Developmental & early childhood · Brookes Publishing
ASQ-3 Age Calculator
Chronological age in total months for the Ages & Stages Questionnaires, Third Edition, with the published age range 1 – 66 months checked for you.
ASQ-3 at a glance
| Full name | Ages & Stages Questionnaires, Third Edition |
|---|---|
| Publisher | Brookes Publishing |
| Published age range | 1 – 66 months |
| Age format | Total months |
| Corrected age | Corrected age is used to select the questionnaire interval for babies born more than 3 weeks preterm, until 24 months of age. |
| Discipline | Developmental & early childhood |
The ASQ-3 is a parent-completed developmental screener used across paediatrics, home visiting and Head Start. Because the parent completes it, the wrong interval is the single most common administration error, and it comes straight from a mis-computed age.
How the ASQ-3 wants the age entered
Age in months selects the questionnaire interval (there are 21 intervals). Each interval has a defined administration window, so the age at completion — not the age today — decides which form is valid.
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 ASQ-3 manual asks for without recalculating:
- Years;months.days — the full record-form format, e.g. 1;7.17
- Years;months — complete units only, e.g. 1;7
- Complete months — e.g. 19 months
- Rounded to nearest month — e.g. 20 months (1;8), 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 January 29, 2025 and assessed on September 15, 2026:
- Chronological age: 1 year, 7 months, 17 days
- Record on the ASQ-3 as: 19 months
- Day remainder: 17 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 ASQ-3
Corrected age is used to select the questionnaire interval for babies born more than 3 weeks preterm, until 24 months of age.
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 ASQ-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 |
| BDI-3 | Birth – 7:11 | 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.
ASQ-3, Brookes Publishing 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.