How to Calculate Chronological Age

Method 1: Calculate Chronological Age by Hand

The manual method for calculating chronological age uses subtraction with borrowing — the same technique taught in elementary math, applied to dates. This is the method required on many standardized test protocols where clinicians must show their work.

Here is the step-by-step process:

1. Write the test date (or today's date) in three columns: Year, Month, Day.
2. Write the date of birth directly below it in the same format.
3. Subtract the days. If the test day is smaller than the birth day, borrow 30 from the month column — add 30 to the day column and subtract 1 from the month column.
4. Subtract the months. If the test month is now smaller than the birth month (after any borrowing), borrow 12 from the year column — add 12 to the month column and subtract 1 from the year column.
5. Subtract the years to get the final chronological age.

The borrowing convention of using 30 days per month is standard across Pearson, Brigance, and most other test publishers. Some protocols may use the actual number of days in the relevant month, but 30 is the accepted default.

Example 1: No Borrowing Needed

Test date: October 20, 2025   Date of birth: March 8, 2018

In this example, the test day (20) is larger than the birth day (8), and the test month (10) is larger than the birth month (3). No borrowing is needed.

Chronological age: 7 years, 7 months, 12 days

Example 2: Borrowing Required

Test date: February 5, 2026   Date of birth: September 18, 2019

Here, the test day (5) is smaller than the birth day (18), so we need to borrow. After borrowing days, the test month becomes 1, which is still smaller than the birth month (9), so we need to borrow from years as well.

Chronological age: 6 years, 4 months, 17 days

The borrowing logic is the trickiest part of manual calculation. If the concept feels confusing, don't worry — our calculator handles it automatically.

Method 2: Use Our Free Calculator

The fastest and most reliable way to calculate chronological age is to use our Chronological Age Calculator. Simply enter the date of birth and the test date, and the tool instantly returns the age in years, months, and days — plus total months, total weeks, and total days.

The calculator uses the same borrowing logic described above, handles leap years correctly, and provides the result in the exact format used by standardized test protocols. It is 100% free, requires no signup, and stores no personal data.

For assessments that require Pearson-specific formatting (such as CELF-5, PLS-5, or PPVT-5), visit our Pearson Age Calculator, which outputs the age in the semicolon-separated format (e.g., 7;4) used on Pearson protocols.

Method 3: Calculate Age in Excel

Microsoft Excel does not have a built-in "age" function, but the DATEDIF function can calculate the difference between two dates in years, months, and days. Here is the formula:

Here's what each part of the formula does:

• DATEDIF(B2,C2,"Y") — Returns the number of complete years between the two dates.
• DATEDIF(B2,C2,"YM") — Returns the number of remaining months after full years are accounted for.
• DATEDIF(B2,C2,"MD") — Returns the number of remaining days after full months are accounted for.
• The & operator concatenates text and numbers into a readable string.

Important note: DATEDIF is an undocumented function in Excel — it will not appear in the function autocomplete list or help files, but it works reliably in all versions of Excel (from Excel 2003 through Microsoft 365). It was originally included for Lotus 1-2-3 compatibility.

Result for the example above: 6 years, 4 months, 17 days

Method 4: Calculate Age in Google Sheets

Google Sheets supports the same DATEDIF function with identical syntax. The formula works exactly the same way:

=DATEDIF(B2,C2,"Y") & " years, " & DATEDIF(B2,C2,"YM") & " months, " & DATEDIF(B2,C2,"MD") & " days"

Unlike Excel, Google Sheets actually documents the DATEDIF function in its help resources, making it slightly easier to discover. The behavior and parameters are identical to Excel's implementation.

To calculate the age as of today's date (instead of a fixed test date), replace C2 with TODAY():

=DATEDIF(B2,TODAY(),"Y") & " years, " & DATEDIF(B2,TODAY(),"YM") & " months, " & DATEDIF(B2,TODAY(),"MD") & " days"

Common Mistakes

Even experienced clinicians make errors when calculating chronological age by hand. Here are the most frequent mistakes and how to avoid them:

• Forgetting to borrow: The most common error is subtracting a larger birth day from a smaller test day without borrowing 30 days from the months. Always check: is the test day smaller than the birth day? If yes, borrow before subtracting.
• Using the wrong test date: The chronological age must be calculated based on the date the test was administered, not the date the report is written or the date the referral was made. Using the wrong date can shift the age by days or weeks.
• Off-by-one month errors: After borrowing 30 days, some clinicians forget to subtract 1 from the month column. This results in an age that is one month too high.
• Reversing the subtraction: Always subtract date of birth FROM the test date — not the other way around. Subtracting the wrong direction will produce a nonsensical result.
• Not double-borrowing: Sometimes you need to borrow twice — once from months to days, and again from years to months. Forgetting the second borrow leads to incorrect results.

Practice Problems

Test your skills with these three practice problems. Try calculating the chronological age by hand before revealing the answer.

Problem 1

Test date: April 12, 2026   Date of birth: July 30, 2020

Reveal Answer

Step	Year	Month	Day
Test date	2026	4	12
Borrow days (12 < 30)	2026	3	42
Borrow months (3 < 7)	2025	15	42
Date of birth	2020	7	30
Result	5	8	12

Answer: 5 years, 8 months, 12 days

Problem 2

Test date: November 3, 2025   Date of birth: March 15, 2017

Reveal Answer

Step	Year	Month	Day
Test date	2025	11	3
Borrow days (3 < 15)	2025	10	33
Date of birth	2017	3	15
Result	8	7	18

Answer: 8 years, 7 months, 18 days

Problem 3

Test date: January 8, 2026   Date of birth: December 22, 2018

Reveal Answer

Step	Year	Month	Day
Test date	2026	1	8
Borrow days (8 < 22)	2026	0	38
Borrow months (0 < 12)	2025	12	38
Date of birth	2018	12	22
Result	7	0	16

Answer: 7 years, 0 months, 16 days

Want more practice? Use our chronological age calculator to check your work, or explore the chronological age chart for a quick-reference lookup table.

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