Age calculator by date of birth

How to use the age calculator

1. Enter the date of birth. The answer to "how old am I" appears straight away, in years, months and days.
2. Leave "Age on this date" as today, or change it to work out an age on any other date, past or future. Age today puts today back.

Under the main result you also get the same age in total months, weeks and days, an approximate number of hours, the date and weekday of the next birthday with the days to go, the half birthday, and the day of the week you were born on. On your birthday the calculator wishes you a happy birthday. Copy result puts a short summary on your clipboard. Everything is worked out on your device and nothing is saved.

How age is worked out (the age calculator formula)

In the US and the UK, age is counted in completed years: you are 36 from your 36th birthday until the day before your 37th. The calculator then counts the months and days since your last birthday:

1. Years: this year minus your birth year, minus 1 if your birthday is still to come this year.
2. Months: whole months since your last birthday. A month is complete when the day of the month you were born on comes round again.
3. Days: the days since that last whole month.

Here it is for someone born on May 15, 1990, on October 8, 2026:

So the age is 36 years, 4 months and 23 days. That is also 436 months and 23 days, 1,899 weeks and 2 days, or 13,295 days, about 319,080 hours. The next birthday, the 37th, is Saturday, May 15, 2027, 219 days away, and May 15, 1990 was a Tuesday.

In England and Wales the law says exactly when you reach an age. Section 9 of the Family Law Reform Act 1969 sets it at "the commencement of the relevant anniversary of the date of his birth", so you are 18 from the very start of your 18th birthday, not from the time of day you were born.

How old am I if I was born in a given year?

If you only know the year of birth, the age calculator by year of birth is simple subtraction, but there are two answers in any year: the age before the birthday and the age it turns into on the birthday. For 2026:

For any other year, take the birth year from 2026. Someone born in 1990 turns 36 in 2026 and is 35 until the birthday. Enter the full date of birth above to know which of the two applies today.

Age in months, weeks and days

Babies are usually described in weeks and then months, so the calculator shows both. A baby born on June 20, 2026 is, on October 8, 2026:

To answer "how many days old am I", look at the In days line: it counts every day from the date of birth to the age on date, leap days included, with the day you were born counted as day 0. Hours are days times 24, so they are approximate: the calculator does not ask for a time of birth.

For a baby born early, an age counted from the due date, called the corrected age, is also used. The corrected age calculator works that out.

Age on another date, and age at death

The calculator also works as an age calculator from date to date. Change "Age on this date" to any day after the date of birth:

• A future date: how old you will be on a trip, a wedding or a school start date.
• A past date: how old you were when something happened.
• Age at death: enter the date of birth and the date of death. Someone born on March 3, 1921 who died on January 5, 1999 was 77 years, 10 months and 2 days old.

When the age on date is not today, the next birthday and half birthday are counted from that date instead. If you only need the number of days between two dates, the days between dates calculator is simpler.

February 29 birthdays and the ends of months

If you were born on February 29, your birthday only appears in leap years. The US Naval Observatory gives the rule: "years evenly divisible by 4 are leap years, with the exception of centurial years that are not evenly divisible by 400", so 2028 is a leap year and 2100 is not. In other years this calculator uses February 28, the last day of February, so you complete each year of age on February 28 and are one day older on March 1. In a leap year the birthday is February 29 itself. Some people celebrate on March 1 instead, and where a legal age matters, the law where you live decides which day counts.

The same rule covers months. Someone born on January 31 completes a month on February 28 (or 29), because February has no 31st. Spreadsheets do this differently, as the next section explains.

Age in Excel or Google Sheets

With the date of birth in cell A2, these formulas work in both Excel and Google Sheets. Microsoft notes that DATEDIF "is useful in formulas where you need to calculate an age", and TODAY returns the current date.

DATEDIF with "Y" gives "The number of complete years in the period". You may see =DATEDIF(A2,TODAY(),"MD") for the leftover days, but Microsoft warns that "MD" "may result in a negative number, a zero, or an inaccurate result", which is why the table moves the date of birth on by the whole months with EDATE instead. To get the age on another date, put that date in B2 and use B2 in place of TODAY(). DATEDIF shows #NUM! if the date of birth is after the end date.

DATEDIF counts a month or year only when the same day number is reached. Google says months and years are "only counted if they are equal to or go past the 'day'", so under that rule a February 29 birthday completes a year on March 1 in common years, one day later than in this calculator. Google also notes that TODAY reflects the date "the last time the spreadsheet was recalculated".

Questions and answers

Sources

Checked on 8 October 2026. If a rule has changed, please tell us.

• legislation.gov.uk: Family Law Reform Act 1969, section 9
• Microsoft Support: DATEDIF function
• Microsoft Support: TODAY function
• Microsoft Support: EDATE function
• Google Docs Editors Help: DATEDIF function
• Google Docs Editors Help: TODAY function
• Google Docs Editors Help: EDATE function
• US Naval Observatory: Leap years