To calculate age from a date of birth, compare the birth date with the date you want to calculate against. Work out complete years first, then complete months, then the remaining days. The calendar matters because months have different lengths and leap years add an extra day.
Quick answer: Enter the date of birth and the target date into an age calculator to find the completed years, months, and days between them.
How to calculate age manually
- Subtract the birth year from the target year.
- Subtract the birth month from the target month. If the result is negative, subtract one from the years and add 12 to the months.
- Subtract the birth day from the target day. If the result is negative, borrow the number of days in the previous calendar month and subtract one month.
Worked example
Suppose a person was born on May 14, 1995, and the age is being calculated on February 5, 2025.
- Year difference: 2025 − 1995 = 30. Since February is before the May birthday, reduce this to 29 completed years.
- Month difference after borrowing one year: February (2) − May (5) + 12 = 9 months. Because the day 5 is before day 14, borrow one month, leaving 8 months.
- Day calculation: January has 31 days. Borrowing those days gives 31 + 5 − 14 = 22 days.
The exact calendar age is 29 years, 8 months, and 22 days.
Why birthdays and leap years matter
A person does not complete a new year of age until the relevant birthday has been reached. February may have 28 or 29 days depending on the year. A calendar-based age calculation must account for the actual month and leap-year dates instead of treating every month as 30 days.
Age in days is a different calculation
Age in completed years, months, and days is not the same as a total count of days. A total-day calculation counts the number of calendar days between two dates. The answer can depend on whether the start date is included. When comparing legal, school, travel, or eligibility dates, follow the rule used by the relevant organisation.
Calculate your age instantly
Use the Calclio Age Calculator. Enter your date of birth and the target date, then review the result in years, months, and days. Double-check the date format (month/day versus day/month) before calculating.
Frequently asked questions
Can I calculate age for a future date?
Yes. Use the future date as the target date. The result is the age the person will have reached on that date.
Does a leap-year birthday change age?
Age still increases according to the applicable birthday convention, but some organisations may have specific rules for people born on February 29. Follow the relevant legal or administrative rule when eligibility depends on it.
Why can two websites show a different number of days?
They may use different date formats, time zones, start-date inclusion rules, or day-count conventions. Compare the inputs and counting method.
This tool is for general date calculations. Check official rules where age determines legal rights, eligibility, or deadlines.