Age From Date Of Birth Calculator
Age is the distance between a date of birth and a reference date, and it can be expressed in several ways: whole years, total months, weeks, days, or hours. The calculation is a subtraction once both dates have been converted to a day count.
Age in years
36
- Total months
- 435
- Total weeks
- 1,892
- Total days
- 13,246
- Total hours
- 317,904
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
Days alive = day number of reference date - day number of birth date
- Birth date
- The day the person was born
- Reference date
- The date the age is measured on
- Day number
- A count of days from a fixed epoch
How to check the result by hand
- 1
Convert the birth date to a day number
Apply the Julian day formula to the day, month and year of birth. The result is a single integer that identifies that calendar date.
- 2
Convert the reference date
Apply the same formula to the date the age is being measured on, which is usually today.
- 3
Subtract to get days
Reference day number minus birth day number is the exact number of days alive.
- 4
Divide for the other units
Divide by seven for whole weeks, by 30.436875 for total months, by 365.25 for whole years, and multiply by twenty-four for hours.
- 5
Read the age
The years figure is the age that appears on a form. The days and hours figures are the same span expressed in finer units.
For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Age From Date Of Birth page.