Due date calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Due date = first day of the last period + 280 days (40 weeks), plus or minus the difference from a 28-day cycle.
- Only about 1 in 20 babies arrives on the estimated date. An ultrasound or your healthcare provider can give a more accurate date.
| Inputs | Answer |
|---|---|
| First day of your last period: 2026-03-01; Average cycle length: 28 | Your estimated due date is Sunday, December 6, 2026 (Naegele's rule: 280 days after the last period, adjusted for cycle length). |
| First day of your last period: 2026-06-15; Average cycle length: 32 | Your estimated due date is Friday, March 26, 2027 (Naegele's rule: 280 days after the last period, adjusted for cycle length). |
| First day of your last period: 2026-01-20; Average cycle length: 26 | Your estimated due date is Sunday, October 25, 2026 (Naegele's rule: 280 days after the last period, adjusted for cycle length). |
How to use it
Naegele's rule estimates the due date as 280 days (40 weeks) after the first day of the last menstrual period, assuming a 28-day cycle; the calculator shifts it if your cycle is longer or shorter. Conception is estimated about 14 days after the period started.
Only a small share of babies arrive on the estimated date. A dating ultrasound or your healthcare provider can give a more accurate date. This is general information and an estimate, not medical advice; talk to a healthcare professional about your own situation.
Frequently asked questions
How is the due date calculated?
Add 280 days to the first day of your last period.
What if my cycle is not 28 days?
The calculator adjusts by the difference from 28 days.
How accurate is a due date?
It is an estimate; most births happen in the weeks around it.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
About · How we verify this · Editorial policy · Corrections
General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.