Developmental & early childhood · Riverside Insights
BDI-3 Age Calculator
Chronological age in total months for the Battelle Developmental Inventory, Third Edition, with the published age range Birth – 7:11 checked for you.
BDI-3 at a glance
| Full name | Battelle Developmental Inventory, Third Edition |
|---|---|
| Publisher | Riverside Insights |
| Published age range | Birth – 7:11 |
| Age format | Total months |
| Corrected age | Prematurity adjustment is applied under 2 in most early-intervention protocols; follow the state or agency rule, which sometimes overrides the manual. |
| Discipline | Developmental & early childhood |
The BDI-3 is a full developmental battery used heavily for Part C and preschool special-education eligibility, with adaptive, personal-social, communication, motor and cognitive domains. Many states specify BDI cut-scores in their eligibility rules, which is why the age in months has to be exact.
How the BDI-3 wants the age entered
Enter chronological age in total completed months. The BDI-3 uses age in months to set the entry point in each domain and to select the norm reference.
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 BDI-3 manual asks for without recalculating:
- Years;months.days — the full record-form format, e.g. 1;6.20
- Years;months — complete units only, e.g. 1;6
- Complete months — e.g. 18 months
- Rounded to nearest month — e.g. 19 months (1;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, 2025 and assessed on September 15, 2026:
- Chronological age: 1 year, 6 months, 20 days
- Record on the BDI-3 as: 18 months
- 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.
Corrected age and the BDI-3
Prematurity adjustment is applied under 2 in most early-intervention protocols; follow the state or agency rule, which sometimes overrides the manual.
Corrected (adjusted) age is measured from the due date rather than the birth date. Switch the calculator above to Born preterm and enter the gestational age at birth to see both figures side by side. The dedicated adjusted age calculator adds gestational and postmenstrual age, and the corrected age guide explains the convention for families.
Other instruments
Same calculation, different recording format. These are the instruments most often administered alongside the BDI-3:
| Instrument | Age range | Age format |
|---|---|---|
| Bayley-4 | 16 days – 42 months | Years;months.days |
| Mullen | Birth – 68 months | Years;months.days |
| DAYC-2 | Birth – 5:11 | Total months |
| ASQ-3 | 1 – 66 months | Total months |
| ASQ:SE-2 | 1 – 72 months | Total months |
| Denver II | Birth – 6:0 | Years;months.days |
The full list is on the assessment instrument reference, and the assessment age calculator covers any instrument not listed individually.
BDI-3, Riverside Insights 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.