Chronological Age Calculator
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How to Calculate Chronological Age (By Hand, for OT, and for Forms)

Chronological age is the actual time someone has lived from birth to a given date, usually in years, months, and days. It's the same idea as "chronological aging"—age in terms of time. In everyday talk we might say "she's 5" or "he turned 3 in March," but many forms, assessments, and policies want the exact breakdown. Schools, occupational therapy, early intervention, research, and compliance forms often ask for "age as of [date]." Calculating chronological age—or finding chronological age—means getting that number. Here's how to do it by hand, when you need it, and when a calculator is better.

How to Calculate Chronological Age by Hand

You subtract the birth date from the "as of" date (e.g. test date or today), in three steps: years, then months, then days, with borrowing when a subtraction is negative.

  1. Years: Assessment year minus birth year.
  2. Months: Assessment month minus birth month. If negative, subtract 1 from years and add 12 to months.
  3. Days: Assessment day minus birth day. If negative, subtract 1 from months and add the number of days in that month (watch for different month lengths and leap years).

Example. Birth date: March 15, 2020. As-of date: February 10, 2026. The child hasn't had their sixth birthday yet, so chronological age is 5 years. For months: February (2) − March (3) is negative, so 5 − 1 = 4 years and 2 + 12 = 14 months. For days: 10 − 15 is negative, so 14 − 1 = 13 months and 10 + 31 = 41 days (January has 31). Result: 5 years, 11 months, 26 days (after adjusting 41 days into 1 month and 26 days). That's calculating age from date of birth by hand—doable, but easy to slip on leap years or month lengths. For forms or assessments, using an age calculator by date of birth is more reliable.

When You Need Exact Age: Occupational Therapy and More

How to calculate chronological age for occupational therapy: Many OT evaluations, tool choices, and forms need "age at evaluation" in years, months, and days. Developmental norms and eligibility often depend on it. Enter the child's birth date and the evaluation date into a calculator; use the result in your documentation or to select the right protocol.

Preschool and enrollment often use age cutoffs or bands. A "preschool age calculator" need is the same: birth date plus cutoff or referral date → chronological age for eligibility or placement. Finding chronological age for these cases is just that one calculation.

Premature babies: Many forms ask for adjusted age. Use a "premature adjusted age calculator" approach: in our tool, use the due date (not birth date) as the start date. You get adjusted age in years, months, and days—what early intervention and developmental screenings often want.

For any form, HR requirement, or compliance need that asks for "age in years, months, and days," calculating chronological age for testing or paperwork is the same: two dates in, one result out. An age calculator by date of birth handles it in seconds.

Chronological Age vs. Biological Age

Chronological age is time since birth—what we're calculating here. How to calculate biological age is different: it's usually based on biomarkers, fitness, or other health measures and comes from specific tests or models, not from birth date and a calendar. When a form or assessment asks for "age in years, months, and days," they mean chronological age. Biological age is a separate concept and outside the scope of this article.

When to Use a Calculator Instead of Doing It by Hand

Doing the math by hand is error-prone: leap years, varying month lengths, and borrowing rules can lead to being off by a few days. For scoring, eligibility, or anything that might be checked, a free chronological age calculator is safer. Enter the two dates, get years, months, days, and total days, and copy the result. Our tool also supports adjusted age (use due date as start), so it works for OT, preschool, prematurity, and testing in one place.

Bottom Line

Calculating chronological age from date of birth has a clear formula and you can do it by hand. But for OT, preschool, prematurity, assessments, and forms, using an age calculator by date of birth is faster and less error-prone. Get the exact age you need, then focus on the evaluation or paperwork. If you're using specific assessments like Pearson, Brigance, or Cornell, see our guide to using one chronological age calculator for testing across all of them.