BASC-3 Age Calculator

The BASC-3 is the most widely used multi-rater behaviour rating system in school psychology, with teacher, parent and self-report components. Because each rater form is age-banded, one age error can invalidate an entire rater set.

Record chronological age as years;months. Age selects the form level — preschool, child, adolescent or young adult — for each rater.

The underlying calculation is the same for every instrument — complete years, complete months, then the leftover days. What differs is which part of it goes on the form. The calculator above reports all four figures at once, so you can copy the one the BASC-3 manual asks for without recalculating:

• Years;months.days — the full record-form format, e.g. 3;6.20
• Years;months — complete units only, e.g. 3;6
• Complete months — e.g. 42 months
• Rounded to nearest month — e.g. 43 months (3;7), using the 15-day threshold

The exact date arithmetic, including month-end clamping and leap-day handling, is documented in the calculation methodology.

A child born February 26, 2023 and assessed on September 15, 2026:

• Chronological age: 3 years, 6 months, 20 days
• Record on the BASC-3 as: 3;6
• Day remainder: 20 days — 15 or more, so a nearest-month rounding rounds up

Enter these exact dates above to reproduce the result. Every figure on this page is computed by the same library that runs the calculator, so the worked example and the tool can never disagree.

Same calculation, different recording format. These are the instruments most often administered alongside the BASC-3:

The full list is on the assessment instrument reference, and the assessment age calculator covers any instrument not listed individually.