The Age Calculator converts a date of birth into an exact age in years, months and days, using the real length of each calendar month rather than a 30-day average. Enter a birth date to find age today, or set any other date to find age at that moment. The calculator also works backwards, finding a date of birth from a known age, and reports total days lived, the weekday someone was born on, and the days remaining until the next birthday.
Calculate age from a date of birth
Age is the elapsed time between a birth date and a reference date, expressed as whole years, then whole remaining months, then remaining days. The reference date defaults to today and can be changed to any past or future date.
Enter the birth date in whichever format you use, and the Age Calculator confirms the date it read back to you in full before calculating.
The order matters. Age is not a single number divided into units; it is a subtraction performed on the calendar. Someone born on 30 January is 1 month old on 28 February in a common year, not 29 days old and not 1.03 months old. Working down from years to months to days is the only method that matches how people and legal systems actually count age.
Three date formats are supported: mm/dd/yyyy, dd/mm/yyyy and yyyy-mm-dd. A date like 03/04/2026 is 3 April in most of the world and 4 March in the United States, so the calculator always echoes the parsed date in long form beneath the field. Check it before reading the result.
Enter a birth time for precise age
Adding a birth time refines the final component of the result from whole days to days and hours. The field accepts either a 12-hour time with AM or PM, or a 24-hour time. Leave it blank and the calculator treats the birth as occurring at midnight, which is the standard convention and the one every legal age threshold uses.
Birth time changes the answer only near a birthday boundary. Someone born at 11:00 PM on 25 July is, at 9:00 AM on their birthday, still a few hours short of the full year. Most contexts ignore this. Neonatal records, astrological charts and some medical dosing calculations do not, which is why the field exists.
Apply the age calculation method
The calculation subtracts the birth date from the reference date field by field, borrowing where a field goes negative. Borrowing uses the true length of the preceding month, taken from the calendar, never an average.
years = referenceYear − birthYear
months = referenceMonth − birthMonth
days = referenceDay − birthDayif days < 0: months = months − 1 days = days + (number of days in the month before the reference month)
if months < 0: years = years − 1 months = months + 12 ```
The borrow is where hand calculations and simpler calculators go wrong. Borrowing a flat 30 days produces an answer that drifts by one to three days depending on which month the borrow crossed. Borrowing 31 days from a January, 28 from a common-year February and 29 from a leap-year February produces the answer a court, a school registrar or a passport office would agree with.
Find age on 25 July 2026 for a 10 November 1991 birth
Someone born on 10 November 1991 is 34 years, 8 months and 15 days old on 25 July 2026. The subtraction needs one borrow, in the months column. 1. Subtract each field.
year month day
2026 7 25 reference date
− 1991 11 10 birth date
─────────────────────────
35 −4 152. Months is negative, so borrow a year. 35 − 1 = 34 years −4 + 12 = 8 months
3. Read the result. 34 years, 8 months, 15 days
The days column needed no borrow here because 25 is larger than 10. Had the birth day been 26, the calculator would have borrowed 30 days from June (the month preceding July), leaving 34 years, 7 months, 29 days.
That person has lived 12,675 days, or 1,810 weeks and 5 days. They were born on a Sunday. Their next birthday falls on 10 November 2026, a Tuesday, 108 days away.
Calculate a date of birth from a known age
The calculator runs in reverse when you know an age but not a birth date. Enter the age and a reference date, and it returns the range of birth dates that produce that age, because a whole-number age covers a full year of possible birth dates.
Someone who is 34 on 25 July 2026 was born between 26 July 1991 and 25 July 1992. If you also know the month and day, the calculator returns the exact year instead of a range. This is the mode that answers "what year was someone born if they are 40 today", which is a common genealogy and records-checking question.
Calculate age at death
Setting the reference date to a date of death returns a lifespan rather than a current age. The arithmetic is identical; only the label changes. Enter a birth date of 15 March 1934 and a death date of 2 September 2019, and the result is 85 years, 5 months and 18 days.
Genealogists use this in the opposite direction as well. Headstones and death records often state an age but not a birth date, so entering the death date and the stated age returns the birth window to search.
Handle a 29 February birthday
A person born on 29 February has a real birthday only in leap years. In the three years between, there is no matching date, and different systems resolve it differently. The Age Calculator uses 1 March in common years, so a leapling turns a year older on 1 March rather than 28 February.
