How our tools work
Methodology: How Our Age Calculators Work
The exact methods, formulas and sources behind every calculator on this site, so you can check our results and know when to rely on them.
By Sandeep · Updated November 2026
Our Principles
- Published methods only. Every calculator follows a method from a test manual, medical guideline or peer-reviewed study, named below.
- Real calendar math. We use actual month lengths and leap years, not averages such as 30 days per month or 365.25 days per year.
- Show the working. Where it helps, results include the steps, so you can check them by hand.
- Private by design. Calculations run in your browser. The dates and values you enter are not sent to us or stored.
Chronological Age
Our chronological age calculator subtracts the date of birth from the test date in years, months and days. If the days are negative, it borrows the number of days in the month before the test month; if the months are negative, it borrows 12 months from the years. This matches the method taught in test manuals and used by most scoring software. Dates are handled in UTC so time zones and daylight saving cannot shift a result by a day. See how to calculate chronological age.
Some tests round the age (for example, 15 days or more rounds up a month) and others never round. Our testing tools show both the exact and rounded age; always follow your test manual. See age rounding rules.
Corrected Age
Corrected age = chronological age − (40 weeks − gestational age at birth). We apply it for babies born before 37 weeks, as is standard in pediatric practice, and show a reminder that most clinicians stop correcting at about 24 months.
Age in Months, Weeks and Days
Total months = years × 12 + months, using the same borrowing method. Weeks and days are counted from the exact number of days between the two dates.
School Age
The school age calculator compares a child’s birth date with the kindergarten cut-off date you choose. Cut-off dates vary by state and district, so we list common examples and recommend checking with your local school.
Developmental Age and Percent Delay
Percent delay = (chronological age − developmental age) ÷ chronological age × 100, with ages in months and corrected age used for premature babies. This is the formula used by many US early intervention programs; eligibility thresholds vary by state.
Mental Age and Ratio IQ
Ratio IQ = mental age ÷ chronological age × 100 (Stern, 1912; Terman, 1916). Because the ratio is not valid for adults, we cap adult chronological age at 16, as early Stanford-Binet editions did, and explain that modern tests use deviation IQ instead.
Biological Age (PhenoAge)
Our biological age calculator uses the Phenotypic Age model from Levine and colleagues (Aging, 2018), which combines chronological age with nine blood markers: albumin, creatinine, glucose, C-reactive protein, lymphocyte percentage, mean cell volume, red cell distribution width, alkaline phosphatase and white blood cell count. Units are converted to those used in the original paper before the formula is applied. The result is an estimate for education, not a diagnosis.
How We Test Our Calculators
- Each calculator is checked against worked examples, including leap years, month-end birthdays and dates in the same month.
- Where a test publisher or study gives sample results, we confirm that our output matches.
- Inputs are validated, for example years must be between 1900 and 2100 and the test date must be after the birth date.
Limitations
Our tools are for information and education. They do not replace a test manual, a qualified examiner or medical advice. If a result will be used for a diagnosis, eligibility decision or legal purpose, confirm it with the official method or a professional. If you find an error, please tell us on the contact page; see our editorial policy for how we handle corrections.