How to Calculate Age in Years, Months and Days
Ask someone their age and they'll give you whole years — "I'm 36." But forms, schools and official deadlines often need the exact age: years, months and days. Doing that by hand is a small piece of calendar arithmetic. This guide teaches the borrowing method with a fully worked example — and if you'd rather skip the arithmetic, the free AgeCalc calculator does exactly these steps in your browser.
The method, in three subtractions
Write the two dates in year – month – day order, with the date of birth underneath the date you are calculating to (usually today). Then subtract column by column, starting from the right:
- Days: subtract the birth day from the current day. If the result is negative, borrow one month: add the number of days in the month before the current month, and remember you owe a month in the next step.
- Months: subtract the birth month from the current month (minus the one you borrowed, if any). If the result is negative, borrow one year: add 12 months, and remember you owe a year.
- Years: subtract the birth year from the current year (minus the one you borrowed, if any).
The three results are the exact age. The secret is the borrowing: months have different lengths, so when you borrow a month you must add the length of the specific month you are borrowing into — the month just before the current one.
A worked example
Date of birth: June 24, 1990. Calculating on: October 22, 2026.
| Step | Calculation | Result |
|---|---|---|
| Days | 22 − 24 = −2 → borrow September's 30 days: 22 + 30 − 24 | 28 days (owe 1 month) |
| Months | 10 − 6 − 1 (borrowed) | 3 months |
| Years | 2026 − 1990 | 36 years |
Exact age: 36 years, 3 months, 28 days. Quick sanity check: June 24, 1990 + 36 years = June 24, 2026; + 3 months = September 24, 2026; + 28 days = October 22, 2026. It lands exactly on the day — the method works.
Notice why the borrowed month mattered: we added 30 days because September, the month before October, has 30 days. Had we been calculating in March, we would have borrowed February's 28 (or 29) days instead.
The trap in the quick method
The shortcut "this year minus birth year" is only right after the birthday has happened this year. Someone born in December is still the younger age for most of the following year. That is why the months step matters: if the current month-and-day falls before the birth month-and-day, the person has not yet had this year's birthday, and the borrowing method handles it automatically — the years column quietly comes out one lower.
Leap years and February 29 birthdays
For age in years, months and days, leap years need no special handling: the method follows the calendar, so a February with 29 days simply gets borrowed correctly when it comes up. Leap days do show up in totals like "total days old", which is why dividing total days by 365 only ever gives an approximate age — roughly one day in every four years is a leap day.
People born on February 29 only get a "real" birthday every four years. In everyday life most celebrate on February 28 or March 1 in ordinary years; legally, the date on which they turn a new age depends on the country — some use February 28, others March 1. The AgeCalc calculator observes the birthday on March 1 in non-leap years.
Calculating age on a different date
The same three steps work for any end date, not just today — and that's the version life actually asks for. Will a child be 5 by the school cutoff of September 1? Was an applicant 18 on the closing date? How old will you be on your wedding day, or on the date you plan to retire? Replace "today" with the target date in the subtraction — in the calculator, that's the "Age at date" field — and follow the identical borrowing steps.
Common questions
- How do I express the answer in months only? Finish the exact calculation, then convert the years: total months = years × 12 + months, with days left over. This is how babies' ages are usually quoted — 1 year 3 months is "15 months old".
- Does the time of day matter? For everyday and legal purposes, no — age changes at the start of the birthday, not at the exact hour of birth (a couple of legal traditions differ, but forms work in whole days).
- Why do different calculators occasionally differ by a day? Usually the February-29 convention, or a tool that borrows a flat 30 days for every month instead of the real month's length. Calendar-accurate tools agree with the method above.