What next?
A due date anchors a lot of dates. These work out the rest.
How to use this calculator
- Enter the first day of your last period.
- The calculator adds 280 days (Naegele’s rule).
- Read your estimated due date and how far along you are.
What your result means
The date assumes a regular 28-day cycle, and only about 1 in 20 babies actually arrive on it. A dating scan is more accurate. Treat it as a helpful window rather than a fixed appointment.
Why this one is different
Your own cycle length is an input, not assumed to be 28 days, which moves the estimate by days for many people. Naegele's rule is named and shown rather than hidden behind a date picker, and the page is clear that a due date is a midpoint, not an appointment. Nothing is sent anywhere.
You're "pregnant" two weeks before you conceive
It's one of pregnancy's odd quirks: doctors count from the first day of your last menstrual period, not conception — which usually happens about two weeks later. So at "4 weeks pregnant" you actually conceived around two weeks ago.
The classic estimate, Naegele's rule, adds 280 days (40 weeks) to that first day. This tool also adjusts for your cycle length, since a longer or shorter cycle shifts ovulation. Only about 1 in 20 babies actually arrive on the exact date — treat it as a target window.
How it works
Due dates are counted from the first day of your last menstrual period, not from conception — which is why you are counted as two weeks pregnant before an egg is fertilised. That convention exists because the LMP date is one almost everyone can recall, while the moment of conception almost nobody can.
The estimate adds 280 days (40 weeks) to that date. If your cycle is not 28 days, ovulation lands earlier or later than day 14, so the calculator shifts the whole estimate by the difference: a 32-day cycle pushes the date four days later, a 25-day cycle pulls it three days earlier. Conception is estimated at day 14 of the cycle, adjusted the same way.
How far along you are is simply today's date minus your LMP, expressed in completed weeks and days. First trimester runs to the end of week 12, second to the end of week 26, third from week 27 on.
Worked example
Last period started 5 January 2026, average cycle 32 days:
Due date = 5 Jan + 280 + 4 = Fri 16 October 2026
Conception ≈ 5 Jan + 14 + 4 = Fri 23 January 2026
Only about 4% of babies arrive on the estimated date itself. Roughly 8 in 10 arrive within the two weeks either side, which is why midwives talk about a due window rather than a due day.
Frequently asked questions
How accurate is this estimate?＋
It's a well-established estimate, but only a dating ultrasound can confirm your due date. Just 4–5% of babies are born on their exact predicted day.
Is my data stored?＋
No. The calculation runs in your browser and nothing you enter is sent or saved anywhere.
Which dating method does this use?＋
Naegele's rule: 280 days from the first day of your last menstrual period, assuming a 28-day cycle with ovulation around day 14.
What if my cycle is not 28 days?＋
Adjust the result by the difference. A regular 32-day cycle typically means ovulation is about four days later, so the estimate shifts roughly four days back.
How does a dating scan compare?＋
An early ultrasound, usually between 8 and 14 weeks, measures the fetus directly and is more accurate than a date-based estimate. If a scan date differs from this one, use the scan.
Do many babies arrive on the due date?＋
Very few. The due date is the midpoint of a range, and most births happen in the weeks either side of it.
Related calculators
Assumptions & limitations
Every figure here comes from a simplified model. Keep these limits in mind when reading your result:
- Based on Naegele’s rule — 280 days from your last period, assuming a 28-day cycle.
- Only about 4% of babies arrive on the exact estimated date.
- An early dating scan is more accurate, especially with irregular cycles.