Due Date Calculator
Estimate your due date from your last period — plus how far along you are and which trimester you're in.
Reviewed by the OmniCalc teamMethod verified 2026-07-01
—estimated due date
An estimate (Naegele’s rule) — only about 1 in 20 babies arrive on the exact date. Not medical advice.
How to use the due date calculator
- 1Enter the first day of your last period.
- 2Adjust the cycle length if yours isn’t 28 days.
- 3Read your estimated due date and how many weeks along you are.
An estimate, not a diagnosis
This is a rule-of-thumb date to plan around. For a confirmed due date — and any questions about your pregnancy — see a doctor or midwife; an early scan is the most accurate dating tool.
Frequently asked questions
How is the due date calculated?
By Naegele's rule: 280 days (40 weeks) from the first day of your last menstrual period, shifted by any difference from a 28-day cycle. It assumes conception around two weeks after that date.
How accurate is it?
It's an estimate. Only about 1 in 20 births happen on the exact predicted day; most arrive within a week either side. An early ultrasound gives a more precise date, especially if your cycle is irregular.
What if my cycle isn't 28 days?
Enter your average cycle length and the estimate shifts accordingly — a longer cycle usually means a slightly later due date, because ovulation happens later.