Age Calculator

The age of a person is counted differently in different cultures. This calculator uses the most common system, the one where age increases on a person's birthday. Someone who has lived 3 years and 11 months is 3, and turns 4 a month later. Most western countries count this way, and so does almost every form you will ever fill in.

The calculator takes a date of birth and an end date, defaulting to today, and reports the gap in seven different units at once: years, months and days, then total months, weeks, days, hours, minutes and seconds. All seven describe the same interval.

Other ways of counting age

In some cultures age is expressed by counting years with or without the current year included. A person who is twenty years old under one system is in their twenty-first year under the other, which is the same fact stated from either end.

One of the traditional Chinese systems goes further: people are born at age 1, and age increases at the Traditional Chinese New Year rather than on the birthday. A baby born one day before that New Year is 2 years old two days later, at two days of age. Korea used a similar reckoning for generations before standardising on the international count in June 2023.

None of this makes any one system correct. It does mean that an age quoted without saying which system it came from can be out by up to two years, which matters more often than it sounds: medical records, immigration paperwork and genealogy all run into it.

Where months get ambiguous

The months and days part of an age result can be confusing, particularly when the start date sits at the end of a month.

Nobody disputes the simple case: February 20 to March 20 is one month. But there are two defensible readings of February 28, 2022 to March 31, 2022. Count February 28 to March 28 as one month, and the answer is one month and 3 days. Treat both February 28 and March 31 as the end of their months, and the answer is one month exactly. Both are reasonable.

The same fork appears for April 30 to May 31, and for May 30 to June 30, and it comes from nothing more than the uneven number of days in different months. This calculator uses the former method, matching day-of-month to day-of-month first and counting the leftover days afterwards. So:

• February 28 to March 31 gives 1 month 3 days
• April 30 to May 31 gives 1 month 1 day
• May 30 to June 30 gives 1 month

If you need a figure nobody can argue with, use the total days. It is the one line in the results that no convention can shift.

How the calculation runs

Years come off first, then months, then days. Subtract the birth year from the end year, then the birth month from the end month, then the birth day from the end day. If the day count comes out negative, borrow the length of the month immediately before the end date and reduce the month count by one. If the month count then goes negative, add twelve and reduce the years by one.

Working the default example through: March 30, 1985 to August 13, 2026 gives 41 years, 5 months and −17 days. Borrowing the 31 days of July turns that into 41 years, 4 months and 14 days, which is the headline answer.

Total days is computed separately, as a straight difference between the two dates, which is why it stays exact regardless of how the months were apportioned. Hours, minutes and seconds follow from whole days: 15,111 days is 362,664 hours, 21,759,840 minutes and 1,305,590,400 seconds. Time of day is not part of the calculation, so a birth at breakfast and a birth at midnight give the same answer.

Leap years, and February 29

A year is a leap year when it divides by 4, except for century years, which have to divide by 400. 2000 was a leap year; 1900 was not, and 2100 will not be. That correction keeps the calendar aligned with the solar year, which is closer to 365.2425 days than to 365.25.

Leap days are quietly doing work in every long age. Someone born in March 1985 has lived through ten February 29ths by 2026, and those ten days are already inside the 15,111 total. Nothing needs adding by hand.

Being born on February 29 is the awkward case. Three years in four have no such date, and jurisdictions differ on whether the legal birthday falls on February 28 or March 1. This calculator does not take a position: it counts to whatever end date you give it, and a leap-day birth reaching February 28 of a common year comes back as 11 months and 30 days rather than a full year, which is arithmetically correct even where the law would say otherwise.

When exact age matters

Legal thresholds are the obvious case. Voting, driving, drinking, signing a contract and drawing a pension all turn on a date rather than a rounded number of years, and being three days short is the same as being three years short.

Medicine runs on precision too. Paediatric dosing is often set by age band as well as weight, vaccination schedules are written in weeks and months for the first two years, and gestational and corrected age matter for babies born early.

Sport and school use cut-off dates, which is why age is so often quoted "as at" a fixed day rather than today. A child born a week either side of a cut-off can land in a different school year or a different competition group for the rest of their time in it. Setting the end date rather than accepting today is exactly what that field is for.

Employment and finance add their own thresholds: retirement dates, service milestones, insurance bands and annuity rates all key off an exact age at an exact date.

Which unit to quote

The seven lines in the result are not interchangeable in practice, because different fields have settled on different units.

Weeks are the unit for newborns and for pregnancy. Health visitors, growth charts and vaccination schedules all work in weeks for the first couple of months, which is why the weeks-and-days line sits high in the list rather than buried at the bottom.

Months take over from roughly two months to three years, where "18 months" carries more information than "1 year 6 months" for anyone reading a development chart.

Years, months and days is the general-purpose form, and the one legal and administrative paperwork expects.

Total days is the researcher's unit. It is exact, it sorts and subtracts cleanly, and it sidesteps every month-length convention on this page.

The two calendars underneath the result exist for the same reason: seeing both dates marked makes it obvious when a birthday has or has not passed yet in the end month, which is the single most common source of an age that looks a year out.

Common mistakes

The first is treating a year as 365 days. Over 41 years that assumption loses ten leap days, so a total-days figure derived that way is out by more than a week.

The second is dividing days by 30 to get months. It is a reasonable rough guide and it is never exactly right, because no month has 30 days more than four times a year.

The third is mixing up "age" with "the year you turn". Someone born in December is a full year younger than someone born in January of the same year for eleven months out of twelve, which is why school and sport use cut-off dates rather than birth years.

The fourth is forgetting the time zone. This calculator works in whole calendar dates rather than timestamps, which avoids the problem entirely, but a birth recorded at 11pm in one zone can be the following date in another, and the paperwork will follow whichever was written down.