How age is calculated
Exact age is found by subtracting the date of birth from the current date one unit at a time — days, then months, then years — "borrowing" from the next unit when you need to, just like subtraction with carrying.
Example: born 15 May 1990; today is 26 September 2026.
- Days: 26 − 15 = 11 days
- Months: 9 − 5 = 4 months
- Years: 2026 − 1990 = 36 years
- Age: 36 years, 4 months, 11 days
When the day of the month is smaller than the birth day, borrow one month: the calculator adds the number of days in the previous calendar month (28–31) so the result is exact.
Chronological age for testing
Psychologists, speech-language pathologists and teachers need a child's chronological age (CA) on the testing date to look up norms for standardized assessments. CA is written in years and months, often as 7;4 (7 years, 4 months) or with days as 7-4-12.
Most test manuals use this method:
- Write the test date and birth date as year, month, day.
- Subtract days. If the birth day is larger, borrow 1 month from the test month and add 30 days.
- Subtract months. If needed, borrow 1 year and add 12 months.
- Subtract years.
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 3 → 2 | 10 → 40 |
| Birth date | 2018 | 11 | 22 |
| After borrowing a year (2 → 14 months) | 2025 | 14 | 40 |
| Chronological age | 7 | 3 | 18 |
Result: 7;3 (7 years, 3 months, 18 days). Rounding rules vary by test. Some manuals tell you to round up to the next month when days are 15 or more (which would make this 7;4); others, including many Pearson assessments, say never to round. Always follow the manual for the specific test you are scoring. The calculator shows both the unrounded CA and the days so you can apply the right rule.
Leap-year birthdays
People born on 29 February have a calendar birthday only once every four years. For legal purposes in most US states, and in the UK, a "leaplings" birthday in a common year is treated as 1 March when counting age for things like driving or drinking age — though many people celebrate on 28 February. This calculator counts an age as complete on 1 March in non-leap years.
Other ways of counting age
- East Asian age reckoning: traditionally a baby is 1 at birth and everyone gains a year on Lunar New Year. South Korea officially switched to international age for legal purposes in June 2023.
- Gestational vs. corrected age: for babies born prematurely, pediatricians use a "corrected age" — chronological age minus the weeks born early — to track development until about age 2.
- Age in days and weeks: used for newborns, medication dosing charts and milestones. See the totals above.
Frequently asked questions
How do I calculate my exact age?
Subtract your birth date from today's date: days first, then months, then years, borrowing a month (or a year) when needed. The calculator does this automatically and shows the working.
How do you calculate chronological age for testing?
Subtract the birth date from the test date as year-month-day, borrowing 30 days for a month and 12 months for a year when needed. Report the result as years;months (for example 8;5), and follow your test manual's rounding rule for the leftover days.
How many days old am I?
Enter your date of birth and the calculator shows the total number of days you have lived, including leap days.
What day of the week was I born?
It's shown in the results as "Born on a".
When does someone born on February 29 turn a year older?
In common years, age is generally considered to increase on March 1 for legal purposes, although many people celebrate on February 28.