–12)

Year of your last period (e.g. 2026)

Days between the first day of one period and the next (21–45)

Results update as you type.

Results

Estimated due date
8 June 2027
Gestational age today
7 weeks 4 days
Days remaining
227
Calculated as of
24 October 2026

How is the Due Date calculated?

Enter the day, month and year of the first day of your last period, plus your typical cycle length in days — 28 is the textbook average, and anywhere from 21 to 45 is normal. The calculator applies Naegele's rule with a cycle adjustment: add 280 days to the LMP, plus one extra day for every day your cycle runs over 28. It returns the estimated due date, your gestational age in weeks and days as of today, and the days remaining until (or since) that date.

Formula

Due date = LMP + 280 days + (cycle length − 28) days

where:
LMP = first day of the last menstrual period
cycle length = days from one period's first day to the next's (21–45)

280 days is 40 weeks of gestation. The cycle adjustment shifts the date one day per day because ovulation happens about 14 days before the next period, not always on day 14.

Example

Last period: 1 January 2026 · Cycle: 28 days
Due date = 1 Jan + 280 days = 8 October 2026
On 15 February 2026 that is 45 days of gestation — 6 weeks 3 days — with 235 days remaining. With a 30-day cycle the same LMP gives 10 October 2026 instead: two extra days of cycle, two extra days of waiting. The dating scan your clinic performs will usually land within a week or two of this figure.

Things to remember

Treat the due date as a window: the 39th to 40th weeks are full term and induction usually comes by 42. Irregular cycles make LMP arithmetic drift, so a dating scan anchors it better. Take it to your appointment, not your calendar — and contact your provider promptly with any bleeding, severe pain or reduced movement.

Frequently Asked Questions

How accurate is a due date calculated from the last period?

For regular 28-day cycles it lands within about a week, but spontaneous births spread across at least five weeks around that date and very few babies arrive exactly on it. First-trimester ultrasound dating is more precise, typically within 3–5 days, which is why scans often confirm or adjust the LMP estimate.

What if my cycle is not 28 days long?

Add one day to the due date for every day your cycle runs over 28 — a 32-day cycle shifts it about four days later, and this calculator makes that adjustment for you. Very irregular cycles, varying by more than a week or two, make LMP dating unreliable; a dating scan becomes the better anchor.

Can the due date change during pregnancy?

Yes. A first-trimester ultrasound can move the date by a week or more from the LMP estimate, while later scans are less reliable for dating and usually leave it alone. Once set, the date tends to stay fixed because every screening window and growth check that follows is measured against it.

Is 40 weeks counted from conception or the last period?

From the first day of the last menstrual period. Ovulation typically happens around day 14 of a 28-day cycle, so a baby conceived on day 14 is already counted as two weeks old — obstetricians have dated pregnancies this way for over a century, which is why '40 weeks pregnant' means about 38 weeks since conception.

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