How to Calculate Age Exactly: Years, Months and Days

By Syed M. Tahir · Published · 6 min read

Most of the time, knowing someone's age in whole years is enough. But forms, school admissions, sports categories, insurance, visas and many other situations ask for an exact age on a particular date, sometimes down to the day. Working that out by hand is easy to get slightly wrong, because months have different lengths and leap years add an extra day. This guide shows a reliable method, explains the awkward cases and gives spreadsheet formulas you can use.

The basic method

An exact age is counted in three steps, in this order:

  1. Whole years. Count how many birthdays have happened up to the date you are measuring to. If the birthday this year has not happened yet, it does not count.
  2. Whole months. From the most recent birthday, count how many times the same day number has come round in later months.
  3. Remaining days. Count the days from that point to the end date.

Worked example

Someone was born on 15 May 1990. How old are they on 22 October 2026?

  • Their most recent birthday was 15 May 2026, their 36th. So: 36 years.
  • From 15 May 2026, the 15th has come round in June, July, August, September and October. So: 5 months, reaching 15 October 2026.
  • From 15 October to 22 October is 7 days.

The exact age is 36 years, 5 months and 7 days. Our age calculator gives the same answer and also shows the total in days, weeks and months.

A second example where the day "borrows"

Born 28 August 2010. What is the age on 3 March 2026?

  • The 2026 birthday (28 August) has not happened yet, so the last birthday was 28 August 2025: 15 years.
  • From 28 August 2025, the 28th comes round in September, October, November, December, January and February: 6 months, reaching 28 February 2026.
  • From 28 February to 3 March 2026 is 3 days (2026 is not a leap year, so March 1 is the next day after February 28).

Answer: 15 years, 6 months and 3 days. Notice that we counted forward from the birthday rather than subtracting day numbers. Subtracting 28 from 3 would give a negative number and force you to guess how many days to "borrow" from the month, which is where many manual mistakes happen.

Month-end dates

What if someone was born on the 31st, and a later month has only 30 days, or February has only 28? A common, sensible rule, and the one our calculator uses, is that the month is complete on the last day of the shorter month. So from 31 January, one month is complete on 28 February (or 29 February in a leap year), and two months are complete on 31 March.

Different software can handle this edge case differently, which is why two calculators sometimes disagree by a day or two for these dates. The difference only affects the months-and-days part. The number of whole years, and the total number of days, are the same in every correct method.

Leap years and 29 February birthdays

In the Gregorian calendar used in most of the world, a leap year has 366 days, with 29 February added. The rule is:

  • A year divisible by 4 is a leap year,
  • except years divisible by 100, which are not,
  • except years divisible by 400, which are.

So 2000 and 2024 were leap years, 2026 is not, and 2100 will not be.

People born on 29 February only have that date on their calendar about once every four years. For counting age in common years, there are two conventions: treat 28 February as the birthday, or treat 1 March as the birthday. Our calculator uses 28 February, which matches the month-end rule above. Some laws and organisations use 1 March for things like reaching a legal age. If a decision depends on the exact day, check the rule that applies in your country or with the organisation concerned.

Counting total days

Sometimes you need an age or a period as a single number of days, for example for a contract, a medical form or simply out of curiosity. The total is the number of calendar days between the two dates. You do not count the start date as day one: on the day someone is born, they are 0 days old. Every leap day in between adds one to the total.

For long periods, counting by hand is impractical, so use a calculator or a spreadsheet. As a rough check, multiply the years by 365.25. For 36 years that gives about 13,149 days, and the exact figure for the example above is 13,309 days, because it also includes the extra 5 months and 7 days.

Calculating age in Excel or Google Sheets

Put the date of birth in A2 and the "as of" date in B2 (or use TODAY()). The DATEDIF function, available in Excel and Google Sheets, returns each part:

  • Whole years: =DATEDIF(A2,B2,"Y")
  • Months after the last birthday: =DATEDIF(A2,B2,"YM")
  • Days after the last whole month: =DATEDIF(A2,B2,"MD")
  • Total days: =B2-A2 (format the cell as a number)

Microsoft's documentation warns that the "MD" option can give inaccurate results in some cases, particularly around month ends, so double-check those dates. For whole years, "Y" is reliable and is the most common need.

Common mistakes to avoid

  • Subtracting birth years only. 2026 − 1990 = 36, but if the birthday has not happened yet this year, the person is still 35.
  • Dividing total days by 365. This drifts by about a day every four years because of leap years, and can give the wrong age close to a birthday.
  • Mixing date formats. 03/04/2026 is 3 April in many countries and 4 March in the United States. Using the year-month-day order (2026-04-03) avoids confusion.
  • Using the wrong reference date. Many applications ask for age "on" a specific date, such as 1 September or the closing date, not today.
  • Ignoring time zones for births near midnight. For almost every purpose, the date on the birth certificate is what counts, regardless of where you are now.

Different ways of counting age

The method above is the international convention: age starts at zero and increases by one on each birthday. Some traditional systems count differently. For example, in some East Asian traditions a person is considered one year old at birth and gains a year at the new year rather than on their birthday. Official documents in most countries use the international method, which is what this guide and our calculator follow.

Sources

This guide is general information for sellers and is not financial, tax or legal advice. Marketplace rules and fees change, so always confirm in your own seller account. DailyToolLab is not affiliated with Etsy Inc., Shopify Inc. or eBay Inc.