Begin with a reference date
An age calculation needs two dates. The first is the date of birth. The second is the date you want the age on. Today is convenient, but a school form, old photograph or future celebration may call for a different reference date. Write both dates in full before doing the arithmetic. A short date such as 03/04 can mean different things in different countries.
Count completed years first
Start with the difference between the two year numbers, then check whether the birthday has arrived in the reference year. Someone born on 20 November 2000 is 25 completed years old on 5 November 2026. Their 26th birthday is still ahead. This check is why subtracting the birth year alone sometimes gives an answer one year too high.
After counting completed years, move forward from the birth date by that number of years. This gives an anniversary that sits on or before the reference date. Then count complete calendar months from that anniversary. Finally, count the remaining calendar days. The calculator follows this order so each part has a clear meaning.
A worked example
For a birth date of 15 June 2000 and a reference date of 20 August 2025, the completed years take you to 15 June 2025. Two completed months take you to 15 August. Five more calendar days reach 20 August. The result is 25 years, 2 months and 5 days.
Why dividing by 365 gives a different answer
A decimal age based on total days divided by 365 or 365.25 is a different measurement. It may be useful for a rough comparison, but it does not give a person's completed calendar years and months. Leap years and varying month lengths make a fixed divisor unsuitable for that job. The total-days figure on our result is counted separately.
Use the result in context
Our tools compare calendar dates. They do not ask for birth location, historical time-zone rules or the time on a birth certificate. If an organisation needs age at a particular time or applies its own eligibility rule, use that organisation's instructions. For ordinary calendar age, choose the date you need and keep it with the result so another person can reproduce the calculation.