How do I calculate age from a date of birth?

Subtract the birth year from the current year, then adjust down by one if the birthday has not yet occurred this year. That gives your age in completed years.

Why does my age change on the actual birthday?

Legal and most everyday age counts completed years. Your age only increases on the date of birth each year, because that is when you complete another full year of life.

Do leap years affect my age calculation?

Only in the total day count. Because a leap year has 366 days, someone born on February 29 has an official birthday every four years but celebrates on February 28 or March 1 in other years.

How do I show age in years, months and days?

Subtract the birth date from the current date column by column: years first, then months, then days. If a column is negative, borrow from the larger unit above it.

How to Calculate Age

Knowing your exact age is useful well beyond filling out forms. It matters for school admissions, job eligibility, retirement planning, medical dosing, and even legal documents. But calculating age from a date of birth is not always as simple as subtracting one year from another — months and days, and even birthdays, get in the way. This guide shows you how to calculate age accurately, how to express it in years, months, and days, and why leap years matter.

On the surface it seems trivial: subtract the birth year from the current year. But that only gives a rough figure. If your birthday has not happened yet this year, the simple subtraction overstates your age by one year. And if you want a precise figure — like a doctor's exact age or a document that asks for days — you need a more careful method. Let's build from the simplest approach up to the most precise.

Age in Years, Months, and Days

The most accurate way to state an age is to break it into three parts. To do this, subtract the birth date from today's date column by column — years first, then months, then days.

Suppose someone was born on 15 March 1995 and today is 5 September 2026.

Years: 2026 − 1995 = 31
Months: 9 (September) − 3 (March) = 6
Days: 5 − 15 = −10

Because the days are negative, you need to borrow. Borrow one month (August) which has 31 days, so 5 + 31 = 36, minus 15 gives 21 days. You then reduce the month count by one, from 6 to 5. The result is 31 years, 5 months, and 21 days.

This borrowing is the trickiest part of the calculation. The number of days in the month you borrow from matters — July has 31 days, April has 30, and February has 28 or 29. If you borrow from a 30-day month, you add 30; if you borrow from February in a leap year, you add 29. Keep that in mind, and the column-by-column method always works.

Age expressed this way is precise and is how most official documents state it. If you only want the number of completed years — the way most people say their age — you simply use whole years and ignore the months and days.

How to Calculate Age from Date of Birth

For a quick "completed years" figure, use this simple method:

Age = Current year − Birth year, then subtract 1 if the birthday has not happened yet this year.

Let's use the example above. Born in 1995, and today is in 2026. That is 31 years. But since their birthday was in March and today is September, they have already turned 31 — so the age is 31.

If today were instead 5 February 2026, the birthday would not yet have passed. Then the age would be 2026 − 1995 = 31, minus 1, giving 30. That single adjustment is how you avoid the most common age-calculation mistake.

Here is the key point: your age only increases on the actual birthday. Your "birthday age" is the number of complete years you have lived. Even though a newborn is technically in their first year of life, their stated age is 0 until their first birthday. That is why a person born in December may still be counted as one year younger than someone born in January of the same year, even before either has celebrated a birthday.

Performing this subtraction by hand, especially with complex dates and month-length variations, is easy to get wrong. Our age calculator handles the full years, months, and days breakdown instantly and accurately.

Why Leap Years Matter

A leap year adds one day to the calendar — February 29 — every four years (with a few exceptions at the turn of centuries). Most of the time leap years don't change your age in years or months. But they do affect two things.

Total day count: If you calculate age in total days, someone born on a leap day will have lived through a 366-day year. Most calculators simply count each calendar day from birth to today, and leap years naturally add one extra day.
February 29 birthdays: People born on February 29 have an official birthday only every four years. In the intervening years they typically celebrate on February 28 or March 1. The same rule applies to the borrowing step: if you subtract a February 29 birthday in a non-leap year, you may need to borrow a day, which can shift the day count by one.

So when you state an age in days, a leap year quietly makes the number slightly larger. When you state an age in years, it has no effect at all. Understanding how to calculate age — in years, in months, or in total days — helps you pick the right measure for the job, whether you are filling out a form or planning for the long term.