How To Calculate Age From Date Of Birth

Age is not a single number. It can be stated in whole years, in total months, in weeks, in days, or in hours, and each form answers a different question. A passport wants whole years; a hospital chart may want days.

The reliable way to compute any of them is to convert both dates to a day number first. A day number is an integer that counts the days from a fixed epoch, so the difference between two day numbers is exactly the number of days between the dates.

The standard day count is the Julian day number, a continuous count of days used in astronomy since the sixteenth century. It runs from a single epoch, so it handles leap years, century rules and month lengths automatically.

Converting a calendar date to a Julian day number is a short formula using only integer division. It takes the day, the month and the year, shifts the year and month so that March starts the year, and applies the leap-year corrections.

Once both dates are converted, the day difference is a plain subtraction. That single figure drives every other output: dividing by seven gives whole weeks, dividing by the average month length gives total months, and multiplying by twenty-four gives hours.

Whole years are computed by dividing the day count by three hundred and sixty-five point two five, the average length of a Gregorian year. The result is correct for every day except the birthday itself, when it rounds down by one.

Total months use an average month length of about thirty point four four days. This is an average rather than a calendar count, so it is a good summary figure but not the same as the familiar years-and-months answer.

The familiar years-and-months-and-days form is harder, because calendar months vary between twenty-eight and thirty-one days. It requires borrowing, exactly like subtraction with a base of twelve months and varying day counts.

Leap days matter at the margins. A person born on 29 February technically has a birthday only in leap years, and the choice of 28 February or 1 March as the anniversary shifts the day count by one.

Time zones and the exact moment of birth are ignored. The calculation treats both dates as whole calendar days, which is the convention for almost every form that asks for an age.

Age in hours is only meaningful over short spans. Over decades the figure is large enough to be interesting but it ignores daylight saving, which is why it is shown as a curiosity rather than a precise measure.

The same subtraction works for anything that has a start date: the age of a building, the duration of a contract, or the number of days a policy has been in force.

The calculator takes both dates as day, month and year numbers. It does not use a calendar picker, so enter each part in the box provided and read the results below.