Step 1: Get the whole years right
Start with the year. Subtract the birth year from the year you're measuring in. Then ask one question: has the birthday happened yet this year? If not, take one off.
Take someone born October 20, 1990. On August 10, 2026, 2026 minus 1990 gives 36. October 20 hasn't arrived, so they're 35. On October 20 they turn 36. That one check is where most quick answers go wrong.
Formula notes
Age = year now - birth year - (1 if the birthday is still ahead this year)
Step 2: Count the months and days
For an exact age, start from the last birthday. Count forward in whole months until the next step would pass the target date. Then count the days that are left.
Someone born March 25, 2000 had their last birthday on March 25, 2026. On August 10, 2026, four whole months take you to July 25. From July 25 to August 10 is 16 days. Their age is 26 years, 4 months and 16 days.
Always count forward from the birthday, never backward from today. Months have 28 to 31 days, so counting in the other direction can give a different day count.
Three worked cases
These cases cover the situations people trip over most. Each one was checked against the calendar.
- Birthday already passed: born January 15, 1995, measured August 10, 2026. 31 years, then 6 months to July 15, then 26 days. Answer: 31 years, 6 months, 26 days.
- Birthday still ahead: born October 20, 1990, measured August 10, 2026. 35 full years, 9 months to July 20, then 21 days. Answer: 35 years, 9 months, 21 days.
- Birthday today: born August 10, 2008, measured August 10, 2026. Exactly 18 years, 0 months, 0 days, and 6,574 days old.
Leap-day birthdays need a rule
If you were born on February 29, three years out of four have no matching date. You have to pick a stand-in: February 28 or March 1. Both are in use, and official rules don't all agree.
The pick matters right around the birthday. Born February 29, 2000 and measured February 28, 2001, you're 1 year old under the February 28 rule. Under the March 1 rule you're 11 months and 30 days old, one day short. When it counts for something official, find out which rule that office uses.
Doing it in Excel or Google Sheets
Spreadsheets have a function for this. With the birth date in A1, =DATEDIF(A1,TODAY(),"Y") gives whole years. Use "YM" for the months after the last birthday and "D" for total days.
Microsoft's own documentation warns against the "MD" unit for leftover days because it has known problems and can return wrong or negative numbers. If you need the days, count them from the last full month instead, or check the answer with our age calculator.
Formula notes
Years: =DATEDIF(A1, TODAY(), "Y")Months after last birthday: =DATEDIF(A1, TODAY(), "YM")Total days: =DATEDIF(A1, TODAY(), "D")
Common mistakes
A few habits give answers that look right and aren't. Watch for these.
- Subtracting years without checking whether the birthday has passed.
- Dividing total days by 365. It drifts by about a day every four years because of leap days.
- Treating every month as 30 days when you want calendar months.
- Mixing up exact age (years, months, days) with age at last birthday (whole years).
- Assuming a school, insurer or benefit office counts age the same way you do.
Frequently asked questions
What is the formula to calculate age?
Subtract the birth year from the current year, then subtract one more if the birthday hasn't happened yet this year. That gives completed years, the age most forms want.
How do you calculate age in months?
Multiply completed years by 12, then add the full months since the last birthday. Someone 31 years and 6 months old has lived 378 full months.
Why is dividing days by 365 wrong?
Leap years add a day roughly every four years, so 365 slowly overstates your age. Dividing by 365.2425 is closer, but it's still an average, not your calendar age.
Does a birthday count on the day itself?
Yes, for ordinary age. You turn a new age on your birthday. Some programs, like U.S. Social Security, treat you as reaching an age the day before.
How do I calculate age in Google Sheets?
Use =DATEDIF(A1, TODAY(), "Y") with the birth date in A1. It works the same way as in Excel.
References
These sources support the method or guidance used for Age Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Age Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.