Autism · PRO-ED

GARS-3 Age Calculator

Chronological age in years;months for the Gilliam Autism Rating Scale, Third Edition, with the published age range 3:0 – 22:0 checked for you.

Birth Date

Test / Assessment Date

GARS-3 at a glance

Full name	Gilliam Autism Rating Scale, Third Edition
Publisher	PRO-ED
Published age range	3:0 – 22:0
Age format	Years;months
Corrected age	Not applied in standard scoring.
Discipline	Autism

The GARS-3 is a caregiver and teacher rating scale used to quantify autism-related behaviours and support, not to diagnose on its own. It is frequently paired with an observational measure such as the ADOS-2.

How the GARS-3 wants the age entered

Record chronological age as years;months. Norms are reported by age band across the full range.

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 GARS-3 manual asks for without recalculating:

Years;months.days — the full record-form format, e.g. 4;6.20
Years;months — complete units only, e.g. 4;6
Complete months — e.g. 54 months
Rounded to nearest month — e.g. 55 months (4;7), using the 15-day threshold

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

Worked example

A child born February 26, 2022 and assessed on September 15, 2026:

Chronological age: 4 years, 6 months, 20 days
Record on the GARS-3 as: 4;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.

Other instruments

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

Instrument	Age range	Age format
ADOS-2	12 months – adult	Years;months
WISC-V	6:0 – 16:11	Years;months.days
WPPSI-IV	2:6 – 7:7	Years;months.days
WAIS-5	16:0 – 90:11	Years;months.days
SB5	2:0 – 85:0 and older	Years;months.days
KABC-II NU	3:0 – 18:11	Years;months.days

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

Checked against the documented calculation method and its automated test suiteLast reviewed July 26, 2026Who publishes this

GARS-3, PRO-ED are trademarks of their respective owners. AgeCalc is not affiliated with, endorsed by, or sponsored by any of them. Brand names are used descriptively to identify the assessment formats this calculator supports.

Frequently Asked Questions

Subtract the date of birth from the test date to get complete years, complete months, and the leftover days. For the GARS-3 the age is recorded as years;months. Record chronological age as complete years and complete months (e.g. 10;11). The day remainder is not carried onto the form. A child born February 26, 2022 and tested on September 15, 2026 is 4;6.20 — 4 years, 6 months, 20 days.

PRO-ED publishes the Gilliam Autism Rating Scale, Third Edition for ages 3:0 – 22:0. Outside that range the instrument has no norms, so a score cannot be computed even though the chronological age can. Always confirm the range in the edition of the manual you are scoring from — publishers revise ranges between editions.

No. For the GARS-3 the partial month is not rounded up — record complete years and complete months, and report the leftover days separately where the form has a place for them. Rounding up moves the examinee into the next norm band and changes the standard score.

Prematurity correction is not part of standard GARS-3 scoring — use chronological age from the date of birth. If a clinical team decides to report a corrected age alongside it for context, say so explicitly in the report so the score is not misread.

Standardized scores are read from age-banded norms. A one-month error can move an examinee into a neighbouring band and shift a standard score by several points — enough to change an eligibility decision at a cut-off. Recomputing the age from the birth date and test date, rather than reusing an age from an earlier report, is the reliable check.