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Chronological age
calculator
Get a child's exact age in years;months;days for test scoring, with both borrowing methods
Written and maintained by Christin Bowles, founder of TidyCalcs.
Fill this in only for babies born before 37 weeks to also get corrected (adjusted) age.
Please enter both dates.
Both borrowing methods
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What chronological age means in testing
Chronological age (often shortened to CA) is how long a child has been alive on the day they take a test. Standardized tests used by school psychologists, speech-language pathologists, occupational therapists and special education teachers compare a child's score with other children of the same age, so the norms table you use depends on getting the age exactly right. It's written as years;months;days, so 8;3;14 means 8 years, 3 months and 14 days.
How to calculate chronological age by hand
Write the test date on top and the birth date underneath, each as year, month, day. Subtract the days first, then the months, then the years.
- Days: if the test day is smaller than the birth day, borrow 1 month from the test month and add 30 days to the test day.
- Months: if the test month is now smaller than the birth month, borrow 1 year from the test year and add 12 months.
- Years: subtract the birth year from what's left of the test year.
Example: a test on 2026-10-05 for a child born 2019-03-20.
30 days or actual days? Check your manual
Many test manuals and record forms say to borrow 30 days whenever you need more days. Online scoring software and date calculators usually borrow the actual number of days in the previous month (28, 29, 30 or 31). The two answers can differ by a day or two, which only matters when a child sits right on the edge of an age band. The calculator above shows both, so use whichever your test manual tells you to.
Rounding
Some tests look up norms by completed years and months and simply drop the days. Others round to the nearest month, where 16 or more days rounds up to the next month (a few use 15). Don't round unless your manual says to. Both versions appear in the results.
Corrected age for premature babies
A baby born before 37 weeks is often assessed using corrected (adjusted) age, which subtracts the weeks they were born early from their chronological age. Full term is counted as 40 weeks, so a baby born at 30 weeks is 10 weeks early. Pediatricians and early intervention teams commonly use corrected age for developmental checks until about age 2. Enter the weeks of pregnancy at birth above to see it.
Common questions
What is chronological age?
Chronological age is the exact amount of time a person has been alive, counted from their date of birth to a specific date. In testing it's written as years;months;days, for example 7;4;12 means 7 years, 4 months and 12 days.
How do you calculate chronological age by hand?
Write the test date on top and the birth date below it as year, month, day. Subtract days, then months, then years. If the days come out negative, borrow one month and add 30 days (or the actual days in the month, depending on your manual). If the months come out negative, borrow one year and add 12 months.
Should I borrow 30 days or the actual days in the month?
It depends on the test. Many manuals say to borrow 30 days, while calendar-based scoring software uses the actual number of days in the previous month. The calculator shows both so you can match your manual.
Do you round chronological age up?
Some tests use completed years and months with no rounding, while others round to the nearest month (often 16 or more days rounds up). Always follow the scoring rules in your test manual.
What is corrected age for premature babies?
Corrected (adjusted) age subtracts the weeks a baby was born early from their chronological age. A baby born at 32 weeks is 8 weeks early, so at 6 months old their corrected age is about 4 months. It's commonly used for development checks until about age 2.
Sources and further reading
For independent, authoritative information on this topic, these are good places to start:
- HealthyChildren.org (American Academy of Pediatrics): Corrected age for preemies — how and when to use adjusted age.
- CDC: Developmental milestones — what most children do by each age.