How to Calculate Chronological Age (Years, Months and Days)

Published 2026-10-25

Chronological age is simply how old someone is, measured exactly from their date of birth to a given date. It sounds trivial, but in schools, speech therapy, psychology and pediatrics it has to be precise to the month and day, because test scores are compared with children of the same age. A mistake of one month can shift a score.

Here is how to calculate it by hand, how to handle the borrowing step, and how to check your answer.

How it’s written

Chronological age (often shortened to CA) is written as years, months and days. Testing forms usually record it as years and months with a semicolon or hyphen: 8;3 or 8-3 means 8 years and 3 months. Don’t write it as a decimal: 8.3 years is not the same as 8 years 3 months.

The method

Write the test date on top and the date of birth underneath, as year, month and day. Then subtract each column, starting with the days.

25 − 12 = 13 days, 10 − 6 = 4 months, 2026 − 2018 = 8 years. The child is 8 years, 4 months and 13 days old, recorded as 8;4.

When you need to borrow

If the day or month on the bottom is bigger than the one on top, borrow from the next column, just like subtraction with numbers:

• Days too small: take 1 from the months column and add the days of a month to the days column. Many test manuals tell you to add 30 days.
• Months too small: take 1 from the years column and add 12 months to the months column.

Example with both: test date October 5, 2026, birth date December 20, 2019.

The 5 days are smaller than 20, so borrow a month: 10 months becomes 9, and 5 days becomes 35. Then 9 months is smaller than 12, so borrow a year: 2026 becomes 2025, and 9 months becomes 21. Now subtract: 35 − 20 = 15 days, 21 − 12 = 9 months, 2025 − 2019 = 6 years. The age is 6 years, 9 months and 15 days, recorded as 6;9.

Should you round the days?

It depends on the test. Some manuals say to ignore the days and never round up; others say to round up to the next month when the days are 15 or more. In the example above, that rule would give 6;10 instead of 6;9. Always follow the instructions for the specific assessment you’re scoring, because the norm tables were built using that rule.

Checking your answer

Hand calculations are easy to slip on, especially when borrowing twice. Check with the age calculator: enter the date of birth, then set “Age on this date” to the test date. It gives years, months and days instantly.

It counts real calendar months rather than a fixed 30 days, so when the birthday falls near the end of a month, its answer can differ from the 30-day method by a day or two. If the two disagree, use the method your test manual specifies.

For the total number of days or weeks between two dates, for example to track time between assessments, use the date difference calculator.

Adjusted age for premature babies

For babies born early, pediatricians and therapists often also use an adjusted (or corrected) age: the chronological age minus the number of weeks the baby was born early. A baby born 8 weeks early who is 6 months old has an adjusted age of about 4 months. Adjusted age is commonly used for development checks in the first two to three years; your child’s doctor or therapist will say which age to use.

Common questions

• Is chronological age the same as birthday age? Yes in meaning, but more precise. “Seven” on a birthday card could be anywhere from 7;0 to 7;11, while chronological age pins it down to the month and day.
• What about leap years? Counting by calendar dates handles them automatically. A child born on February 29 still gains one year of age each year.
• Why not just count total days and divide? Months have different lengths, so dividing days by 30 or 365 gives slightly wrong results. The column method, or a calendar-based calculator, avoids that.