Common formats
- Years;months
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- Years-months-days
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- Total months
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- Total weeks
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- Total days
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- Decimal years
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Chronological Age Calculator
Find a child’s exact age on a test or admission date, using calendar or 30-day months, with corrected age for babies born early.
Chronological age
–years
–months
–days
Enter a date of birth and a test date.
Corrected age
- Corrected age
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- Born early by
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- Due date
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For babies born before 37 weeks, add the gestational age at birth to see their corrected (adjusted) age.
What is chronological age?
Chronological age is simply the time since birth: years, months and days on the calendar. In psychology, education and speech and language therapy it has a precise job. Most standardised tests are norm-referenced: a child’s raw score is compared with scores from children of the same age, often in bands of one to four months. Get the age wrong by a few days near the edge of a band and you may read the wrong norm table.
That’s why test manuals spell out exactly how to calculate it, and why this calculator lets you match the method your manual uses.
How to calculate chronological age by hand
Write the test date above the date of birth, as year, month and day, and subtract from right to left. Here a child born on 20 November 2018 is tested on 5 March 2026:
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 3 | 5 |
| Date of birth | 2018 | 11 | 20 |
| Age (30-day method) | 7 | 3 | 15 |
- Days: 5 − 20 is negative, so borrow a month. With the 30-day method, 5 + 30 = 35, and 35 − 20 = 15 days. The test month drops from 3 to 2.
- Months: 2 − 11 is negative, so borrow a year: 2 + 12 = 14, and 14 − 11 = 3 months. The test year drops to 2025.
- Years: 2025 − 2018 = 7 years.
The chronological age is 7;3 (7 years, 3 months and 15 days).
Calendar months or 30-day months?
The one step where manuals differ is borrowing. The 30-day method treats every borrowed month as 30 days, which makes hand calculation quick and consistent. The calendar method counts real months: for the same child it counts from 20 February to 5 March 2026, which is only 13 days because February 2026 has 28 days, so the age is 7;3 and 13 days.
Most of the time the two agree on years and months and differ by a day or two at most. They can only change the years;months figure when the days are close to a month boundary. Always use the method in your test’s manual; if it doesn’t say, check with the publisher and be consistent.
Rounding to whole months
Some tests ask for age in completed years and months, with the days dropped. Others round to the nearest month, usually rounding up when the remaining days are 15 or more. A few use 16. The calculator’s rounding option uses the 15-day rule. If your manual says nothing about rounding, don’t round.
Corrected age for babies born early
For a baby born before 37 weeks, developmental assessments usually use corrected age (also called adjusted age), counted from the due date instead of the birth date. The American Academy of Pediatrics explains it simply: take the baby’s age and subtract the weeks they were born early, where full term is 40 weeks. A baby born at 32 weeks is 8 weeks early, so at 16 weeks old their corrected age is 8 weeks (HealthyChildren.org, AAP).
Enter the gestational age at birth in weeks (and extra days if you have them) and the calculator shows the due date and the corrected age on the test date. Corrected age is normally used for the first two years. After that, chronological age is used, but your assessment’s own guidance takes priority.
Checking an age against a school cut-off date
School admissions often hinge on a child reaching a certain age by a cut-off date. To check, enter the cut-off date as the test date. The result is the child’s exact age on that day, so you can see straight away whether they meet the requirement. Cut-off dates and rules vary by country, state and school, so confirm the exact rule with the school or education authority.
Last reviewed on by the Ageculator Team. See how we calculate.
Frequently asked questions
How do you calculate chronological age for testing?
Write the test date and the date of birth as year, month and day, then subtract column by column, starting with the days. If the days come out negative, borrow one month from the months column; if the months come out negative, borrow one year (12 months). The result is the age in years, months and days.
Should I borrow 30 days or a calendar month?
Follow the manual for the test you’re giving. Many test publishers tell examiners to count every month as 30 days when borrowing; others expect calendar months. The two methods can differ by a day or two near the end of a month, which occasionally moves a child into a different norm table.
Do I round chronological age to the nearest month?
Only if the test manual says so. Some instruments round up when the leftover days are 15 or more (or 16, in some manuals); others never round. Tick the rounding option only when your manual asks for it.
What is corrected age, and when should I use it?
Corrected (or adjusted) age is a premature baby’s age counted from their due date instead of their birth date. It’s used when comparing development with babies born at term, usually for the first 2 years. Enter the gestational age at birth to see it.
What does an age written as 8;4 mean?
It means 8 years and 4 months. Test reports often use a semicolon (or sometimes a colon or hyphen) between years and months, so it can’t be confused with a decimal like 8.4 years.