Age Calculator

How to use it

1. Enter a date of birth. The age appears immediately, measured to today.
2. Change “Age on” to work out an age at any other date — a deadline, an application cut-off, a date in the past.
3. Switch to Age difference to compare two dates of birth and get the gap between two people.

Nothing you type is sent anywhere, and unlike the other date tools on this site, nothing is written to the address bar either. A date of birth is personal data, and a querystring ends up in browser history and referrer headers.

How age is calculated

Count the whole years from the birth date, then the whole months, then the days left over. You have your birthday when the anniversary arrives, not before — so on 14 August someone born on 15 August 2000 is still 25, and the next day they are 26.

That sounds obvious and it is where most of the disagreements come from.

The month-end problem

Here is the case that makes calculators disagree, and it has no universally correct answer.

Someone is born on 31 January. What is their age on 28 February the following year?

Their yearly anniversary is 31 January, so that part is easy: one year. But the monthly anniversary of 31 January is meant to be 31 February, which does not exist. So the arithmetic has to clamp somewhere.

This tool clamps to the last day of the shorter month, so 28 February counts as one whole month. The answer is 1 year, 1 month. That matches what date libraries do — it was checked against python-dateutil across 2,009 cases — and it means the monthly anniversary of a 31st always falls on the last day of a short month rather than spilling into the next one.

The alternative convention rolls forward to 3 March and reports 1 year, 0 months, 28 days. Neither is wrong. What is wrong is not telling you which one you got.

Using the age difference calculator

Switch to the second tab, enter both dates of birth, and the tool reports the gap. The order does not matter — it works out which is older and tells you.

Most age calculators simply do not do this. calculator.net has a second date field, but it means “age at this date”, not “the other person”. So the usual workaround is calculating two ages and subtracting them by hand, which goes wrong exactly at the month ends described above.

The gap is a fixed quantity, incidentally. Two people born five years and three months apart are always five years and three months apart, at every point in both their lives. It does not narrow.

Chronological age, and who needs it

Chronological age is age counted straight from the date of birth — as opposed to a developmental, biological or bone age, which are estimates of something else.

It is written in a specific notation: years;months;days, semicolons included. So 11;5;5 is eleven years, five months and five days.

That precision exists for a reason. Educational and psychological assessments — reading tests, cognitive assessments, speech and language screens — publish their norms in bands a few months wide. A child of 11;5 and a child of 11;11 are compared against different reference groups. Round to “11 years” and you can place a child in the wrong band, which changes their score and possibly what happens next.

The tool shows this notation alongside the plain answer. Set “Age on” to the date of the assessment rather than today.

Corrected age for premature babies

A baby born at 32 weeks arrived about 8 weeks before their due date. At 6 months old by the calendar, they have had roughly 4 months of development since the point a full-term baby would have started.

Corrected age — also called adjusted age — counts from the due date instead of the birth date:

corrected age = chronological age − (40 weeks − weeks at birth)

Clinicians use it when interpreting developmental milestones, generally until around age 2, after which the difference stops mattering for most children. Tick “Born prematurely” and enter the weeks of gestation at birth.

This is arithmetic and nothing more. It does not tell you whether a child is developing typically, and it is not a screening tool. Questions about how a child is doing go to their paediatrician, who has the context a calculator does not.

Leap-day birthdays

People born on 29 February have a birthday in only one year out of four. The other three years, something has to give, and there are two conventions:

We use 28 February, which keeps the birthday inside the birth month and means the age increments on the same date every year. It matters for things like when someone legally turns 18, and jurisdictions genuinely differ — so if it is consequential, check the rule that applies rather than trusting any calculator.

How cultures count age differently

The method above is the international standard, but it is not universal.

The traditional East Asian system counts a baby as one year old at birth and adds a year at the lunar new year, so a child born in December could be considered two years old a few weeks later. South Korea used a version of this until 2023, when it standardised on the international method for legal and administrative purposes.

If you are converting between systems, the difference is one or two years depending on the birth date and the time of year.

Doing it elsewhere

Two warnings about DATEDIF. It is undocumented and does not appear in Excel’s autocomplete, though it works. And its "MD" unit has known bugs around month ends, occasionally returning negative numbers — which is exactly the case this page spends a section on.

Related

The date duration calculator answers the same question about any two dates and shows both the inclusive and exclusive day counts. For deadlines that skip weekends and public holidays, use the business days calculator. And if the two dates come from different countries, the time zone converter will tell you whether a daylight saving change sits between them.