How to Calculate Your Exact Age in Years, Months, and Days
Most people know their age in years, but calculating your exact age down to months and days involves a few steps that can trip people up. This post explains the right way to count from a birth date to today.
Why Simple Subtraction Is Not Quite Enough
If someone was born on March 15, 1990, and today is February 18, 2026, subtracting the years gives 36. But the person has not yet reached their birthday this year, so they are still 35. Simply subtracting birth year from the current year ignores whether the birthday has occurred yet in the current calendar year.
The same issue arises with months and days. Months have different lengths, and leap years add an extra day to February every four years. These variations mean you cannot multiply or divide uniformly to go from days to months or from months to years.
A proper age calculation requires working through years, months, and days as separate components, adjusting each one based on whether the current date has passed the corresponding component of the birth date.
Step-by-Step: Years First
Start by subtracting the birth year from the current year. This gives you a preliminary year count. Then check whether the birthday has already occurred this calendar year by comparing the birth month and day to the current month and day.
If the current month is earlier than the birth month, or if the current month matches but the current day is earlier than the birth day, the birthday has not yet happened this year. In that case, subtract 1 from your preliminary year count. Otherwise, keep the preliminary count as is.
For the example of a March 15 birthday and a current date of February 18, the current month (February, month 2) is earlier than the birth month (March, month 3), so the birthday has not occurred yet. The correct age in years is 2026 minus 1990 minus 1, which equals 35.
Counting the Remaining Months
After determining the full years, find how many complete months have passed since the last birthday. Start from the last birthday month and count forward to the current month. If the current day of the month is before the birth day, subtract one month because the current month is not yet complete.
In the example, the last birthday was March 15, 2025. From March to February of the following year spans 11 months if counted forward. However, since February 18 is after the 15th, the month of February is complete, giving 11 full months since the last birthday.
This step is where most manual calculations go wrong. People forget to check whether the current day has passed the birth day within the current month, which can cause an off-by-one error in the month count.
Counting the Remaining Days
Finally, count the remaining days after the last complete month. If today is the 18th and the birth day is the 15th, the remaining days since the last complete month began are 18 minus 15, which equals 3 days.
When the current day is before the birth day, you need to borrow from the previous month. Take the number of days in the previous month, add the current day number, and subtract the birth day. This handles the irregularity of month lengths correctly.
Combining the three components gives the full exact age. For the example, the person is 35 years, 11 months, and 3 days old as of February 18, 2026. Age calculators online automate all of these steps, but understanding the manual process helps you verify results and spot errors.
When Leap Years Matter
People born on February 29 during a leap year face a unique situation in non-leap years. Since February 29 does not exist in most years, their birthday is celebrated on either February 28 or March 1 depending on convention. Most legal systems and age calculators treat February 28 as the legal birthday in non-leap years.
Leap years also affect date differences more broadly. A period spanning multiple February months will include different numbers of days depending on whether a leap year falls within it. This matters most for precise calculations involving time elapsed in days.
Age calculators handle leap year adjustments automatically. If you are doing calculations by hand for a leap year birthday, check whether a leap year falls within the period you are calculating and add the extra day where appropriate.