ToolPorch / Age Calculator
Chronological Age Calculator for evaluations
Exact years;months;days between birth date and test date — the unrounded borrow-method age that SLPs, OTs, and school psychologists compute for every evaluation. With corrected age for prematurity, and a copy button that pastes straight into your report.
Enter both dates — the age computes instantly.
The borrow method, worked through
Chronological age is a column subtraction with borrowing, done right-to-left: days, then months, then years. Example: born 2019-09-24, tested 2026-08-19.
Days: 19 − 24 is negative, so borrow a month. The month before August has 31 days, so days = 19 + 31 − 24 = 26, and the month column drops by one.
Months: (8 − 1) − 9 is negative, so borrow a year: 7 + 12 − 9 = 10, and the year column drops by one.
Years: (2026 − 1) − 2019 = 6. Chronological age: 6;10;26 — six years, ten months, twenty-six days.
The one subtlety worth knowing is the borrow amount for days: it's the length of the month before the test month (borrowing from July means 31 days; borrowing from March means 28 or 29). Getting that wrong by hand is the classic one-day error — and one day can matter at a norm-table boundary. This calculator uses the true calendar length every time, leap years included.
Rounding: what this tool does and why
It never rounds. Some legacy test protocols instructed examiners to round ages of 15 days or fewer down and 16 or more up to the nearest month; most current manuals want the exact age. Reporting the unrounded value is the safe default because you can always apply a manual's rounding rule to an exact age, but you can't un-round a rounded one. Whatever your test requires, the number you start from should be this one.
Corrected age for prematurity
If a child was born before 37 weeks, many developmental assessments (and most NICU follow-up clinics) use corrected age: chronological age minus the weeks of prematurity, where prematurity is measured against a 40-week term. Enter the gestational age at birth and both numbers appear side by side, computed the same borrow-method way. Convention is to correct until around 24 months of chronological age; whether and how long to correct is ultimately a clinical and test-manual decision, so this tool shows the math and leaves the judgment to you.
Frequently asked questions
How is chronological age calculated?
By the standard borrow method: subtract days first, borrowing a month if needed (using the actual length of the month before the test month), then subtract months, borrowing a year if needed, then years. The result is exact years;months;days with no rounding — this calculator never rounds a day up or down.
Why does "no rounding" matter for testing?
Many standardized assessments select norm tables by exact age bands, and being one month off can shift a raw score into the wrong band. Some older manuals used day-rounding conventions; most current ones don’t. This tool always reports the unrounded age — if your specific test manual prescribes a rounding rule, apply the manual’s rule to this exact result.
What happened to the Pearson age calculator?
Pearson retired its standalone chronological age calculator; its old page now redirects visitors elsewhere. Evaluators who relied on it need an exact, unrounded years;months;days calculation with a test-date field — which is precisely what this calculator does, free, with nothing to install.
How does corrected (adjusted) age work?
For children born prematurely, corrected age subtracts the weeks of prematurity (40 weeks minus gestational age at birth) from chronological age. Enter gestational age and the calculator reports both ages. Corrected age is conventionally used until about 24 months chronological — and whether to use it at all is a clinical decision that belongs to your assessment’s manual and your judgment.
Is anything I enter stored or sent anywhere?
No. The math runs entirely in your browser; birth dates are never transmitted or saved. Refreshing the page clears everything.