Age Calculator
Work out your exact age in years, months, and days, or check the age of anyone else, in a couple of seconds.
Enter the dates
Choose a date of birth and, if you like, a different reference date to measure against.
Your result will appear here once you calculate an age.
- Total days lived
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- Total weeks lived
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- Total months lived
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- Day of the week born
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- Days until next birthday
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What is this calculator?
An age calculator takes a date of birth and works out exactly how much time has passed since then, broken down into years, months, and days rather than just a single number. Most people can tell you their age in years, but the tool on this page goes further by showing the extra months and days that a birthday alone doesn't capture, along with totals like the number of days, weeks, and months someone has been alive.
This is useful for far more than curiosity. Parents track a child's age in months for pediatrician visits, HR teams confirm eligibility dates for benefits, schools calculate cutoff ages for enrollment, and legal or immigration paperwork often asks for an age as of a specific date rather than today. Because this calculator lets you set any reference date instead of always defaulting to "today," you can figure out how old someone was, is, or will be on a date that matters for your situation.
Every result is calculated using standard calendar math, which means leap years are automatically taken into account, and there's no rounding shortcut that quietly makes the answer wrong by a day or two. Whether you're checking your own age down to the day or working out a precise figure for an official form, the numbers here are built to match how age is actually counted on a calendar.
How to use it
Using the calculator only takes two steps. First, enter the date of birth in the "Date of birth" field using the date picker or by typing the date directly. Second, check the "Calculate age as of" field, which is automatically filled in with today's date but can be changed to any other date if you want to measure age at a different point in time.
Once both dates are set, select "Calculate age" or press Enter on your keyboard. The result card will update instantly with the age broken into years, months, and days, plus a set of extra details underneath. If you want to start over, select "Reset" to clear both fields and return the tool to its default state.
How the calculation works
The calculator compares two calendar dates: the date of birth and the reference date you've chosen. Rather than simply subtracting one date from the other and dividing by 365, it walks through the calendar the same way a person counting on their fingers would, which is what keeps the result accurate.
It starts by comparing the year, month, and day of the reference date against the year, month, and day of the birth date. If the day-of-month on the reference date is earlier than the day-of-month on the birth date, that means a full month hasn't been completed yet, so the day count borrows from the previous month and the month total is reduced by one. The same borrowing happens between months and years if a full year hasn't yet passed since the most recent birthday.
Because this method checks actual calendar days rather than assuming every month has the same length, it automatically handles short months, long months, and leap years without any special-case rules needing to be typed in by the user. February 29 is treated as a real, valid date, and any date range that crosses a leap year is measured using the correct number of days for that year, not an estimated average.
The calculator also works out totals across the entire lifespan, such as the number of complete days, weeks, and months lived, by measuring the full gap between the two dates rather than just the year-month-day breakdown. This is why you'll see both a "3 years, 2 months, 14 days" style result and separate totals like "1,170 days lived" in the same result card.
Formula
The core idea can be written as a simple calendar comparison. In plain terms, the calculator finds the difference between the reference date and the birth date, then adjusts the month and day counts so that only complete units are counted.
Years = ReferenceYear − BirthYear
Months = ReferenceMonth − BirthMonth
Days = ReferenceDay − BirthDay
If Days < 0:
Months = Months − 1
Days = Days + (days in the month before the reference month)
If Months < 0:
Years = Years − 1
Months = Months + 12
Total Days Lived = (Reference date − Birth date), in whole days
The "borrowing" steps in the formula are what make the result match a real calendar instead of a rough estimate. They ensure that, for example, someone born on March 20 turns a new age on March 20 every single year, regardless of how many days were in the months in between.
Practical example
Suppose someone was born on March 20, 2000, and you want their age as of June 5, 2024.
Comparing the dates: the years go from 2000 to 2024, which is 24 years. The month difference is June (6) minus March (3), which is 3 months. The day difference is 5 minus 20, which is negative, so one month is borrowed: the month total drops to 2, and the day count becomes 16 (the number of days remaining after borrowing from May).
The final result is 24 years, 2 months, and 16 days. Alongside that, the calculator would also report a total of roughly 8,842 days lived, around 1,263 completed weeks, and that the person was born on a Monday.
Common real-world uses
- Medical and pediatric forms: confirming a patient's exact age in years and months for dosage charts or developmental checkpoints.
- School and program enrollment: checking whether a child meets an age cutoff by a specific enrollment date.
- HR and benefits administration: calculating age as of a plan start date for eligibility rules tied to age thresholds.
- Legal and immigration paperwork: working out age as of a filing date or event date rather than the current date.
- Personal milestones: finding out how many days old you are, or counting down to a milestone birthday.
- Genealogy and record-keeping: figuring out someone's age at the time of a historical document or event.
Frequently Asked Questions
How accurate is this age calculator?
It's accurate to the day. The tool compares the exact calendar dates you enter rather than using an average like 365.25 days per year, so leap years, short months, and long months are all handled correctly. As long as the date of birth and reference date are entered correctly, the year, month, and day breakdown will match what you'd get counting manually on a calendar.
Can I calculate someone's age on a date other than today?
Yes. The "Calculate age as of" field defaults to today's date, but you can change it to any past or future date. This is useful for figuring out how old someone was on a specific event date, or how old they will be on a future date such as a graduation, retirement, or contract renewal.
How does the calculator handle leap years?
Leap years are handled automatically because the calculation is based on real calendar dates rather than a fixed number of days per year. If the date range includes a February 29, that extra day is simply counted like any other day in the total, and it doesn't distort the years, months, and days breakdown.
What happens if someone was born on February 29?
A birth date of February 29 is treated as valid, since it's a real calendar date in leap years. When calculating age in years, month, and days against a non-leap year, the calculator counts to the nearest equivalent point in the calendar, so the age breakdown stays consistent and continues to add up correctly year over year.
Why does my age in months look different from years times twelve?
Total months lived isn't simply years multiplied by twelve, because it also includes any additional completed months beyond the last full year. For example, someone who is 5 years and 3 months old has lived 63 total months, not 60, since the 3 extra months since their last birthday are counted too.
Does the calculator count the day of birth itself?
Yes, the day someone is born counts as day one of their life for the purposes of total days lived. The years, months, and days breakdown measures completed units of time between the two dates, so a full day only gets added once 24 hours on the calendar have passed.
Can I use this to find out how many days until my next birthday?
Yes. Once you calculate an age, the result card shows the number of days remaining until the next birthday based on the reference date you entered. If a birth date falls on February 29, the countdown uses February 28 in non-leap years so the figure stays accurate and easy to plan around.
Why do I need to enter a reference date if I just want my current age?
You don't have to change anything. The reference date field is automatically filled in with today's date, so if you only want your current age, you can leave it as is and just enter your date of birth. The field is only there for the times you need an age calculated for a different date.
Is there a limit to how far back or forward the dates can go?
The calculator works with any valid calendar date supported by your browser's date picker, which typically covers a very wide range of past and future years. This makes it suitable for historical research and record-keeping as well as everyday personal or professional use, without running into unexpected date restrictions.
Does this calculator store or share the dates I enter?
No. All calculations happen directly in your browser using the dates you type in, and nothing is saved, transmitted, or stored anywhere. Once you leave the page or reset the form, the information is gone, so there's no record kept of any date you've entered.