Chronological age is the exact age on the day of a test: subtract the date of birth from the test date, column by column — days, then months, then years — borrowing from the next column when needed. A child born on June 18, 2018 and tested on October 30, 2026 has a chronological age of 8 years, 4 months and 12 days, written 8;4;12 or, in years and months, 8;4.
The calculator shows the working the way a scoring manual sets it out, and gives the answer under both borrowing rules — real month lengths, or a flat 30 days — so you can follow the manual for your test.
Worked example
Date of birth: June 18, 2018Test date: October 30, 2026When borrowing a month: Use real month length
Chronological age on Oct 30, 2026: 8 years, 4 months, 12 days
No borrowing is needed here: 2026 − 2018 = 8 years, October − June = 4 months, 30 − 18 = 12 days, written 8;4;12 (or 8;4 in years and months).
Open this example in the calculator →How to use the Chronological Age Calculator
- Enter the date of birth.
- Enter the test date (the day the test was given, not the day you score it).
- Choose how a borrowed month is counted — check your test manual; many say 30 days.
- Read the age in years, months and days, the working, and the short form such as 8;4.
How to calculate chronological age by hand
Write the test date above the date of birth as year, month and day, and subtract from right to left:
- Days. If the test day is smaller than the birth day, borrow one month from the month column and add its days to the day column.
- Months. If the test month (after any borrowing) is smaller than the birth month, borrow one year — 12 months — from the year column.
- Years. Subtract.
Example with borrowing. Test date 2026-03-05, born 2016-01-20. Days: 5 − 20 will not go, so borrow a month: with the real length of February 2026 that is 28 days, 33 − 20 = 13; with a 30-day borrow it is 35 − 20 = 15. Months: 2 − 1 = 1. Years: 2026 − 2016 = 10. The age is 10;1;13 by real month lengths, or 10;1;15 counting 30 days.
30 days or the real month?
Test publishers do not all agree. Many scoring manuals tell examiners to count a borrowed month as 30 days, because it is quick and consistent by hand; others, and most software, use the real length of the month. The two can differ by a day or two when the birth day is later in the month than the test day — which can matter when a norms table changes at a month boundary. Use the method in the manual for the test you gave; the calculator shows the other one too so you can see whether it makes a difference.
Rounding
Some tests are normed in years and months only. Most manuals say to drop the days rather than round, so 8;4;29 is still 8;4; a few say to round up at 15 days or more. The rounded figure is shown separately for that case — do not use it unless the manual asks you to.
Common uses
Chronological age is the starting point for standard scores on cognitive and achievement tests, speech and language assessments, developmental screenings and many school forms. For a baby born early, developmental screening may use the corrected age instead.
Frequently asked questions
How do you calculate chronological age?
Subtract the date of birth from the test date: days first, then months, then years, borrowing a month or a year when a column will not go. Born June 18, 2018 and tested October 30, 2026 gives 8 years, 4 months and 12 days.
Do you borrow 30 days or the actual days in the month?
It depends on the test publisher. Many manuals say 30 days; others and most software use the actual month. The calculator shows both — follow the manual for your test.
What does 8;4 mean?
Eight years and four months. The semicolon separates years from months (and months from days in 8;4;12).
Should I round chronological age?
Only if the test manual says to. Most say to drop the days rather than round.
Is chronological age the same as age?
Yes — it is the actual age since birth, as opposed to a mental age, a reading age or a corrected age for a premature baby.