Developmental & early childhood · Pearson
Bayley-4 Age Calculator
Chronological age in years;months.days for the Bayley Scales of Infant and Toddler Development, Fourth Edition, with the published age range 16 days – 42 months checked for you.
Bayley-4 at a glance
| Full name | Bayley Scales of Infant and Toddler Development, Fourth Edition |
|---|---|
| Publisher | Pearson |
| Published age range | 16 days – 42 months |
| Age format | Years;months.days |
| Corrected age | Corrected age is used for children born before 37 weeks gestation, generally until 24 months. Record which age was used; the difference commonly moves a composite by several points. |
| Discipline | Developmental & early childhood |
The Bayley-4 is the reference developmental assessment for infants and toddlers and is central to early-intervention eligibility. Its norm intervals are the narrowest of any instrument in common use, which makes it the least forgiving of age-entry errors.
How the Bayley-4 wants the age entered
Enter the exact chronological age in years;months.days — at this age the norm intervals are only days to weeks wide, so the day component is not optional.
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 Bayley-4 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 Bayley-4 as: 1;6.20
- 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 Bayley-4
Corrected age is used for children born before 37 weeks gestation, generally until 24 months. Record which age was used; the difference commonly moves a composite by several points.
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 Bayley-4:
| Instrument | Age range | Age format |
|---|---|---|
| Mullen | Birth – 68 months | Years;months.days |
| DAYC-2 | Birth – 5:11 | Total months |
| BDI-3 | Birth – 7:11 | 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.
Bayley-4, Pearson 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.