About the Age Calculator
An age calculator works out exactly how old someone is from their birth date, in years and months. It is useful for forms, eligibility checks, medical records, and any situation where an approximate age is not sufficient.
The formula
Age = current date − birth date, in years and monthsFull years are counted first, then the remaining months. The calculation accounts for whether the birthday has already occurred this year.
How to use this calculator
- 1Enter your Birth Year. The field starts at
1990, which you can overwrite. - 2Enter your Birth Month (1-12). The field starts at
5, which you can overwrite. - 3Enter your Current Year. The field starts at
2026, which you can overwrite. - 4Enter your Current Month. The field starts at
8, which you can overwrite. - 5Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Birth Year | 1990 |
| Birth Month (1-12) | 5 |
| Current Year | 2026 |
| Current Month | 8 |
Result
Age: 36 years old
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
The subtlety is that a year is not a fixed number of days. Leap years mean the same calendar interval can be 365 or 366 days, so age is calculated in calendar units rather than by dividing a day count. Someone born on 29 February has a birthday that exists only every four years, which different jurisdictions handle differently for legal purposes.
Age reckoning is also not universal. The Western convention counts completed years from birth, so a newborn is zero. Traditional East Asian age reckoning counted a baby as one at birth and added a year at the lunar new year, which could make someone two days after birth. South Korea formally moved to the international system in 2023.
Things worth knowing
- For legal purposes, age is normally reckoned as completed years — you are 18 on your eighteenth birthday, not before.
- Leap year birthdays on 29 February are treated as 28 February or 1 March depending on jurisdiction.
- Paediatric medicine uses age in months up to two or three years, because development changes quickly.
- Age of a historical figure at a past date needs the date of that event, not today's date.
- Insurance and actuarial calculations sometimes use nearest birthday rather than last birthday, which can differ by a year.
Frequently asked questions
How do I calculate age exactly?+
Count complete years from the birth date to today, then the additional complete months. Whether this year's birthday has already passed determines the year count.
How are leap year birthdays handled?+
Someone born on 29 February usually celebrates on 28 February or 1 March in non-leap years. Legal treatment varies by jurisdiction, so official documents may specify one or the other.
Why do some countries count age differently?+
Traditional East Asian age reckoning counted a newborn as one year old and incremented at new year, making a person up to two years older than the international count. South Korea standardised on the international system in 2023.
How is a baby's age measured?+
In days and weeks for the first month, then in months up to two or three years. Development milestones change quickly enough that years would be too coarse a unit.