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Chronological Age Calculator
Enter a child’s date of birth and the test date to get their chronological age in years, months and days — the figure psychologists, speech-language pathologists and teachers need before scoring a standardized test. Choose how days are borrowed and whether to round, so the answer matches the rule in your test manual.
- Years;months
- 8;2
- Age in months
- 98 months
- Age in decimal years
- 8.25 years
- Age in days
- 3,012 days
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Formula
How to use it
- Enter the date of birth.
- Enter the date the test was given; it defaults to today.
- Choose the borrowing rule and rounding your test manual specifies.
- Copy the years;months figure or the age in months into the scoring software or norm tables.
Worked examples
Born July 20, 2018, tested March 5, 2026: borrow February’s 28 days, so 5 + 28 − 20 = 13 days
- Chronological age
- 7 years, 7 months, 13 days
- Years;months
- 7;7
- Age in months
- 91 months
- Age in days
- 2,785 days
- Age in decimal years
- 7.62 years
The same child with 30-day borrowing: 5 + 30 − 20 = 15 days, which rounds up to 7;8
- Chronological age
- 7 years, 7 months, 15 days
- Years;months
- 7;8
- Age in months
- 92 months
Born November 28, 2015, tested February 3, 2026 (30-day months): borrow a month and a year
- Chronological age
- 10 years, 2 months, 5 days
- Years;months
- 10;2
- Age in months
- 122 months
Born January 10, 2020, tested October 11, 2026 — no borrowing needed
- Chronological age
- 6 years, 9 months, 1 day
- Years;months
- 6;9
- Age in months
- 81 months
Working it out by hand
Write the test date above the birth date as year, month, day and subtract from the right. For a test on 2026-03-05 and a birth date of 2018-07-20: 5 days minus 20 does not go, so borrow a month and the days become 5 + 30 = 35 (or 5 + 28 using February’s real length), leaving 15 (or 13). The test month drops from 3 to 2, which is less than 7, so borrow a year: 2 + 12 = 14 months, and 14 − 7 = 7 months. The year drops from 2026 to 2025, and 2025 − 2018 = 7 years. The age is 7 years, 7 months and 15 (or 13) days.
The two borrowing rules can differ by a day or two whenever borrowing happens, which matters only when the days sit near a rounding point.
Rounding and the years;months notation
Reports often write age as years;months, so 7;7 means 7 years and 7 months — not 7.7 years. Norm tables are usually arranged in month bands, so whether 15 days counts as a month can move a child into the next band.
Test publishers do not all agree: some say never round, some round up from 15 days, others from 16. The calculator does whichever you choose; the manual for the test you are scoring is the authority.
Questions people ask
How do I calculate chronological age for testing?
Subtract the date of birth from the test date, starting with the days. Borrow a month (30 days in most manuals) when the days are short, and a year (12 months) when the months are short. Born 2018-07-20 and tested 2026-03-05 gives 7 years, 7 months, 15 days with 30-day borrowing.
What does 7;4 mean in a report?
7 years and 4 months. The semicolon separates years from months; it is not a decimal.
Do you round chronological age up?
Only if the test manual says to. A common rule rounds up to the next month when 15 or 16 days or more remain; other tests use completed months only.