Pregnancy Due Date Calculator
Estimate your baby's due date from the first day of your last menstrual period (LMP). The calculator also shows how many weeks pregnant you are today, your current trimester, and key dates along the way.
How to use this calculator
- Enter the first day of your last menstrual period — not the day it ended.
- Adjust your average cycle length if it's not 28 days (this shifts the estimate).
- Press Calculate to see your due date, current week, trimester and key milestones.
- Confirm the date with your doctor — an early ultrasound gives the most accurate dating.
Formula used
This is Naegele's rule, the standard method used by clinicians: pregnancy is counted as 40 weeks (280 days) from the first day of the last period. Longer or shorter cycles shift ovulation, so the formula adjusts by the difference from a 28-day cycle.
Example calculation
LMP on 1 January with a 28-day cycle: due date = 1 January + 280 days = 8 October.
With a 32-day cycle, ovulation happens ~4 days later, so the due date shifts to 12 October.
How is a due date calculated?
Pregnancy is dated from the first day of your last period — about two weeks before conception actually occurs — because that date is usually known precisely while conception rarely is. A full-term pregnancy is counted as 40 weeks from LMP, which is where the 280-day rule comes from.
The due date is a midpoint, not a deadline: only about 1 in 20 babies arrives on it, and anything from 37 to 42 weeks is considered normal timing. Doctors often refine the estimate with a first-trimester ultrasound, which measures the embryo directly and is accurate to within about 5 days.
Why use this calculator?
- Get an instant estimate of your due date and see exactly how far along you are today.
- Know when each trimester begins for planning scans, tests and announcements.
- Adjust for your real cycle length — a detail many simple calculators ignore.
Frequently asked questions
How accurate is a due date calculator?
LMP-based dating is accurate to within about a week for regular cycles. Only ~4–5% of babies arrive on the exact date, but around 90% arrive within two weeks either side. A first-trimester ultrasound is the most accurate dating method and may shift the estimate.
How many weeks pregnant am I?
Count from the first day of your last period, not from conception. If your LMP was 10 weeks ago, you are 10 weeks pregnant — even though conception occurred about 8 weeks ago. The calculator shows today's exact week and day.
What if my cycles are irregular?
LMP dating becomes unreliable with irregular cycles because ovulation timing varies. Enter your best average, but rely on an early ultrasound for dating — mention the irregularity to your doctor.
Can the due date change during pregnancy?
Yes. If an early ultrasound measurement differs from your LMP-based date by more than about 5–7 days, clinicians usually re-date the pregnancy to match the scan, which then becomes the official due date.