How to Calculate Age From a Date of Birth by Hand
Most people can say their age without thinking, but forms, eligibility rules and records often ask for something more exact: how many years, months and days old someone is on a given date. The arithmetic is ordinary subtraction with one twist, because years, months and days are not the same size.
This guide shows the quick way to get whole years, the borrow method for the full result, a cross-check using total days, and the edge cases that make two calculators disagree.
Whole years: subtract, then check the birthday
Subtract the birth year from the current year. That gives the age the person will turn this year. If this year's birthday has not happened yet, take one away.
Worked example: born 28 November 1990, and today is 19 October 2026. The year difference is 2026 − 1990 = 36. The birthday on 28 November is still ahead, so the age is 35. A person only turns 36 on 28 November 2026.
- Compare month first. If the current month is earlier than the birth month, the birthday has not happened.
- If the months match, compare the day of the month.
- On the birthday itself, the person has already reached the new age.
Years, months and days with borrowing
For an exact age, write both dates as year, month and day, then subtract each column. It works like column subtraction on paper: when a column goes negative, borrow from the next column to its left.
age = (end year − birth year) years, (end month − birth month) months, (end day − birth day) daysFix any negative column by borrowing, starting with days.
Worked example, same dates: 2026-10-19 minus 1990-11-28 gives 36 years, −1 month and −9 days.
- The days column is negative, so borrow a month. Add the length of the month before the end date's month. That is September, which has 30 days: −9 + 30 = 21 days, and the months column drops to −2.
- The months column is now negative, so borrow a year. Add 12 months: −2 + 12 = 10 months, and the years column drops to 35.
- The result is 35 years, 10 months and 21 days.
To check, start at the birth date and add the pieces back: 28 November 1990 plus 35 years is 28 November 2025; plus 10 months is 28 September 2026; plus 21 days is 19 October 2026. It lands exactly on the end date.
Cross-checking with total days
Total days lived is a useful second opinion. From 28 November 1990 to 19 October 2026 there are 13,109 days, which is 1,872 weeks and 5 days.
You can sanity-check that without counting every day. The age 35 years, 10 months and 21 days is about 35 + 10/12 + 21/365, or 35.89 years. Multiply by the average calendar year of 365.2425 days and you get roughly 13,109. If your two answers disagree by hundreds of days, one of the subtractions has a slip.
A days calculator gives the exact day count between any two dates, including the leap days in between.
Birthdays on 29 February
A person born on 29 February has a true birthday only in leap years. Someone born on 29 February 2000 had actual 29 February birthdays in 2004, 2008, 2012, 2016, 2020 and 2024: six of them, even though they are 26 in October 2026.
In other years the birthday is usually treated as 28 February or 1 March. Which one counts depends on the country and on the purpose, such as a school entry date or a legal age threshold, so check the rule that applies to your situation rather than assuming.
Why two age calculators can differ
Whole years always agree. Months and days can differ because months have different lengths, and there is no single universal rule for borrowing when the birth day is the 29th, 30th or 31st.
- Some tools borrow the length of the month before the end date's month; others use the length of the birth month.
- A birth day that does not exist in the target month (31 in a 30-day month) has to be rounded somehow.
- Some tools count the end date as an extra day; most do not.
- Time zones can matter for births close to midnight, because the date depends on where the clock is.
Some traditional age systems also count a newborn as age 1 or add a year at the new year rather than on the birthday. The method in this article is the common international one.
For a quick answer, an age calculator does the borrowing for you, and a chronological age calculator is useful when you need the age on a specific past or future date.
- 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.
- Days Calculator: Use the days calculator to find how many days lie between two dates, or to add days to a date and see the result with its day of the week. Weekdays, weeks and months are reported too.
- Chronological Age Calculator: Enter a date of birth and, if you like, the date to measure the age on. The calculator returns the exact age in years, months and days along with the totals in months, weeks and days.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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