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Age Calculator
Exact age in years, months and days — plus how long you have lived and when the next birthday lands.
Enter a date of birth.
How this calculator works
Age is calendar arithmetic, not division. You turn 26 the moment your twenty-sixth birthday arrives, regardless of how many days that took — and it varies, because leap years exist. Dividing days lived by 365.25 gets the right answer most of the time and the wrong one within a few days of a birthday, which is exactly when anyone checks.
So this walks whole months forward from the date of birth and counts the remaining days, the way you would on paper. The result is exact on the day, the day before, and the day after.
It measures to today unless you enter a date to measure to. That is useful for working out how old someone was on a particular day, or will be at some point in the future.
A February 29 birth date has no birthday at all in a common year, and which day substitutes is a matter of jurisdiction rather than arithmetic. This uses February 28, the usual rule in the US and the UK, and says so on the result rather than leaving it implied.
The formula
Exactly what happens to your numbers, step by step.
Age
walk whole months from birth until one more would pass the date, then count the remaining daysNot (days ÷ 365.25), which drifts near a birthday.
Next birthday
days = the next anniversary − today
A worked example
Born July 4, 1990, measured on September 10, 2026.
What you enter
- Date of birth
- 1990-07-04 (a Wednesday)
- Measured to
- 2026-09-10
Step by step
- Whole years
- 36 (last birthday July 4, 2026)
- Then whole months
- 2 (to September 4)
- Then days
- 6 (to September 10)
- Next birthday
- July 4, 2027
36 years, 2 months and 6 days
Dividing 13,217 days by 365.25 gives 36.19 years, which rounds to the same answer here — but a week either side of a birthday that method disagrees with the calendar, and the calendar is what counts.
Assumptions
Every result here rests on these. Change your inputs and the result changes with them.
- Age is counted in whole calendar days, with no time of birth.
- A birthday arrives on the same month and day each year.
- For a February 29 birth date, the anniversary falls on February 28 in a common year.
What this cannot tell you
- Time of birth is not used. Someone born at 11pm and someone born at 1am the next morning are treated as a day apart, which is right for every purpose except the first day of life.
- Time zones are not modeled. A birth recorded in one time zone and an age measured in another can differ by a day at the boundary.
- The February 29 rule is a convention, not a fact. This uses February 28; some jurisdictions and some legal contexts use March 1, and for anything with legal weight the local rule is what matters.
- It cannot tell you a legal age of majority, school year, or eligibility date. Those depend on cut-off rules that vary by jurisdiction and are not simple date arithmetic.
Questions people ask
How do I calculate my exact age?
Count whole years from your date of birth to the last birthday you have had, then whole months, then the remaining days. Enter your date of birth above and it does exactly that — not a division that drifts near a birthday.
Why not just divide days by 365.25?
Because it disagrees with the calendar near a birthday, which is when the answer matters. Leap years are not evenly spaced from any given birth date, so the average year length is an approximation and your birthday is not.
What if I was born on February 29?
You have no birthday at all in a common year, and the substitute is a matter of convention. This uses February 28, which is the usual rule in the US and the UK. Some jurisdictions use March 1, and for anything legal the local rule is the one that counts.
How many days have I been alive?
The total is shown above alongside the age, along with weeks and hours. It counts real calendar days including every leap day, so it is exact rather than an average.
Can I work out how old someone was on a particular date?
Yes — enter the date you want to measure to and it works out the age on that day rather than today. It works forwards as well, so you can find an age on a future date.
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Sources and review
This calculator uses standard arithmetic with no external rules or published rates, so there is nothing to cite beyond the formulas shown above.
Methodology version 1.0.0 · Last reviewed
Cite this calculator
For coursework, a resource list or your own site. How every calculator is built and checked is set out on the methodology page.
Citation
Chalk Calculator. (2026, September 20). Age Calculator (Methodology version 1.0.0). https://chalkcalculator.com/age-calculator/Link to it from your site
<a href="https://chalkcalculator.com/age-calculator/">Age Calculator</a> from Chalk Calculator