Working out how old someone is sounds trivial until you need the exact answer in years, months and days. This guide explains the method step by step, shows two worked examples and covers the cases that trip people up, such as birthdays on February 29.
The basic idea
Your age is the length of time between your date of birth and a second date, usually today. The full answer has three parts: complete years, then complete months, then the remaining days. A person born on March 14, 1990 is, on October 15, 2026, 36 years, 7 months and 1 day old.
The important word is complete. You become one year older on your birthday, not gradually, and in the same way a month is only counted once its day-of-month has arrived. Everything left over after the last complete month is counted as days.
Step by step
The method that works every time is to count forward from the birth date in the largest units first:
- Add whole years to the birth date until adding one more would pass the target date. The number of years you added is the age in years.
- From that date, add whole months in the same way. The number of months added is the month part.
- Count the days from there to the target date. That is the day part.
Counting forward avoids the usual mistake of subtracting numbers column by column, where you have to borrow from months of different lengths and easily make an error of a day or two.
Worked example 1: a simple case
Born March 14, 1990, today is October 15, 2026. Adding 36 years gives March 14, 2026, and adding 37 would give March 14, 2027, which is after today, so the age is 36 years. From March 14, 2026, adding 7 months gives October 14, 2026, and an eighth month would pass today. Finally, October 14 to October 15 is 1 day. The result is 36 years, 7 months and 1 day.
Worked example 2: when the day of the month is higher than today's
Born November 25, 1988, today is October 10, 2026. The birthday this year has not happened yet, so the last birthday was November 25, 2025, which means 37 complete years. From November 25, 2025, adding 10 months gives September 25, 2026, and an eleventh month would give October 25, which is after today. From September 25 to October 10 is 5 days to the end of September plus 10 days in October, so 15 days. The age is 37 years, 10 months and 15 days.
If you subtract the numbers directly (2026 − 1988 = 38 years, October − November = −1 month, 10 − 25 = −15 days) you have to borrow a year, a month and a month's worth of days. That is where mistakes creep in, because the number of days you borrow depends on which month it is.
Birthdays on February 29
People born on a leap day, known as leaplings, only have an actual birthday every fourth year. In the other years the birthday is celebrated on February 28 or March 1, and the rules differ between places for legal purposes. For counting age, a common convention is to treat the birthday as arriving on March 1 in years without a leap day, so a person born on February 29, 2000 is counted as turning 26 on March 1, 2026, although many people celebrate on February 28.
A calculator has to pick one convention and stick with it. The important thing is to be aware that the answer for those born on February 29 can differ by a day between systems.
Age in months, weeks and days
Sometimes the answer you want is not years but a single unit. To get age in days, count the days between the two dates, which includes every leap day in between. To get age in weeks, divide the days by 7. Age in months is the years times twelve plus the extra months, which is exact as long as you count complete months.
Parents of young children often count in weeks and then months. Doctors and schools use the date of birth directly rather than a rounded age, since a child who is 11 months and 28 days is quite different from one who is 12 months.
Common mistakes
- Subtracting only the years. This is right only if the birthday has already happened this year, and it is wrong for about half the year.
- Forgetting leap years when counting days. Between two dates 40 years apart there are about ten leap days, and ignoring them is a ten-day error.
- Counting the end date and the start date both. When you count days between two dates, one of the two ends should be left out, or the answer is one day too high.
- Using the wrong time zone near midnight. Someone born at 23:30 in Oslo was born on the next calendar day in Tokyo, and legal records use the place of birth.
Age in different cultures
Most of the world counts age the same way, from zero at birth. Until June 2023 South Korea also used a traditional system where a baby was one year old at birth and everyone added a year on January 1, which made people up to two years older than in international counting. The country moved to the international standard in 2023 for most legal and administrative purposes.
Knowing the convention matters when you compare ages across sources, for example in sports, school enrollment or travel rules.
Use the calculator
If you would rather skip the arithmetic, the age calculator on this site gives the exact result in years, months and days for any date of birth, and the days-old tool shows how many days you have lived. Both run in your browser, so the dates you type stay on your device.