How to calculate chronological age by hand
Write the test date above the date of birth as year, month, day, and subtract from the right:
- Days: if the test day is smaller than the birth day, borrow one month from the test month and add 30 days.
- Months: if the test month is now smaller than the birth month, borrow one year and add 12 months.
- Years: subtract the birth year from the test year.
Example: tested on 2026-03-05, born 2018-07-20. The days 5 − 20 need a borrow: 35 − 20 = 15 days, and the test month becomes 2. Months 2 − 7 need a borrow: 14 − 7 = 7 months, and the year becomes 2025. Years 2025 − 2018 = 7. Chronological age: 7 years, 7 months, 15 days (7;7).
Chronological age calculator: worked examples
| Born | Tested | Calendar method | 30-day method |
|---|---|---|---|
| July 20, 2018 | March 5, 2026 | 7y 7m 13d | 7y 7m 15d |
| February 10, 2015 | November 12, 2026 | 11y 9m 2d | 11y 9m 2d |
| January 31, 2020 | March 1, 2026 | 6y 1m 1d | 6y 1m 0d |
| September 28, 2012 | September 27, 2026 | 13y 11m 30d | 13y 11m 29d |
Frequently asked questions
What is a chronological age calculator?
A chronological age calculator works out a person's exact age in years, months and days on a given date, usually the date of a test or assessment. Psychologists, speech pathologists, teachers and doctors use it to pick the right norms table when scoring a test.
Why do the two methods sometimes give different days?
Doing it by hand, test manuals tell you to borrow 30 days when the test day is smaller than the birth day. The calendar method borrows the real length of the month instead (28 to 31 days). The years and months always agree; the days can differ by one or two near the end of a month. Use whichever method your test manual specifies.
Should I round the chronological age?
Follow the test manual. Most current tests, including the Wechsler scales, say not to round days up to the next month. Some older tests rounded 16 days or more up to a whole month, so check before you look up norms.
How do I write chronological age?
Usually as years;months or years-months-days, for example 7;7 or 7-7-15 for 7 years, 7 months and 15 days.
Need the age in weeks or days, or a birthday countdown? Use the age calculator or the birthday countdown.