An age calculator works out exactly how old someone is from their date of birth, expressed in years, months and days rather than years alone. It accounts for the messy parts of the calendar automatically: months of different lengths, leap years, and whether this year's birthday has already passed. The result is the precise figure required by school enrolment, medical assessments, visa applications and legal forms.
How to Use This Age Calculator
Enter the date of birth.Use the date picker so the format is unambiguous. Day-month-year and month-day-year conventions differ by country, and picking the date avoids the mix-up entirely.
Set the target date if needed.It defaults to today. Change it to find someone's age on a future date, such as the first day of a school term or a visa deadline.
Read the full breakdown.You get completed years, months and days, which is the format official forms ask for.
How Age Is Actually Calculated
Age uses completed units, not rounded ones. Someone who is 29 years and 11 months old is 29, not 30. This sounds obvious, but it is the source of most manual errors, because the calculation borrows across units in a way ordinary subtraction does not.
Years = target year - birth year Months = target month - birth month Days = target day - birth day
If days are negative: borrow the number of days in the previous month, then reduce months by 1
If months are negative: add 12 and reduce years by 1
The borrowing step is what separates a correct answer from an approximation. Because months vary between 28 and 31 days, you must borrow the actual length of the preceding month, not a generic 30.
Worked Example: Step by Step
Born 22 September 1998, calculated on 8 March 2026
The month before March is February 2026 with 28 days
Days: -14 + 28 = 14
Months: -6 - 1 = -7
Step 3: Borrow months from years
Months: -7 + 12 = 5
Years: 28 - 1 = 27
The person is 27 years, 5 months and 14 days old
Leap Years and Why They Matter
A leap year adds 29 February to keep the calendar aligned with the Earth's orbit, which takes roughly 365.2422 days. The rule has three parts, and the third catches people out.
Rule
Test
Example
Divisible by 4
Leap year
2024, 2028 are leap years
Divisible by 100
Not a leap year
1900 was not a leap year
Divisible by 400
Leap year after all
2000 was a leap year
The 1900 problem
Some older spreadsheet software still treats 1900 as a leap year because of a compatibility decision made decades ago. Anyone calculating ages for people born before 1901 with those tools may see a one-day discrepancy. Modern date libraries follow the correct rule.
Where Exact Age Is Required
School enrolment and cut-off dates
Schools set an age cut-off on a specific calendar date. A child born days either side of it starts a full year apart, so parents need the age on that exact date rather than today's age.
Medical and developmental assessment
Paediatric growth charts, vaccination schedules and developmental screenings are banded by month. A child of 23 months and one of 25 months sit in different bands even though both round to two years.
Legal thresholds and official forms
Pension eligibility, driving licences, employment law and immigration all hinge on exact age at a given date. Forms usually ask for years and months, not a rounded age.
Calculating age on a future date
Set the target date rather than today's date to answer questions like "how old will my child be when term starts" or "will I meet the age requirement by the application deadline". The arithmetic is identical, only the target changes.
Common Sources of Error
Date format confusion. 03/09/2026 means 3 September in most of the world and 9 March in the United States. Using a date picker removes the ambiguity.
Borrowing a generic 30 days. Borrowing must use the real length of the preceding month, which changes the day count.
Ignoring the birthday test. Subtracting birth year from current year alone overstates the age of anyone whose birthday has not yet arrived this year.
Time zone drift. A tool running in a different time zone may use yesterday's or tomorrow's date as "today".
People Also Ask
Age is counted as completed years since birth. Subtract the birth year from the current year, then subtract one more if the birthday has not yet occurred this year. A person born on 15 June 1990 is 34 on 1 June 2025 but turns 35 on 15 June. Months and days are counted the same way, borrowing from the next larger unit when needed.
Two things usually cause it. The first is time zones: a tool using UTC may treat your local date as yesterday or tomorrow. The second is how the tool handles the birth day itself, since some count the birth date as day zero and others as day one. A one-day gap is almost always one of these, not an error in the arithmetic.
In non-leap years, most legal systems treat the birthday as falling on 1 March, though some jurisdictions use 28 February. Either way the person gains a year at the same point in the calendar. Leap-day births are the main reason age tools that simply count days can disagree with tools that count calendar years.
Chronological age is a person's exact age expressed in years and months, such as 7 years 4 months. Educational and speech assessments require this precision because test norms are banded by month, and a single month can change which band a child falls into. Our chronological age calculator gives this breakdown directly.
Count the days between the birth date and today, including every leap day in that span. Divide by seven for weeks. A person aged exactly 30 years has lived roughly 10,957 days, though the exact figure depends on how many leap years fell within their lifetime.
Age results are calculated from the dates you enter. For legal, immigration or medical purposes, confirm the figure against official documentation and the requirements of the relevant authority.