Calculate Age From a Date of Birth: A Step-by-Step Walkthrough

Most people never calculate an age from scratch — they know their own and guess everyone else's. But forms, eligibility rules and family arguments occasionally demand an exact answer, and that is where a clear method helps.

The three numbers you need

An exact age is really three numbers: completed years, completed months, and leftover days. To find them you compare two dates: the date of birth and the "as of" date (usually today). Write each as day, month and year.

A worked example

Suppose someone was born on 23 August 1994 and you want their age on 9 March 2026.

1. Subtract the years: 2026 − 1994 = 32.
2. Subtract the months: 3 − 8 = −5. A negative number means the birthday hasn't arrived yet this year, so borrow one year: years become 31, months become −5 + 12 = 7.
3. Subtract the days: 9 − 23 = −14. Negative again, so borrow one month: months become 6, and add the length of the month before the as-of month. February 2026 has 28 days, so days become −14 + 28 = 14.

Result: 31 years, 6 months, 14 days.

Which month length do you borrow?

This is the step that trips people up. When you borrow days, you use the length of the month preceding the as-of date's month — here February, not March. Using the wrong month can shift the answer by up to three days, which rarely matters socially but matters a great deal when a cutoff falls in that window.

Check your work by counting forward

A good sanity check is to reverse the calculation. Start from the birth date, add 31 years (23 August 2025), add 6 months (23 February 2026), then add 14 days (9 March 2026). If you land on the as-of date, the answer is right.

Where people go wrong

• Forgetting to borrow a year when the birthday hasn't happened yet.
• Treating every month as 30 days.
• Ignoring leap years when the range spans 29 February.
• Mixing up day/month order when reading dates written as 03/09/1994.

When to skip the manual method

Working through this once is a good way to understand it, but for anything official, use a tool that applies the same rules without slips. Enter the date of birth, choose the as-of date, and read off the years, months and days.

Get an exact age instantly with Age Calculator →