Quick answer: To calculate age, subtract the birth year from the current year, then subtract one if the birthday has not happened yet this year. For an exact age in years, months and days, subtract each part separately and borrow from the month or year when a part goes negative. An age calculator does the borrowing for you and can also give an age on any chosen date.
Try the free Age Calculator
Free, no sign-up, and built by the author of this guide.
Open the Age CalculatorThe method in three steps
Write both dates as year, month and day. Then work from right to left:
- Days: subtract the birth day from the end day. If the result is negative, borrow one month: subtract 1 from the months and add the number of days in the month before the end date's month.
- Months: subtract the birth month from the end month. If the result is negative, borrow one year: subtract 1 from the years and add 12 to the months.
- Years: subtract the birth year from the end year, after any borrowing.
For age in whole years only, the shortcut is simpler: end year minus birth year, minus one if the end date falls before the birthday in that year.
How to use the free Age Calculator
- Pick a date in the Date of birth field.
- Optionally pick a date in the Calculate age on field. Leave it empty to calculate the age as of today.
- Click Calculate age.
- Read the result, shown as years, months and days, for example 24 years, 8 months and 18 days.
- Click Copy result to copy the text.
If the date of birth is missing, invalid or later than the end date, the tool shows Enter a valid date of birth. The calculator gives an exact age in years, months and days. It does not show a total in days or weeks, the next birthday, or a zodiac sign. The calculation runs in your browser, so the dates are not sent anywhere.
Worked example
Someone was born on 20 June 2001. How old are they on 10 March 2026?
| Step | Working | Result |
|---|---|---|
| Subtract days | 10 − 20 | −10, so borrow a month |
| Borrow a month | February 2026 has 28 days: −10 + 28 | 18 days |
| Subtract months | 3 − 6 − 1 (borrowed) | −4, so borrow a year |
| Borrow a year | −4 + 12 | 8 months |
| Subtract years | 2026 − 2001 − 1 (borrowed) | 24 years |
The answer is 24 years, 8 months and 18 days, which matches the tool when you enter 2001-06-20 and 2026-03-10. You can check it by counting forward: 24 years after 20 June 2001 is 20 June 2025, 8 months later is 20 February 2026, and 18 days later is 10 March 2026.
Leap years and month ends
Months have 28, 29, 30 or 31 days, so “one month” is not a fixed length. That is why the borrowing step uses the actual length of the month before the end date. Under the Gregorian calendar, a year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400. So 2000 was a leap year and 1900 was not.
People born on 29 February have a real birthday only in leap years. Using the method above, someone born on 29 February 2004 is 20 years, 11 months and 30 days old on 28 February 2025 and 21 years, 0 months and 0 days on 1 March 2025. For legal purposes, the date a leap-day birth counts as a new year of age depends on the law of each country, so check the rule that applies to you.
Birthdays on the 29th, 30th or 31st can also give different day counts in different calculators when the end date falls just after a short month. If the exact number of days matters, check the result by counting forward from the birth date as in the worked example.
Common mistakes
- Subtracting years only. 2026 − 2001 = 25, but the person is still 24 until their birthday in June.
- Assuming every month has 30 days. This gives day counts that are off by one or two around short and long months.
- Mixing date formats. 03/04/2026 is 3 April in many countries and March 4 in the United States. The tool's date picker avoids this, but check dates copied from documents.
- Choosing an end date before the birth date. The calculator rejects this and asks for a valid date of birth.
- Forgetting the as-of date. For an eligibility cut-off, enter that date in Calculate age on instead of using today.
When an exact age matters
Exact ages are used for school admission cut-offs, sports age groups, insurance, pensions, visas and medical dosing. In each case, the organization sets the reference date, for example the age on 1 September or on the day of the event. Enter that date in Calculate age on and keep a copy of the result together with the dates you used, so anyone can repeat the calculation.
Frequently asked questions
How do I calculate my age in years only?+
Subtract your birth year from the current year, then subtract one more if your birthday has not happened yet this year.
Can I calculate age on a future or past date?+
Yes. Enter the date in the Calculate age on field. Leave it empty to calculate age as of today.
How is age calculated for someone born on 29 February?+
The tool counts the new year of age from 1 March in common years. Legal rules for leap-day birthdays vary by country, so check the rule that applies to you.
Why does the calculator say Enter a valid date of birth?+
The date of birth is missing, is not a valid date, or is later than the end date. Correct the dates and click Calculate age again.
Does the tool show age in total days?+
No. It shows age as years, months and days only.