Age Calculator
A birth date, and optionally a date to measure to.
Why age is counted the way it is
You are not one year old until your birthday arrives. That sounds obvious and it is the reason age cannot be worked out by dividing days by 365.
The method is: count whole years first, and only count one if the birthday has already passed this year. Then count whole months from the last birthday. Then whatever days are left over.
This is why the same person can be "34 years, 2 months and 28 days" one day and "34 years and 3 months" the next — the month completed, and the leftover days reset.
The borrowing problem
When the day of the month has not been reached, a month is borrowed — and how many days that supplies depends on which month. Born on the 31st and measuring in March, you borrow 28 or 29 days from February. Measuring in May, you borrow 30 from April.
This is the part naive age code gets wrong, and it is why two calculators can disagree by a day.
29 February
Born on a leap day, you get a real birthday every four years. In practice most legal systems treat 1 March as the anniversary in non-leap years, though a few use 28 February. It matters for things like reaching the age of majority on an exact date.
Age is counted differently in some places
The system above is the international norm, but it is not universal. Korea traditionally counted everyone as one at birth and added a year each New Year, so a baby born in December could be two years old a few weeks later. Korea moved to the international system for most official purposes in 2023. Several other East Asian traditions have similar reckonings.
Common uses
- Exact age for a form that wants years and months.
- Age on a specific date — school entry, an eligibility cutoff, a policy start.
- Age gaps between two people.
- How long since any other event.
The working, step by step
Questions people ask
How is exact age calculated?
Whole years first, counting one only if the birthday has passed. Then whole months, then the remaining days.
Why can two age calculators disagree by a day?
Because of how they borrow days when the day of the month has not been reached. How many days that supplies depends on the length of the previous month.
What about people born on 29 February?
A real birthday comes every four years. Most legal systems treat 1 March as the anniversary in other years, though some use 28 February.
Can I calculate age on a past or future date?
Yes. Leave the second box blank for today, or fill it in for any other date.
Is age counted the same way everywhere?
No. Korea traditionally counted everyone as one at birth and added a year each New Year, though it moved to the international system for most official purposes in 2023.
Worked out in the page
The arithmetic runs in your browser. No date you type is sent anywhere, which matters for the ones that are nobody else's business — a birth date, a due date, a notice period. There is no server here to receive them. The about page lists every request the site makes.