Due date calculator

How it works

Enter the first day of your last period and your usual cycle length.

The formula

Naegele's rule: due date = first day of last period + 280 days, adjusted by (cycle length − 28) days. Weeks pregnant = days since that date ÷ 7.

Worked example

Last period started 1 March 2026 with a 28-day cycle: 1 March + 280 days = 6 December 2026.

Good to know

• Only a few babies arrive on the exact due date.
• An early ultrasound dates a pregnancy more accurately; your doctor is the reference.