How to Calculate Age From Date of Birth

The Correct Method

Chronological age is the number of complete years that have passed since birth. The word complete is doing the work: a birthday only counts once it has actually arrived.

1. Subtract the birth year from the current year.
2. Check whether this year’s birthday has happened yet.
3. If it has not, subtract one.

That single check is what the year-subtraction shortcut leaves out, and it is wrong for anyone whose birthday falls later in the year than today’s date.

Worked Example

Born 15 September 2000. Today is 4 August 2026.

• 2026 − 2000 = 26
• Has 15 September arrived in 2026? No — today is 4 August, which is earlier.
• So subtract one: the person is 25 years old, not 26.

They turn 26 on 15 September 2026. Until then the naive subtraction overstates their age by a year, which is exactly the error that shows up on badly built forms and spreadsheets.

Age in Months and Days

For a precise age, subtract the full dates rather than just the years, borrowing where a column goes negative in the same way you would with ordinary subtraction.

Take a birth date of 20 June 1990 and today of 4 August 2026. Years: 2026 − 1990 = 36. Months: August is month 8, June is month 6, so 8 − 6 = 2. Days: 4 − 20 is negative, so borrow a month. That month has 31 days, giving 31 + 4 − 20 = 15 days, and the month count drops from 2 to 1. The result is 36 years, 1 month, and 15 days.

The borrowing is where hand calculations go wrong, because the number of days you borrow depends on which month you borrow from. This is precisely the kind of edge case an online calculator handles correctly every time.

Leap-Year Birthdays

People born on 29 February have a birthday that only exists once every four years, which raises a genuine question: when do they age in common years?

There is no single universal answer, and it is more a legal and social convention than a mathematical fact. Most people treat 1 March as the day they add a year in non-leap years, since that is the day after their birthday would have fallen. Some jurisdictions use 28 February for legal purposes such as reaching the age of majority. Either way, the person still ages exactly one year per calendar year; only the label on the celebration moves.

For everyday age calculation, counting complete years from 29 February 2000 to a date in 2026 gives 26 as long as 1 March has passed, which matches how almost everyone treats it.

How Other Cultures Count Age

The method above is international, or Western, age. It is not the only system, and the differences are larger than most people expect.

The traditional East Asian system counted a person as one year old at birth and added a year at each lunar new year rather than on the birthday, which could make someone born just before new year two years old within days. South Korea used a version of this in daily life for generations, then standardised on international age in law in 2023 to remove the confusion of people effectively having several different ages at once.

These systems are not wrong, they simply answer a different question — which calendar year of life you are in, rather than how many complete years have elapsed. If you are calculating age for an official form, use international age unless the form specifically asks otherwise.

Common Mistakes

1. Subtracting years alone and skipping the birthday check, which overstates age for anyone whose birthday has not yet passed this year.
2. Borrowing a fixed 30 days regardless of the actual month, which throws the day count off.
3. Counting the birth day itself inconsistently. By convention you are zero years old on the day you are born.
4. Applying one culture’s age system where another is expected, most often on international documents.
5. Assuming a leap-year birthday breaks the calculation. It does not; only the celebration date is a convention.

Frequently Asked Questions

How do I calculate age from a date of birth?

Subtract the birth year from the current year, then subtract one more if this year’s birthday has not yet arrived. Someone born in September is still the lower number until September comes around, which is the step the simple year subtraction misses.

Why does subtracting the years give the wrong age?

Because it ignores the birthday. Year subtraction assumes the birthday has already happened, which is only true for part of the year. For anyone whose birthday falls after today’s date, the plain subtraction is one year too high until that date passes.

How do I calculate age in months and days?

Subtract the full dates, borrowing when the day or month column goes negative. When borrowing a month, add the number of days that month actually has, not a fixed 30. The borrowing rules are why an online calculator is more reliable for exact ages.

When does a person born on 29 February age up?

By convention, most treat 1 March as the day they add a year in non-leap years, though some legal systems use 28 February. The person still ages one year annually; only the celebration date is a matter of convention rather than mathematics.

What is the difference between Korean age and international age?

The traditional Korean system counted you as one at birth and added a year each new year, making people one or two years older than their international age. South Korea moved to international age in law in 2023. International age counts complete years elapsed since birth.

How old is someone on the day they are born?

Zero years old, by international convention. Age counts complete years that have passed, and none have passed at birth. This is why age calculators return zero years for a date of birth equal to today, then count upward from there.

Does the age calculation change across time zones?

For everyday purposes, no. Age is measured in whole days from a birth date, so the exact hour and time zone of birth rarely matter unless you are comparing two people born hours apart. Calculators use the calendar date rather than the precise moment.

The Bottom Line

Age from a date of birth is year subtraction plus one crucial check: has the birthday happened yet this year? Miss that step and you are wrong for everyone born later in the year than today. For an exact figure, subtract the whole dates and borrow the correct number of days for the month you are borrowing from. Remember that leap-year birthdays and other cultures’ counting systems are conventions layered on top, not flaws in the maths. For an instant, exact answer, the age calculator handles every edge case, and the date difference calculator covers the gap between any two dates.