Pregnancy is counted from the first day of the last period, about two weeks before conception. Only around 4–5% of babies arrive on their exact due date.
How to use the Pregnancy Due Date Calculator
- First day of last period
- Average cycle length
Results update as you type, and every field accepts the units shown in its menu.
How it works
Add 280 days to the last period, plus or minus the difference between your cycle and 28 days.
Inputs explained
| Field | What to enter |
|---|---|
| First day of last period | A date |
| Average cycle length | In days |
What each result means
- Estimated due date = [First day of last period] + 280 + ([Average cycle length] – 28)
- Estimated conception = [First day of last period] + 14 + ([Average cycle length] – 28)
- Weeks pregnant today = (today() – [First day of last period] – ([Average cycle length] – 28)) ÷ 7
- Days until due date = [Estimated due date] – today()
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Due date = LMP + 280 days + (cycle length − 28)Gestational age = today − LMPWorked example
Inputs — First day of last period: 2026-03-01; Average cycle length: 28 days.
Results — Estimated due date = Sun, December 6, 2026; Estimated conception = Sun, March 15, 2026; Weeks pregnant today = 34 weeks; Days until due date = 42 days.
Key terms
- Naegele’s rule
- Due date = LMP + 280 days.
Frequently asked questions
How accurate is the due date?
Most babies are born within two weeks either side; an early ultrasound gives a more accurate date.
What if my cycle is irregular?
An ultrasound dating scan is more reliable than the LMP method.
What formula does the Pregnancy Due Date Calculator use?
Due date = LMP + 280 days + (cycle length − 28).