How this is calculated
Start with the difference in years. If the current month and day fall before the birth month and day, subtract one — the birthday has not happened yet this year. Months and days are then derived the same way, borrowing from the larger unit when the subtraction would go negative.
Worked example for a birth date of 15 June 1990 evaluated on 1 August 2026: the year difference is 36, and since 1 August is after 15 June, no adjustment is needed. Months from 15 June to 15 July to 1 August gives 1 month and 17 days. Exact age: 36 years, 1 month, 17 days.
Total days lived is a straight difference in calendar days, which correctly accounts for every leap day in between. Someone aged 36 has lived roughly 13,150 days — a number that surprises people because 36 × 365 understates it by about nine days of accumulated leap years.
Why month arithmetic needs borrowing
Months have different lengths, so 'one month later' is not a fixed number of days. Going from 31 January to the same date in February is impossible, and different systems resolve it differently — some clamp to 28 or 29, some roll over into March.
This calculator borrows from the month when the day difference would be negative, using the actual length of the preceding month. It is the convention most people intuitively apply, and it is why the days figure can range from 0 to 30 depending on which months the calculation spans.
Leap years and the 29 February problem
A leap year occurs every four years, except century years, except those divisible by 400. So 2000 was a leap year and 1900 was not — a rule that exists because the solar year is about 365.2422 days rather than 365.25.
People born on 29 February have a birthday in only about one year in four. Legal systems vary on which date counts in common years: some treat 28 February as the anniversary, others 1 March, and the distinction genuinely matters for things like reaching the age of majority. Total days lived is unaffected, since it counts real elapsed days regardless of which calendar date is designated the birthday.
How to use the age calculator
- Enter your date of birth. Year, month and day. The calculation runs against today's date automatically.
- Read the exact age. Years, months and days — not the rounded year figure that subtracting birth years would give.
- Check the birthday countdown. Days until your next birthday, accounting for whether this year's has already passed.