Free tool

Chronological age calculator

Enter a date of birth and test date to get chronological age in years, months and days, with a worked example.

Date of birthTest dateGestational age at birth (weeks, optional)

Enter a date of birth to see the age.

Years;months format, e.g. 4;7 is 4 years 7 months. Days borrow from the actual previous month; some test manuals borrow 30 days or round days differently, so follow your test's own rules for scoring.

The essentials

How is chronological age calculated?

Subtract the date of birth from the test date: days, then months, then years, borrowing as needed.

If the test day is smaller than the birth day, borrow one month and add that month's days to the test day.
If the test month is then smaller than the birth month, borrow one year and add 12 to the test month.
Subtract each column. The result is years, months and days.

Write the test date above the date of birth as year, month and day, and subtract from right to left.

Worked example

A child born 19 November 2021 and tested 5 August 2026 is 4 years, 8 months and 17 days.

A fictional child, P., was born on 19 November 2021 and tested on 5 August 2026. Days: 5 is less than 19, so borrow July's 31 days; 36 minus 19 is 17. Months: 7 is less than 11, so borrow a year; 19 minus 11 is 8. Years: 2025 minus 2021 is 4. P. is 4;8.

Why do borrowing and rounding rules matter?

Manuals differ on borrowing days and rounding. Always follow the rule in your test's manual.

This calculator borrows the actual days in the month before the test month. Some manuals borrow 30 days, which can shift the day count by one or two. Some drop the days; others round up past a set number. Above, borrowing 30 days gives 16 days, and a manual that rounds up from 15 would record 4;9.

Follow the rule printed in your test manual, and record which convention you used if your clinic asks.

How do you write chronological age?

Write years;months with a semicolon, so 4;8 means 4 years and 8 months.

Do not confuse 4;8 with a decimal: 4.8 years is about 4 years and 10 months. When the exact day count matters, write it in full, such as 4 years 8 months 17 days. The calculator shows both, plus the total in months, which some manuals use for younger children.

When should you use corrected age?

For children born before 37 weeks, when the test manual asks for it, usually up to 24 months.

Many test manuals use corrected age for babies born before 37 weeks, subtracting the weeks born early. Enter weeks of gestation in the optional field to see it beside chronological age. The calculator corrects up to 24 months of chronological age and says so when the child is older. Your manual and clinic decide which age to use.

Is this calculator private?

Yes. Dates stay in your browser and are never sent to healhab.

The dates you enter are used only on this page in your browser and are not sent to healhab. Nothing is saved after you leave.

About the chronological age calculator

Chronological age is the time between a person's date of birth and the date of testing, written in years, months and days. Enter both dates above to get it in years;months format for test forms. Add weeks of gestation at birth to see corrected age for a young child born preterm. The calculation runs in your browser.

Most published tests ask for chronological age on the record form, and many use it to choose a starting point. A slip of a month can send you to the wrong place. This calculator does the date arithmetic so you can check your working.

It works for any age and any test date, including past dates for reports you are writing up later.

Questions

What is chronological age in testing?

The exact time since birth on the day a test is given, in years, months and days. Test manuals use it on record forms and to choose the right starting point or tables.

How do I write 4 years and 8 months?

Write 4;8 with a semicolon. A decimal such as 4.8 means a fraction of a year, which is a different value.

Should I round the days up to a month?

Only if the test manual tells you to. Manuals differ on rounding, so check the instructions for the specific test before you record the age.

Can I calculate age for a past test date?

Yes. Change the test date to any day on or after the date of birth, which helps when writing up an assessment later.

Does the calculator work for adults?

Yes. It works for any date of birth and test date, although corrected age only applies to young children born preterm.

Why does my answer differ from another calculator by a day?

Calculators borrow days differently. This one uses the real length of the month before the test month; others use 30 days. Follow your test manual's method.

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