Age Calculator
Pick a starting point (a birth year, a full date of birth, or a known age) and the calculator works out the rest. With a full date you get an exact age down to the day.
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Created by the Reeliy Editorial Team. Last updated: Oct 19, 2026
How to use the age calculator
The calculator offers three starting points, chosen with the radio buttons at the top of the form.
- Year of Birth. Use this when you only know the year. Enter the birth year and the year you want the age for; the answer comes as two possibilities, depending on whether the birthday has already passed.
- Date of Birth. Enter the full birth date and the date to measure on. Leave the second date empty to measure up to today. This gives the exact age in years, months and days, together with the totals in months, weeks and days.
- Current Age. Enter how old someone is now and the calculator returns the year they were born in.
Working out an exact age from a date of birth
An exact age counts complete years first, then complete months, then the days that are left over. Each unit only ticks over once the matching day of the month has been reached, which is why subtracting the birth year from the current year is wrong for anyone whose birthday is still to come.
Example. Someone born on 15 May 1990 is measured on 19 October 2026. The birthday on 15 May 2026 has passed, so that is 36 complete years. From 15 May to 15 October is 5 more months, and 15 to 19 October adds 4 days. The age is 36 years, 5 months and 4 days.
The same span expressed in single units: 36 × 12 + 5 = 437 months; 13,306 days between the two dates; and 13,306 ÷ 7 = 1,900 whole weeks with 6 days to spare.
age in months = years × 12 + monthsWeeks are the whole-day total divided by 7, rounded down.
Births at the end of a month
Months do not all have the same number of days, so a birth on the 29th, 30th or 31st needs a rule. The calculator adds whole months to the birth date and lets the date roll forward when the day does not exist, then counts the days still left to the target date. For a birth on 31 January 1990 measured on 1 March 2026, one month after 31 January 2026 would be 31 February, which rolls over to 3 March and overshoots. So the answer is 36 years, 0 months and 29 days.
A leap-day birth works the same way. Someone born on 29 February 2000 and measured on 19 October 2026 is 26 years, 7 months and 20 days old: adding 26 years and 7 months lands on 29 September 2026, and 20 days then reach 19 October.
Age from the year of birth alone
With only a year, the age can be off by one, because the birthday may or may not have happened yet. For the current year the calculator shows both outcomes: the lower number if the birthday is still ahead, the higher number if it has already passed. For years in the past it reports the age reached during that year, and for a future year the age that will be reached.
Written as a rule: age = target year − birth year, minus one if the birthday has not yet occurred.
| Born in | Before the 2026 birthday | After the 2026 birthday |
|---|---|---|
| 2015 | 10 | 11 |
| 2005 | 20 | 21 |
| 1995 | 30 | 31 |
| 1985 | 40 | 41 |
| 1970 | 55 | 56 |
| 1950 | 75 | 76 |
Chronological age versus running age
Chronological age (also called completed or international age) is the number of full years lived. It is what passports, school forms and most medical records use. Running age is the number of the year of life someone is currently in, which is one more: a person who has just turned 30 has a chronological age of 30 and is in their 31st year of life.
Some traditional systems in East Asia count a newborn as one year old and add a year at each new calendar year or birthday, which is closer to a running count. The calculator shows both so you can compare, for example, "Turned 30 years old in 2020" alongside "The birthday in 2020 began the 31st year of life".
Finding a birth year from an age
If you know how old someone is today, the birth year is the current year minus the age, or one less if their birthday has not come round yet. In 2026, a 30-year-old was therefore born in 1996 if the birthday has passed, or in 1995 if it has not. The calculator lists both years and lets you pick.
How many days old are you?
Choose Date of Birth and read the "Age in days" row. Round-number milestones are popular to mark; an average Gregorian year lasts 365.2425 days, so the number of days converts to years as follows.
| Days lived | Approximate age in years | Whole weeks |
|---|---|---|
| 1,000 | 2.74 | 142 |
| 5,000 | 13.69 | 714 |
| 10,000 | 27.38 | 1,428 |
| 20,000 | 54.76 | 2,857 |
| 30,000 | 82.14 | 4,285 |
A 10,000th day therefore falls a little over 27 years and 4 months after birth. The birthday calculator can tell you which weekday you were born on and how long until the next birthday.
Ages of buildings, companies and other things
Nothing in the calculation is specific to people. Enter a construction date, a founding date or the day a product was made to find how old a building, a business or a device is. For comparing two ages directly, use the age difference calculator, and for a pet's equivalent human age try the dog years calculator.
Date inputs follow your browser's regional format. If a date looks wrong, check that the day and month are not swapped.
Common mistakes
- Subtracting years only. Born in 1990 does not always mean 36 in 2026; it depends on the birthday.
- Mixing up chronological and running age on forms that ask for "age".
- Assuming every month has 30 days when converting months to days. Use the days total from the calculator instead.
- Entering an end date earlier than the birth date, which the calculator rejects.
For measuring the time between two arbitrary dates rather than a lifetime, see the how many years calculator and the chronological age calculator.
Frequently asked questions
How is age calculated from a date of birth?
Count the complete years between the birth date and the target date, then the complete months, then the leftover days. A year only counts once the birthday has been reached.
How old am I if I was born in 1990?
Subtract 1990 from the current year, then subtract one more if your birthday has not happened yet this year. In 2026 that means 35 before the birthday and 36 after it.
What is the difference between chronological age and running age?
Chronological age is the number of full years lived. Running age is the year of life you are in, which is one higher, so someone aged 30 is in their 31st year.
How can I find out how many days old I am?
Select Date of Birth, enter your birth date and read the Age in days row. Leave the second date empty to measure up to today.
Cite this calculator and page
If you use this page in your work, you can cite it as:
Reeliy Editorial Team. (2026). Age Calculator. Reeliy. https://reeliy.com/calculators/age-calculator“Age Calculator.” Reeliy, 2026, https://reeliy.com/calculators/age-calculator.