Calendar desk
Age Calculator
Calculate age on any date using completed calendar years and total days.
Age Calculator: Find a person's completed years and completed months on a chosen as-of date, together with the total calendar days since birth. The calculation follows birthdays rather than dividing by an average year length. A 29 February birthday uses 28 February as its anniversary in a non-leap year. Runs 100% locally in your browser with zero server file uploads.
- Category
- Calculators
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Completed years
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- Completed months
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- Total days
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Age = as-of date − birth date (calendar semantics)- Age uses calendar dates, not time zones or times of day.
- A February 29 birthday uses February 28 as its anniversary in non-leap years.
Formula and worked example
Age = as-of date − birth date (calendar semantics)
A worked example with sample values:
| Inputs | |
|---|---|
| Birth date | Jan 1, 1990 |
| As-of date | Jan 1, 2025 |
| Results | |
|---|---|
| Completed years | 35 |
| Completed months | 420 |
| Total days | 12,784 |
Birthday formula and leap-day example
Let Yb be birth year and Ya the as-of year. Completed years = Ya − Yb − I, where I = 1 if the anniversary has not yet occurred in the as-of year, otherwise 0. The anniversary day is shortened to the birth month's last day if necessary. Total days = as-of day number − birth day number.
Completed months starts with 12(Ya − Yb) + Ma − Mb, where Ma and Mb are the as-of and birth month numbers. Subtract one if the as-of day is smaller than the birth day shortened to fit the as-of month. This produces a total month count rather than a years-and-months decomposition.
For birth date 2000-02-29 and as-of date 2025-02-28, the shortened anniversary has arrived, so years = 2025 − 2000 = 25. Months = 12 × 25 + 2 − 2 = 300. The day-number difference is 9131. The widget shows 25 completed years, 300 completed months and 9131 total days.
Calendar age and anniversary conventions
The Gregorian leap-year explanation from the US Naval Observatory (https://aa.usno.navy.mil/faq/leap_years) provides the calendar background for leap-day births. Dividing total days by 365 would ignore those extra dates and can disagree with completed birthdays.
Use the year result for an ordinary birthday-based age comparison and the day result for an exact date interval. The page does not select legal age rules for a jurisdiction, an eligibility cutoff time or a March anniversary policy. If a form specifies its own convention for leap-day births, apply that convention instead of assuming it matches this result.
How to use it
- Enter the birth date.
- Check or replace the As-of date, which initially uses today's date on your device.
- Calculate and read completed years, completed months and total days as separate measures.
Privacy & limitations
Dates, times and results stay in your browser. No inputs are uploaded.
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Frequently asked questions
When does a leap-day birthday gain a year here?
In a non-leap year this calculator treats 28 February as the anniversary of a 29 February birth. In a leap year the anniversary remains 29 February. This is the tool's convention, rather than a determination of legal birthday rules.
Are completed months the months left after the years?
No. Completed months is the total count since birth. It includes the months corresponding to all completed years, so do not add it to twelve times the year result.
Can I calculate age at a future event or before birth?
A future as-of date is allowed, so you can check age at an event. An as-of date before the birth date is rejected. Times of birth and local time zones are not part of the calculation.
Embed this tool
Free to use on any website. Paste the code where the tool should appear; the link under it lets readers open the full page.
<iframe src="https://toea.com/embed/age-calculator" title="Age Calculator" width="100%" height="810" style="border:0;max-width:680px" loading="lazy"></iframe>
<p style="margin:4px 0 16px;font-size:13px"><a href="https://toea.com/age-calculator">Age Calculator</a> · TOEA</p>
<script>addEventListener("message",function(e){if(e.origin!=="https://toea.com"||!e.data||e.data.type!=="toea-embed-height")return;document.querySelectorAll("iframe").forEach(function(f){if(f.contentWindow===e.source)f.style.height=e.data.height+"px"})})</script>Free tool · runs in your browser · no account required