This due date calculator estimates your delivery date and current gestational age from the first day of your last menstrual period.
Health
Due Date Calculator
Estimate due date and gestational age from LMP with a 280-day pregnancy.
Calculator
About this calculator
Formula notes
Naegele's rule: due date = LMP + 280 days (40 weeks); gestational age = today − LMP in weeks.
Worked examples
- Example: LMP 2026-01-01 gives a due date of 2026-10-08; about 29 weeks along and 75 days remaining as of mid-July.
- With a last menstrual period of 2026-01-01 and a standard 28-day cycle, the estimated due date is around 2026-10-08 before ultrasound dating.
How to use it
- Enter the first day of your last menstrual period.
- Read the estimated due date.
- Check current gestational weeks and days remaining.
Frequently asked questions
How accurate is the 280-day rule?
Only about 4–5% of babies arrive on the exact due date; most arrive within two weeks either side.
What if my cycle is longer than 28 days?
The standard rule assumes a 28-day cycle; longer cycles shift the estimate — an early ultrasound dates pregnancy more precisely.
Limits and interpretation
- Last-menstrual-period dating assumes a known date and cycle pattern; irregular cycles, recent contraception, or uncertain bleeding reduce reliability.
- An estimated due date is not a prediction of the birth date and clinical dating may be revised using early ultrasound and medical history.