How Many Weeks Pregnant Calculator

Gestational age is counted from the last period, so it is a day count divided into weeks and days.

Days since the last periodDays since the first day of bleeding, which is the clinical start point.

Weeks pregnant

Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.

The formula this calculator uses

weeks = floor(days since last period / 7)

1
Find the first day of the last menstrual period
This is the start point for every clinical estimate. Use the first day of bleeding rather than the last, since the convention is defined from the beginning of the period.
2
Count the days since then
Include today if you count the start day as day zero. A consistent convention matters more than which one you pick, so keep it the same between appointments.
3
Divide by seven and drop the remainder
One hundred and fifty four over seven is exactly twenty two. The whole number is the weeks figure and the leftover days are reported separately.
4
Work out the leftover days
One hundred and forty three over seven is twenty with three days remaining, which is reported as twenty weeks and three days rather than as a decimal.
5
Check the days remaining against the term
Two hundred and eighty minus one hundred and fifty four leaves one hundred and twenty six days, which places the date inside the third trimester.

For worked examples, common mistakes and the limits of this formula, read the full How To Calculate How Many Weeks Pregnant page.