Subtracting one year from another sounds like the whole job, but an exact age is trickier: months have different lengths, leap years add a day, and the answer depends on whether your birthday has happened yet this year. Here is how it works, step by step.
Your age in whole years is the number of birthdays you have had. Start with current year minus birth year, then subtract one if this year's birthday has not yet arrived. Someone born on 20 November 1990 is 35 on 15 October 2026, not 36, because the November birthday is still ahead.
For a precise answer, subtract the birth date from today's date field by field: years, months, then days. When a field comes out negative you borrow from the larger unit, exactly like subtraction on paper.
The only judgment call is which month length to borrow. Most calculators, including ours, use the length of the month immediately before the current month. Different tools can differ by a day or two in edge cases for exactly this reason, and neither is wrong; they are just using slightly different conventions.
A year has 365 days, except leap years, which have 366 because Earth takes about 365.2422 days to orbit the sun. The rule: a year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2000 and 2024 were leap years, but 1900 and 2100 are not.
This affects total-days calculations most. The number of days between your 20th and 30th birthdays depends on how many 29 Februaries fell in between, which is two or three. If you are only asking for years and months, leap years rarely change the answer, but if you want days lived, they matter.
People born on 29 February have a real birthday only every four years. In non-leap years the law differs by place. In many English-speaking jurisdictions a person born on 29 February is treated as turning a year older on 1 March in common years, while others use 28 February. For everyday purposes, either is accepted; for legal ones such as age of majority, check the rule in the relevant jurisdiction.
For a total-days figure, convert both dates to a day count from a fixed reference and subtract. Weeks are that figure divided by 7; hours multiply by 24. Someone who is 10,000 days old is about 27 years and 4 months old, a milestone some people like to celebrate. Because 10,000 ÷ 365.2425 ≈ 27.38, the date falls in the first half of their 28th year.
If a form asks for your age, assume the international method unless it clearly says otherwise.
If your result shows more than 11 months or more than the number of days in a month, something was not borrowed correctly. The date must not be in the future, and very old dates are worth a second look: a year typed with an extra digit gives an absurd result, which is why the calculator rejects years before 1900 or after the current year. You can also check any answer backwards: add the computed years, months and days to the birth date and you should land on today.
Try it yourself: the Age Calculator for an exact breakdown from any birth date →