This is a choice, not a fact, and it is worth knowing which one applies to you:
- 1 March is the common convention in the United Kingdom, Hong Kong and New Zealand, and is what most software uses.
- 28 February applies for legal purposes in several jurisdictions including parts of the United States, Taiwan and Germany, on the reasoning that the person has completed a full year on the last day of February.
The difference matters exactly once, for one day, when a legal threshold like 18 or 21 falls in a common year. A toggle in the calculator switches between the two conventions so you can see both answers.
Leap years occur in years divisible by 4, except century years, unless those are divisible by 400. 2000 was a leap year. 1900 was not. 2100 will not be.
Count days until the next birthday
The calculator reports the next birthday date, the weekday it falls on, and the number of days remaining. It also reports the weekday of the original birth, which is the most-searched secondary question on any age calculator and one nobody can answer from memory.
For a birthday that has already passed this year, the next one is in the following year. For a 29 February birth date in a common year, the next birthday follows the convention set above.
Calculate age from a birth date in Excel
Excel calculates age with the DATEDIF function, which is undocumented in the function list but present in every version. Put the birth date in cell A1 in a date format Excel recognises, then use three formulas.
=DATEDIF(A1, TODAY(), "Y") → completed years
=DATEDIF(A1, TODAY(), "YM") → months after the last whole year
=DATEDIF(A1, TODAY(), "MD") → days after the last whole monthCombine them into one cell with CONCAT:
=CONCAT(B1, " years, ", C1, " months, ", D1, " days")Two warnings. DATEDIF with the "MD" unit returns incorrect results in some Excel versions when the end day is smaller than the start day, which is precisely the borrow case. And TODAY() recalculates every time the file opens, so a saved spreadsheet reports the age as of the last open, not the last save. For a fixed reference date, replace TODAY() with a cell holding that date.
Frequently asked questions
How old am I exactly?
Enter your date of birth and the Age Calculator returns your exact age in years, months and days as of today, along with total days lived and the weekday you were born on. Exact age counts completed years first, then completed months, then the remaining days, using the real length of each calendar month.
How many days old am I?
Total days lived is the count of calendar days from your birth date to today, including leap days. Someone born on 10 November 1991 has lived 12,675 days as of 25 July 2026. The calculator reports this alongside the years-months-days result, and also converts it to weeks, hours and minutes.
What day of the week was I born on?
The Age Calculator reports the weekday of your birth date directly. It comes from the Gregorian calendar, so it is exact for any date after 1582. If you want to work it out by hand, the Day of the Week Calculator shows Conway's Doomsday rule applied step by step to your date.
Why does my age differ by a day from another calculator?
Two causes account for nearly all discrepancies. Some calculators borrow a flat 30 days instead of the real month length, which shifts the days component. Others parse dates in a different format, reading 03/04 as 4 March instead of 3 April. Check the long-form date this calculator echoes back beneath the input field.
Does the calculator account for leap years?
Yes. Leap days are counted in the total days figure, and February's real length is used whenever the calculation borrows across it. Leap years fall in years divisible by 4, excluding century years unless they divide by 400, so 2000 was a leap year and 1900 was not.
When will I turn 18 or 21?
Set the calculator to find a date from an age: enter your birth date and the target age, and it returns the exact date you reach it. For a 29 February birth date the answer depends on the leap-day convention, so check which one applies where you live before relying on it for a legal threshold.
How is age calculated in Korea and East Asia?
Korea used a system in which a baby was one year old at birth and everyone aged a year on 1 January, making a person one or two years older than their international age. South Korea standardised on the international system in June 2023 for most legal and administrative purposes. This calculator returns international age.
Can I calculate the age of something that is not a person?
Yes. The arithmetic is identical for a building, a vehicle, a company or a document. Enter the founding, manufacture or issue date as the birth date. The calculator makes no assumption about what the date represents.
Summary
The Age Calculator returns exact age in years, months and days from a date of birth, borrowing real month lengths rather than a 30-day average so the result matches how ages are legally and conventionally counted.
It works in both directions: forward from a birth date to an age, and backwards from a known age to a birth date or birth-year range.
Total days lived, birth weekday, next birthday and days remaining come with every calculation. Leap days are counted, the 29 February convention is stated and switchable, and the parsed date is always echoed in long form so an ambiguous format never produces a silently wrong answer.