How this calculator works
The exact number of calendar days between the two dates is divided by seven. Because seven divides the day count only when the dates are a whole number of weeks apart, most people get a decimal week count. The weeks-and-days line turns that decimal into a whole number of weeks plus zero to six days, which is easier to picture.
Formula
days = O(as-of) - O(date of birth). weeks = days / 7. Whole weeks = floor(days / 7) and leftover days = days mod 7.
Worked example
Born 1990-05-20 and measured on 2026-11-01, you are 13,314 days old, which is exactly 1,902 weeks and 0 days. A birth on 2000-02-29 measured on 2023-03-01 is 8,401 days, or 1,200.142857 weeks, which is 1,200 weeks and 1 day.
Advertisement
Frequently asked questions
Why is the week count not always a whole number?
A week is a fixed block of seven days, but birthdays and the chosen as-of date rarely fall a multiple of seven days apart. The decimal part is a fraction of a week, not extra days after a decimal point.
Is a month four weeks?
No. Months run from 28 to 31 days, so a month is about 4.35 weeks on average. Dividing an age in weeks by four drifts by several weeks over a few years.
Why do some baby books count weeks from the due date?
Pregnancy weeks are counted from the first day of the last menstrual period, not from the birth date. This page measures elapsed time from the date you enter, so use the start date that matches the question.