How to Calculate Age From a Birth Date
Calculating exact age from a birth date requires more than simple year subtraction. This guide walks through calculating years, months, and days correctly, handling month-end edge cases, and using online tools to verify your result.
What You Need Before You Start
To calculate an exact age, you need two pieces of information: the birth date and the reference date (usually today). The birth date should include the month, day, and year. The reference date is the date you want to calculate the age as of, which is most commonly today but might be a future date for planning purposes or a past date for historical records.
Write both dates in the same format before starting. Using month, day, year consistently prevents confusion. If the birth date is March 15, 1990, write it as 03/15/1990. If today is February 18, 2026, write it as 02/18/2026. Having both dates clearly written out makes the subtraction steps less error-prone.
If you are calculating the age of someone who has not yet been born, or checking whether someone has reached a specific age by a certain date, the same method applies. The only difference is which date serves as the reference point.
Step 1 – Calculate the Years Component
Subtract the birth year from the reference year to get a preliminary year count. For a birth year of 1990 and a reference year of 2026, the preliminary count is 36. However, this count is only correct if the birthday has already occurred in the reference year.
To check whether the birthday has occurred, compare the birth month to the reference month. If the reference month is greater than the birth month, the birthday has passed. If the reference month is less than the birth month, the birthday has not yet occurred. If the months are equal, compare the days: if the reference day is greater than or equal to the birth day, the birthday has passed.
For a birth date of March 15 and a reference date of February 18, the reference month (February, 2) is less than the birth month (March, 3), so the birthday has not occurred yet in the reference year. Subtract 1 from the preliminary count: 36 minus 1 equals 35 years.
Step 2 – Calculate the Months Component
To find the months component, identify the date of the most recent birthday. In the example, the most recent birthday before February 18, 2026 was March 15, 2025. Count the number of complete months from March 15, 2025 to February 18, 2026.
Starting from March 15, 2025, each complete month advances the count by one. April 15 is one month, May 15 is two months, June 15 is three, July 15 is four, August 15 is five, September 15 is six, October 15 is seven, November 15 is eight, December 15 is nine, January 15 is ten, and February 15 is eleven complete months.
Since today is February 18, which is past the 15th, the February month is fully complete. The months component is 11. If today were February 10 instead, February would not yet be complete, and the months component would be 10.
Step 3 – Calculate the Days Component
The days component counts the days since the most recent complete month anniversary. In the example, the most recent complete month anniversary was February 15, 2026 (eleven months after the last birthday). From February 15 to February 18 is 3 days.
When the reference day is before the birth day within the current month, you need to borrow from the previous month. Suppose today were February 10 and the birth day is the 15th. Since 10 is less than 15, count the remaining days in the previous month (January has 31 days, so from January 15 to January 31 is 16 days) plus the current day count (10), giving 26 days.
The complete age in the main example is 35 years, 11 months, and 3 days. To verify, count forward 35 years, 11 months, and 3 days from March 15, 1990: adding 35 years gives March 15, 2025; adding 11 months gives February 15, 2026; adding 3 days gives February 18, 2026. The result matches the reference date.
Using an Age Calculator to Verify
Online age calculators automate all three steps and handle edge cases like leap year birthdays automatically. Enter the birth date and reference date, and the calculator returns the exact age in years, months, and days instantly. These tools are reliable for standard dates and worth using any time precision matters.
When verifying a result, also use the calculator to count forward from the birth date by the calculated age. If the forward-count lands exactly on the reference date, the calculation is correct. If it does not match, there is likely an error in how a month boundary or leap year was handled.
For legal documents, medical records, or official forms requiring exact age, always double-check your calculation using two independent methods. Manual calculation and an online calculator should agree. If they do not, identify the discrepancy before using the number in any formal context.