Age Calculator

How to use this calculator

1. Enter the date of birthThe date picker handles the format. Typing works too, as YYYY-MM-DD.
2. Leave the second date blank for todayFill it in to find age at a past or future date — useful for checking eligibility on a specific day, or working out how old someone will be at a wedding.
3. Read total days for the long viewTotal days, weeks and hours are counted directly from the calendar, so every leap day is included automatically.
4. Check the 10,000-day milestoneIt falls at about 27 years and 4 months, and almost nobody notices it. The date is in the breakdown.

Why dividing by 365.25 gives the wrong answer

The obvious approach to age is to count the days between two dates and divide by 365.25. It gives a number that looks right and is wrong in the detail.

The problem is that months are not equal. Going from 15 January to 15 March is two months, and so is going from 15 March to 15 May — but the first spans 59 days and the second spans 61. Any method that treats a month as a fixed number of days will disagree with the calendar.

The correct approach subtracts each component separately. Years minus years, months minus months, days minus days. When the day-of-month subtraction goes negative — because the current day of the month is earlier than the birth day — you borrow one month and add the number of days in the preceding month. That number is 28, 29, 30 or 31 depending on which month it is, which is exactly the information a division throws away.

A worked case makes it concrete. From 31 January to 1 March: the day subtraction is 1 − 31 = −30, so borrow a month and add February's length. In a common year that is 28, giving −2... which is still negative, and this is where implementations differ and where the ambiguity genuinely lives.

Most of the time it is unambiguous. The edge cases cluster around month ends, and they are the reason two age calculators can disagree by a day on the same inputs.

Leap years handle themselves as long as you count actual calendar days rather than assuming a year length. The 0.25 in 365.25 is an average, and an average is precisely what an exact age is not.

Worked example: born 15 June 1990, measured on 4 August 2026

Subtract each component.

Years: 2026 − 1990 = 36.

Months: August is month 8, June is month 6, so 8 − 6 = 2.

Days: 4 − 15 = −11. Negative, so borrow.

Borrowing reduces the months from 2 to 1, and adds the length of the month preceding August — July, which has 31 days. So the day count becomes −11 + 31 = 20.

Exact age: 36 years, 1 month, 20 days.

Now check what the naive method gives. The total elapsed days are 13,199. Divide by 365.25 and you get 36.14 years. Multiply the 0.14 by 12 and you get 1.7 months — which reads as roughly 36 years and 2 months, and is a month adrift from the correct answer.

The difference matters wherever a date threshold does. Eligibility for a pension, a licence, a school year or a legal right turns on whether you have reached an exact age on an exact day, and "about 36 years and 2 months" cannot answer that.

One UK-specific point worth knowing: a person attains an age at the start of the anniversary of their birth. Someone born on 3 June 2008 is 18 on 3 June 2026, not the day after. That was settled by the Family Law Reform Act 1969, which changed the older common law rule.

Ages that matter in the UK

State Pension age is the one worth checking individually rather than assuming. It has been rising and is scheduled to rise further, and the exact date depends on your date of birth rather than on a simple age. The GOV.UK State Pension age checker gives your specific date.

Leap years, birthdays and the arithmetic of dates

Someone born on 29 February has a real birthday only in leap years, which is roughly one year in four. The rule is that a year is a leap year if divisible by 4, except centuries, unless the century is divisible by 400. So 2000 was a leap year and 1900 was not.

In law, a person born on 29 February attains their age on 1 March in a common year in England and Wales. Socially most people celebrate on 28 February or 1 March according to preference.

The day of the week your birthday falls on moves forward by one each year, and by two across a leap year. That gives the familiar pattern where a birthday drifts through the week and then jumps. Over a 28-year cycle the pattern repeats exactly, so your birthday falls on the same weekday as the day you were born every 28 years — with exceptions around century years.

The 10,000-day milestone arrives at 27 years and about 4 months. The 20,000-day mark is around 54 years and 9 months. Neither is marked by anything, and both are easy to find once you count days rather than years.

Calculating with dates is unusually error-prone in software, which is why so many systems get it subtly wrong. Time zones, daylight saving transitions and the absence of a year zero all introduce off-by-one errors. This calculator works in whole calendar dates with no time component, which avoids the daylight saving problems entirely.

A final note on the Gregorian changeover: Britain adopted the Gregorian calendar in September 1752, and eleven days were removed. Dates before that need care, and any age calculation spanning the change will be out by eleven days unless it accounts for it. This one does not, so treat pre-1752 dates as unreliable.

Assumptions and limitations

• Uses the proleptic Gregorian calendar throughout, so British dates before September 1752 will be out by eleven days
• Works in whole calendar dates with no time of day, so it cannot give age to the hour
• Month-end edge cases are resolved by borrowing from the preceding month, which is a convention rather than a universal rule
• Does not account for time zones — a birth abroad may fall on a different local date
• Treats 29 February birthdays as falling on 1 March in common years, following England and Wales
• Does not check legal age thresholds, which vary between the four UK nations
• Ages in the table are indicative — State Pension age in particular depends on your exact date of birth

Questions people ask about this in United Kingdom