To calculate age from a date of birth, compare the complete birth date with the target date. Start with the difference between the two years, then subtract one year when the birthday has not yet occurred in the target year. For an exact result, continue by counting completed months and the remaining days.
Enter a date of birth and target date to calculate exact age, total days, complete weeks, birth weekday, and the next birthday.
Check whether the birthday has happened by the selected target date before accepting the completed-year result.
The Basic Age Formula
The basic completed-year formula begins by subtracting the birth year from the target year. This gives an initial age, but it is not always the final answer because the birthday may still be ahead.
The birthday adjustment is zero when the birthday has already occurred on or before the target date. It is one when the birthday has not yet occurred.
Step 1: Write the Complete Dates
Use the full date of birth and the full target date, including day, month, and year. The target date may be today, a past date, or a future date.
A complete date is necessary because the day and month determine whether the birthday has been reached during the target year. Using only the two year numbers can make the result one year too high.
Step 2: Subtract the Years
Subtract the birth year from the target year to obtain the initial year difference. For a birth year of 2000 and target year of 2026, the initial difference is 26.
Do not stop here. The next step is to compare the birthday with the month and day of the target date.
Step 3: Check Whether the Birthday Has Passed
Compare the birth month and day with the target month and day. When the target date falls on or after the birthday, keep the initial year difference. When the target date falls before the birthday, subtract one year.
This birthday check is the most important part of a completed-year age calculation. It explains why two dates in the same target year can produce different ages.
Worked Example: Birthday Has Not Happened
Suppose someone was born on December 10, 2000 and the target date is June 24, 2026.
The initial year difference is 2026 minus 2000, which equals 26. But December 10 has not yet arrived by June 24, so one year must be subtracted.
The completed-year age is 25. The exact age is 25 years, 6 months, and 14 days.
Worked Example: Birthday Has Happened
Now use the same date of birth, December 10, 2000, with a target date of December 20, 2026.
The birthday occurred ten days earlier, so no extra year is subtracted. The completed age is 26 years.
How to Calculate Exact Age
Completed years are often enough for ordinary age questions, but an exact age includes years, months, and days.
Count completed years first. From the latest birthday, count completed calendar months. Then count the days remaining after the latest monthly anniversary.
ReadHow to Calculate Age in Years, Months, and Daysfor the detailed calendar method.
Age on a Past or Future Date
The target date does not need to be today. Use a historical date to find how old someone was at an earlier event, or a future date to find how old they will be at a planned event.
The same birthday adjustment applies in every case. The target date must be the same as or later than the date of birth.
For future planning, readAge Calculator for a Future Date.
Month-End and Leap-Day Birthdays
Birthdays on the 29th, 30th, or 31st require care because not every month contains those dates. Calendar calculations commonly use the final valid day of a shorter month when a matching date does not exist.
A February 29 birthday also needs a convention during a non-leap year. This calculator uses the final valid day of February for calendar arithmetic, while official rules may vary.
Completed Age vs Running Age
Completed age counts only full years that have already passed. Running age is an informal expression sometimes used for the year of life a person has entered.
Someone who has completed 25 years is living through their 26th year, but their completed age remains 25 until the next birthday. Forms and eligibility rules normally ask for completed age unless they state a different definition.
Common Mistakes
A common mistake is subtracting only the two year numbers and ignoring whether the birthday has happened. This can make the result one year too high.
Another mistake is assuming every month has 30 days or every year has 365 days. These shortcuts do not reproduce exact calendar age.
Also check that the date fields have not been reversed and that the target date is not earlier than the date of birth.
FAQs
How do I calculate age from a date of birth?
Subtract the birth year from the target year, then subtract one more year when the birthday has not yet occurred.
Why can the year subtraction be one year too high?
The subtraction assumes the birthday has already happened in the target year. When it has not, one year must be removed.
Can I calculate age on a past date?
Yes. Use the past date as the target date, provided it is not earlier than the date of birth.
Can I calculate age on a future date?
Yes. Enter the future date as the target date and apply the same birthday check.
How is a February 29 birthday handled?
This calculator uses the final valid day of February in a non-leap year, while legal or administrative rules may differ.