How to calculate age from a date of birth
By Dexaloop · Updated 27 October 2026 · Checked against the sources listed below
Your age in years is the number of birthdays you have had: subtract your birth year from the current year, then subtract 1 more if this year's birthday has not come yet. For an exact age in years, months and days, count whole months on from your last birthday, then count the days left. The only tricky parts are month ends and February 29 birthdays, and for those the rules are conventions, not math.
This guide is general information. Where it mentions law, it names the place, and the rules were checked on 27 October 2026.
Step 1: whole years
Compare the month and day of the birth date with today's month and day.
- If today's month and day are on or after the birthday, age = this year minus birth year.
- If they are before the birthday, age = this year minus birth year, minus 1.
Example: born March 18, 1994. On October 27, 2026 the birthday (March 18) has passed, so the age is 2026 - 1994 = 32 years. On February 10, 2026 it had not passed, so the age then was 2026 - 1994 - 1 = 31.
Step 2: months and days
Start from the most recent birthday. Count how many whole months fit before today, then count the leftover days.
Worked example 1: no borrowing
Born March 18, 1994. Today is October 27, 2026.
- Last birthday: March 18, 2026. Age in years: 32.
- Whole months from March 18: April 18, May 18 and so on up to October 18. That is 7 months.
- Days from October 18 to October 27: 9 days.
Exact age: 32 years, 7 months, 9 days. In total days, that is 11,911 days.
Worked example 2: borrowing across a month end
Born March 29, 1994. Today is October 27, 2026.
- Last birthday: March 29, 2026. Age in years: 32.
- Whole months: April 29 up to September 29. October 29 has not arrived, so that is 6 months.
- Days from September 29 to October 27. September has 30 days, so that is 1 day to September 30, plus 27 days in October: 28 days.
Exact age: 32 years, 6 months, 28 days. The step people get wrong is the borrowed month. You must use the length of the month you are borrowing from, here September with 30 days, not a fixed 30 or 31.
The month-end problem
Months have 28 to 31 days, so "one month after" is not always a real date. One month after January 31 would be February 31, which does not exist. There is no single law of arithmetic that settles this. Two common conventions are:
- Clamp to the month end: one month after January 31 is February 28 (or 29 in a leap year).
- Roll over: the missing days spill into March, so one month after January 31 is March 3 in a common year.
Check which convention a tool or form uses, and use one consistently. A difference of a day or two only matters for babies' ages in months and for tight legal deadlines.
February 29 birthdays
About one day in every 1,461 is a February 29. People born on it have a real birthday only in leap years. In other years, someone has to decide which day counts. The answer depends on where you are and why you are asking.
What the law says in two places
| Place | Law | What it says about attaining an age |
|---|---|---|
| England and Wales | Family Law Reform Act 1969, section 9 | An age is attained at "the commencement of the relevant anniversary of the date of his birth." It does not spell out the February 29 case. |
| Newfoundland and Labrador, Canada | Age of Majority Act, section 3 | "The relevant anniversary of a person born on February 29 is the last day of the month of February." |
So in Newfoundland and Labrador, a person born on February 29, 2008 reached 19, the age of majority there, on February 28, 2027. Where a law is silent, as the England and Wales section is on this point, other rules or a court may decide. Do not assume your country follows either example.
Everyday practice
For birthday cards and apps, people often choose February 28 or March 1 by preference. For anything with legal effect, such as voting, driving, school entry, pensions or a contract, ask the body that applies the rule which date it uses.
Worked example 3: why dividing by 365.25 fails
Born February 29, 2008. Check the age on February 28, 2026.
- Days between the two dates: 6,574.
- 6,574 / 365.25 = 17.9986. Rounded down, that says 17.
- Under a "last day of February" rule like Newfoundland and Labrador's, the person turned 18 that very day.
The division gives an average-year answer. Birthdays do not follow averages, they follow the calendar. Count years by comparing dates, as in Step 1, and keep total days for when someone actually asks for days.
Common mistakes
- Subtracting years only. 2026 - 1994 is 32 only if the birthday has passed. Before it, the person is 31.
- Using 30 days for every month. When borrowing days, use the real length of that month.
- Dividing total days by 365 or 365.25. It drifts around birthdays and leap years.
- Counting the birth day as day 1. On the day of birth the age is 0 days, not 1. The day count starts the next day.
- Mixing time zones. A birth date is a calendar date. Converting it to another time zone can move it a day. Work with plain dates.
- Assuming one February 29 rule. It differs between places and purposes.
Calculate an exact age: tools.dexaloop.com/age-calculator/. Free, no sign-up, and it runs on your device.
Questions
How do I calculate my age in years?
Subtract your birth year from the current year. If your birthday has not happened yet this year, subtract 1 more. That gives the number of birthdays you have had, which is your age in years.
When does someone born on February 29 turn a year older?
In a leap year, on February 29. In other years it depends on the rule being used. Some laws pick the last day of February, others may use March 1, and many laws do not say. For anything legal, check the rule where you live.
Why not just divide the days by 365.25?
Because birthdays fall on calendar dates, not every 365.25 days. Dividing can give the wrong whole number of years on or near a birthday, especially for people born in or near February.
How many months old is a baby born on January 31?
Count whole months from the 31st. February has no 31st, so a common convention treats the last day of a short month as the monthly anniversary. Check which convention a form or clinic uses.
Does the time of birth matter?
For everyday age, no. Under the law of England and Wales a person attains an age at the start of the anniversary of their birth date, so the whole birthday counts from midnight, whatever time they were born.
Sources
- legislation.gov.uk: Family Law Reform Act 1969, section 9 (England and Wales)
- Newfoundland and Labrador: Age of Majority Act, SNL1995 c. A-4.2
This guide is general information, not legal, tax or financial advice. Rules change; check the official source for your own case. Found a mistake? Email support@dexaloop.com.