How to Calculate Chronological Age by Hand (and the Mistakes to Avoid)

Scoring software calculates age automatically, but examiners still need to calculate it by hand on paper protocols or to double-check a result. The method is simple subtraction with borrowing, much like subtracting numbers in elementary school, except that the "places" are days, months and years.

Set up the problem

Write the test date on top and the birth date underneath, in year, month, day columns:

	Year	Month	Day
Test date	2026	10	9
Birth date	2019	4	23

Subtract, borrowing as needed

Days: 9 is less than 23, so borrow one month from the month column. Add 30 days: 9 + 30 = 39. The test month becomes 9. 39 − 23 = 16 days.
Months: 9 − 4 = 5 months. No borrowing needed.
Years: 2026 − 2019 = 7 years.

Chronological age: 7;5.16.

A second example with year borrowing

	Year	Month	Day
Test date	2026	2	14
Birth date	2016	11	20

Days: 14 < 20, so borrow: 44 − 20 = 24 days; test month becomes 1.
Months: 1 < 11, so borrow a year: 13 − 11 = 2 months; test year becomes 2025.
Years: 2025 − 2016 = 9 years.

Age: 9;2.24. Check it with the chronological age calculator using the test-manual method.

30 days or real days?

Many test manuals specify borrowing exactly 30 days, regardless of the month. The exact-calendar method instead borrows the actual number of days in the month before the test date (28 to 31). The difference is at most a couple of days. Use whichever your test manual specifies; the calculator offers both.

Rounding

Some protocols ask only for years and months. Common rules:

Round up to the next month if the days are 15 or more (or 16 or more, depending on the manual).
Some tests never round and simply drop the days.

In the first example, 7;5.16 rounds to 7;6 under the "15 or more" rule. Only round if the manual tells you to.

Common mistakes

Subtracting the wrong way round: always test date minus birth date.
Forgetting to reduce the month after borrowing days, which adds a month to the result.
Writing the date in day/month order by mistake on forms that expect month/day, or vice versa.
Using today's date instead of the actual test date when scoring later.
Rounding when the manual says not to.

Always double-check

An age error leads to the wrong norm table. Calculate twice, or by hand and with a calculator, especially near band boundaries. For why it matters, read why exact chronological age matters.