Chronological Age Calculator
Enter a date of birth and the date to measure on to get an exact chronological age in years, months and days, plus total months, weeks, days and hours, decimal age and the next birthday.
Defaults to today. Change it for a test date, a school cut-off or any other day.
What a chronological age calculator does
Chronological age is simply the time that has passed since you were born, as opposed to developmental age, bone age or reading age, which describe how someone is doing compared with peers. A chronological age calculator counts that time precisely in years, months and days rather than rounding to the nearest birthday. That precision matters for school cut-offs, standardized tests whose norms are grouped by age in months, speech and developmental assessments, pediatric growth charts and legal paperwork.
The calculation starts at the date of birth and steps forward one calendar year at a time, then one month at a time, for as long as it stays on or before the "age on" date. Whatever is left is counted in days. Everything runs on calendar dates, so time zones and daylight-saving changes cannot shift the answer by a day.
Worked example
A child born June 15, 2018 is tested on October 20, 2026. The last birthday before the test was June 15, 2026, so she is 8 years old. From June 15 to October 15 is 4 whole months, and October 15 to October 20 is 5 days. Her chronological age is 8 years, 4 months, 5 days, written on an assessment form as 8;4. In total that is 3,049 days, 100 whole months, or a decimal age of 8.35 years.
The years;months notation on assessment forms
Speech-language, psychological and educational tests (the PLS, CELF, WISC and similar) report norms by age bands such as 7;0–7;5, where the number before the semicolon is years and the number after is months. Days are dropped, not rounded: a child who is 7 years, 5 months and 29 days old is 7;5, not 7;6. Some test manuals say to round up when the days are 15 or more; follow your manual if it does, since using the wrong band changes the standard score.
Decimal age and total months
Growth charts and research papers often want age as a decimal. The convention used here, and by the CDC growth-chart software, is total days divided by 365.25. Total months (used for infants and toddlers) is the whole number of calendar months that have passed, so a baby born on the 15th is 1 month old on the 15th of the next month regardless of whether that month had 28 or 31 days.
Leap-day birthdays and other edge cases
Someone born February 29 has a real birthday only every four years. This calculator counts February 28 as the birthday in other years, so a child born February 29, 2020 turns one on February 28, 2021. Some jurisdictions and some families use March 1 instead; if that is your rule, the age shown is one day high on that one day. Births on the 31st work the same way: a month that has no 31st counts its last day as the monthly anniversary. If the "age on" date is a future date, the calculator shows how old the person will be then, which is handy for enrollment and eligibility questions.
Frequently asked questions
›What is chronological age?
The time that has passed since birth, counted in whole years, months and days. It is different from developmental, bone or reading age, which compare a person with peers.
›How is age in years, months and days calculated?
Count whole years from the birth date to the last birthday on or before the target date, then whole months, then the remaining days. Someone born June 15, 2018 is 8 years, 4 months and 5 days old on October 20, 2026.
›What does 7;4 mean on an assessment?
7 years and 4 months. Test manuals write chronological age as years;months and drop the days. A child who is 7 years, 4 months and 29 days is 7;4 unless the manual says to round up.
›How is decimal age calculated?
Total days since birth divided by 365.25, the convention used by CDC growth-chart software. 3,049 days is 3,049 ÷ 365.25 = 8.35 years.
›How does the calculator handle a February 29 birthday?
In years without a February 29 the birthday is counted on February 28, so someone born February 29, 2020 turns one on February 28, 2021. Some jurisdictions use March 1 instead, which makes the age one day lower on that day.
›Can I calculate age on a future date?
Yes. Set the age-on date to any future date, such as a school cut-off or test date, and the result shows how old the person will be then.