Pregnancy Due Date Calculator
Estimate due date from LMP and cycle length.
How a due date is estimated
A due date is calculated as 280 days — 40 weeks — from the first day of your last menstrual period, not from conception. That convention exists because the LMP date is something you can usually pin down, while the day of conception rarely is. This calculator applies Naegele's rule and adjusts for cycle lengths other than 28 days, then shows how far along you are today.
How to use the Pregnancy Due Date Calculator
- Enter your LMP Date.
- Enter your Cycle Length (days) so the estimate can be adjusted.
- Read the estimated due date.
- Check the current gestational age in weeks.
Formula and a worked example
Due date = LMP + 280 days + (cycle length − 28)
Naegele's rule adds 280 days to the LMP. A longer cycle means later ovulation, so the due date shifts later by the same number of days. Only about 4% of babies arrive on the exact date — anything from 37 to 42 weeks is full term.
Worked example
An LMP of 1 January with a 30-day cycle gives a due date of about 10 October — two days later than the standard 8 October a 28-day cycle would produce.
Frequently asked questions
How many weeks pregnant am I?
Count from the first day of your last period, not from conception. That is why you are considered two weeks pregnant at the moment of conception.
How accurate is an LMP due date?
Reasonably, if your cycles are regular. A first-trimester ultrasound is more accurate and clinicians will revise the date if the two disagree by more than about a week.
What if I do not know my LMP?
An early ultrasound can date the pregnancy from foetal measurements. Ask your midwife or doctor.
Is the due date a deadline?
No. It is the midpoint of a normal range. Roughly half of babies arrive after it, and 37–42 weeks is all considered term.
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