What does a reverse due date calculator do?

It works backwards from a known due date to estimate the first day of your last period and the likely conception or ovulation date.

Can it tell me exactly when I conceived?

It gives a good estimate based on a 28-day cycle. Conception usually occurs around 14 days after the estimated last period, but cycle length can shift this by several days.

Why is the last period 280 days before the due date?

A typical pregnancy is counted as 280 days (40 weeks) from the first day of the last menstrual period, even though conception usually happens about two weeks later.

Reverse Due Date Calculator

Enter your due date to estimate when your last period started and when conception likely occurred.

Estimated due date

Estimated first day of LMP

Sunday, October 12, 2025

Estimated conception

Sunday, October 26, 2025

This is an estimate for information only — only about 1 in 20 babies arrive on their due date. Always confirm dates with your midwife or doctor.

Related calculators

Due date calculator Conception date Ovulation calculator

Reverse Due Date Calculator

Frequently asked questions

What does a reverse due date calculator do?

Can it tell me exactly when I conceived?

Why is the last period 280 days before the due date?