This due date calculator estimates when your baby will arrive, from the first day of your last menstrual period (LMP) or your conception date. It also shows how far along you are today and the key dates of each trimester.
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What Is a Due Date Calculator?
A due date calculator estimates the day a baby is most likely to be born. Pregnancy is counted from the first day of the last menstrual period — 40 weeks (280 days) for an average cycle — which is why “how far along” is always about two weeks ahead of the conception date. Only about 1 in 20 babies arrive exactly on the due date; the date is the center of a normal window, not an appointment. This tool is part of our Health Calculators collection.
How the Calculation Works (Naegele’s Rule)
- LMP method: due date = LMP + 280 days for a 28-day cycle (equivalently: LMP + 1 year − 3 months + 7 days).
- Longer/shorter cycles: add or subtract the difference — a 32-day cycle shifts the estimate 4 days later (verified example below).
- Conception method: due date = conception + 266 days (38 weeks), for IVF or known ovulation dates.
Worked example (verified): LMP of January 1, 2026 with a 28-day cycle → due date October 8, 2026; estimated conception January 15; first trimester ends March 26. If you are planning rather than expecting, our Ovulation Calculator finds the fertile window for conception, and once baby arrives the Age Calculator tracks their exact age.
How Accurate Is a Due Date?
LMP dating assumes a textbook 28-day cycle and day-14 ovulation, so treat the result as an estimate with a normal range of about ±2 weeks. A first-trimester ultrasound is the most accurate dating method and may move the date — clinicians use it when it differs from LMP dating by more than about a week. Babies born between 37 and 42 weeks are considered term; about half of first-time mothers deliver after their due date.
Related Tools
- Ovulation Calculator — fertile window and cycle dates
- Age Calculator — exact age after the big day
- Health Calculators — the full health collection
Frequently Asked Questions
How is a due date calculated from LMP?
Add 280 days (40 weeks) to the first day of your last period for a 28-day cycle, adjusting by the difference if your cycle is longer or shorter.
Can a due date change?
Yes — an early ultrasound sometimes revises it. After the first trimester, dates are rarely changed because babies’ growth rates diverge naturally.
What if I don’t know my LMP?
Use the conception-date mode if you know it (including IVF transfer dates adjusted by your clinic), or ask your provider for ultrasound dating.
Why is pregnancy counted from before conception?
Because LMP is an observable date and ovulation usually is not. The convention adds the ~2 pre-conception weeks, so “4 weeks pregnant” is roughly when a period is missed.
How many babies are born on their due date?
Only about 4–5%. Most arrive within two weeks either side, which is why the milestone table above shows the full-term window.
Is this due date calculator medical advice?
No — it is an estimate for planning. Your provider’s dates and guidance always take precedence.