How to use this tool
Enter the first day of the last menstrual period, usual cycle length and the as-of date.
Read the calendar estimate alongside its dating limitations. Use the due date established by your maternity team.
Due date = LMP + 280 days + (cycle−28); gestational age = 280 − days until due date
Worked example
12 weeks 0 days · estimated gestational age
Fixed example using the settings below. Monetary examples use USD; dates and times use their stated reference values. This example stays unchanged when you edit your calculation.
Example inputs
- First day of last menstrual period: 2026-08-01
- Usual cycle length (days): 28
- As-of date: 2026-10-24
Method illustration: For LMP 2026-08-01 and a 28-day cycle, the estimated due date is 2027-05-08.
Assumptions and limits
- Calendar estimate only. Assumes a regular cycle with ovulation approximately 14 days before the next period. Menstrual dating is uncertain with irregular cycles.
- Ultrasound and clinical dating can change the estimated due date. Use the date established by your maternity team; a due date is not an exact birth date.
Method reference: NHS due date information
Check or report a result
Check the units, selected calculation and assumptions first. Compare the fixed example above with your inputs to identify what changed.
Report a calculation issue with the page address, hypothetical inputs, result displayed and result expected. Do not include passwords, bank details or other private information.