Pregnancy Due Date Calculator
Estimate a due date from the last menstrual period or conception date, with current gestational age.
Formula
Naegele's rule: LMP + 280 days, adjusted for cycle length
Tips
- Only about 4% of babies arrive on the estimated due date. Around 80% arrive within the two weeks either side of it, which is why the full-term window matters more than the single date.
- Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Adjusting the cycle length above corrects for that, but ovulation timing varies between cycles even for regular ones.
- A dating ultrasound in the first trimester is more accurate than any calculation from dates, and is what your maternity team will use.
- Gestational age is counted from the first day of the last period, not from conception — which is why you are considered "two weeks pregnant" at conception.
Frequently asked questions
How accurate is this?
It is arithmetic on the dates you supply, and inherits any uncertainty in them. First-trimester ultrasound dating is accurate to within about five days and supersedes calculated dates. Use this for orientation, not for decisions — your midwife or doctor will confirm dating.
My cycle is irregular. Does this still work?
Less well. Naegele’s rule depends on predictable ovulation. With irregular cycles the estimate can be out by a week or more, and early ultrasound dating becomes considerably more important.