How to Calculate Your Age in Years, Months and Days

Working out your age sounds like a subtraction, and it nearly is. The catch is that years, months and days are not the same size, so a plain subtraction of two dates can be off by a month or a few days. This guide shows the method that always gives the right answer, and the edge cases that trip people up.

The Method: Subtract, Then Borrow

Write both dates as year, month, day and subtract each part, starting with days.

Take a date of birth of 15 August 1995 and a reference date of 5 October 2026.

• Days: 5 − 15 is negative, so borrow one month. That adds the length of the previous month to the day count. September has 30 days, so 5 + 30 − 15 = 20 days, and the month count drops by one.
• Months: October is month 10, minus the borrowed one is 9, and August is month 8, so 9 − 8 = 1 month.
• Years: 2026 − 1995 = 31 years.

The age is 31 years, 1 month and 20 days. You can check it by counting forward: 15 August 1995 plus 31 years is 15 August 2026, plus 1 month is 15 September 2026, plus 20 days is 5 October 2026.

Why the Borrow Step Matters

The number of days you borrow is the length of the month before the reference date, and that changes the answer. Count from 28 November 2025 to 3 March 2026:

• Months from 28 November to 28 February are 3.
• From 28 February to 3 March in a non-leap year is 3 more days.

So the answer is 3 months and 3 days. If the reference year were a leap year, 29 February exists and the same span would be 3 months and 4 days. Anyone who assumes every month has 30 days will be off by a day or two, which is why it is best to let a tool count the real calendar. Our Age Calculator does this for any date of birth.

Counting Total Days Instead

Sometimes you want a single number, such as days alive or days between two events. The simplest way to check one is to remember that a non-leap year has 365 days and each leap year adds one. From 1 January 2000 to 1 January 2026 is 26 years containing 7 leap days (2000, 2004, 2008, 2012, 2016, 2020, 2024), so 26 × 365 + 7 = 9,497 days.

For the example above, 15 August 1995 to 5 October 2026 is 11,374 days. For any two dates, the Date Difference Calculator returns the span in days, weeks, months and years, which is useful for notice periods, warranty windows and project deadlines.

Leap-Day Birthdays

Someone born on 29 February has a real birthday only in leap years. In other years, the date they are treated as turning a year older depends on where they are: some places use 28 February and others use 1 March. If an exact legal age matters, such as for a contract, an exam cut-off or a pension, check the rule for your own jurisdiction rather than assuming.

Different Ways of Counting Age

Not every culture counts age the same way. Most of the world uses completed years, which is what this guide calculates. Some traditional East Asian systems count a person as one year old at birth and add a year at the new year. South Korea moved its official age to the international standard in June 2023, so documents there now use completed years too. When filling in a form, use the definition the form asks for.

Everyday Uses

• School and exam eligibility: many cut-offs are set as an age on a specific date, not a year of birth, so day-level accuracy matters.
• Insurance and retirement: premiums and eligibility often depend on age on the nearest or last birthday.
• Pets and children: age in months is the useful unit in the first years of life.
• Milestones: working out the date you will turn 10,000 days old, or how many days until your next birthday.

Common Mistakes

• Subtracting only the years and ignoring whether the birthday has happened yet this year.
• Assuming every month has 30 days.
• Forgetting that the time of day can matter when you count hours or minutes.
• Mixing day-month-year and month-day-year formats. 05/10/2026 is 5 October in India and 10 May in the United States.

Quick Checklist

• Write both dates as year, month, day.
• Subtract days. If negative, borrow the length of the previous month.
• Subtract months. If negative, borrow 12 months from the years.
• Subtract years.
• Verify by counting forward from the date of birth.

Or skip the arithmetic and enter the dates into the Age Calculator. It handles borrows and leap years automatically.