Chronological Age Calculator

Chronological age is the time since a person was born, measured on the date you are checking. For an assessment report, use the relevant assessment date. A child's age today may be different from their age when the test took place.

A child born on 15 June 2020 is 6 years, 3 months and 21 days old on 6 October 2026. That is 75 completed calendar months and another 21 days. This tells you the child's age; it does not measure their ability or development.

Count the full years from birth, then the extra full months and remaining days. This calculator keeps each monthly milestone on the original day of birth. If that day is missing from a month, it uses the last day of that month.

For the example above:

Record the birth date and assessment date alongside the age. If “6 years 3 months” is copied into another report on its own, the reader may not know when that age was correct.

Borrowing days when subtracting dates

Suppose the child was born on 15 February 2020 and assessed on 10 March 2026. The child completed six years on 15 February 2026. Another 23 days passed before 10 March, giving an age of 6 years, 0 months and 23 days.

If you subtract the dates in columns, you need to borrow days because 10 is smaller than 15. February 2026 has 28 days, so the day calculation is 10 + 28 − 15 = 23. Borrowing 30 days instead would give a different answer.

Some assessments prescribe their own method for working out age. This can differ from a calendar calculation, especially when you need to borrow days.

For example, Pearson's published WIAT-III guidance uses 30-day months. It also says to use the first session when testing takes place over several sessions and not to round the age up. These instructions apply to that assessment; other tests may use different rules. [S03]

Here is the difference for a birth date of 15 February 2020 and an assessment date of 10 March 2026:

Check the current manual for the exact test and edition you are using. A calculator that shows years, months and days will not necessarily follow that test's method.

To find completed months, multiply the completed years by 12 and add the extra full months.

Completed months = completed years × 12 + additional full months.

For an age of 6 years, 3 months and 21 days, the calculation is 6 × 12 + 3 = 75 completed months. Keep the 21 days in the detailed result. They do not complete a 76th month.

You may see six years and three months written as 6:3 or 6-3 in a manual. These labels show years and months. Decimal 6.3 years means six years plus 0.3 of a year, so it is not the same as 6 years 3 months.

When choosing an age band or norm table, follow the test's instructions on days and rounding. A child who is close to a birthday should not automatically move into the next band. Calculating age and choosing the correct scoring table are separate steps.

Chronological age starts on the actual date of birth. Corrected age allows for the weeks a baby was born early and may be used when looking at development. The American Academy of Pediatrics explains this using a 40-week reference and discusses corrected age during the first two years. [S04]

The calculation in weeks is:

Corrected age = chronological age − (40 − gestational age at birth).

If a baby was born at 34 weeks and is now 20 weeks old, subtract the 6 weeks they were born early. Their corrected age is 14 weeks. Working in weeks avoids the inaccurate assumption that every month is four weeks long.

Use whichever age measure the clinician or assessment manual asks for. Corrected age does not change the baby's date of birth or chronological age.