Quick answer: To calculate age, subtract the birth date from today's date in years, months, and days, borrowing when you need to. Someone born on 15 May 1990 is 36 years, 4 months, and 13 days old on 28 September 2026.
Age looks simple until you try to work it out exactly. Saying "I am 36" is easy, but saying "36 years, 4 months, and 13 days" needs a careful subtraction, because months have different lengths and February changes length in leap years. This guide shows you the step-by-step method, works through a full example, and explains the two cases that catch most people out: borrowing, and leap years.
The Step-by-Step Method: Years, Months, and Days
Calculating age is a subtraction, but it is done in three columns at once: years, months, and days. The rule is to start with the days, then the months, then the years, and to borrow from the next column whenever the top number is smaller than the bottom number.
Step 1: Write the current date and the birth date underneath it, in the order year, month, day. For example, the current date is 28 September 2026, and the birth date is 15 May 1990. Written as year, month, day, that is 2026, 9, 28 on top and 1990, 5, 15 underneath.
Step 2: Subtract the days. If the current day is smaller than the birth day, borrow the number of days in the previous month from the months column, add them to the current day, and reduce the months by one. This is the step most people get wrong, and it is explained in detail in the next section.
Step 3: Subtract the months. If the current month is smaller than the birth month, after any borrowing in step 2, borrow 12 months from the years column, add them to the current month, and reduce the years by one.
Step 4: Subtract the years. The result is the age in whole years. The months and days left over after steps 2 and 3 are the remaining months and days of age.
Worked Example: Born 15 May 1990 on 28 September 2026
Here is the example in full, so you can follow each step and check it against your own calculation.
Step 1: Write the dates. Current date: 2026, month 9, day 28. Birth date: 1990, month 5, day 15.
Step 2: Subtract the days. 28 − 15 = 13. The current day is larger than the birth day, so no borrowing is needed. The days part of the age is 13 days.
Step 3: Subtract the months. 9 − 5 = 4. The current month is larger than the birth month, so no borrowing is needed. The months part of the age is 4 months.
Step 4: Subtract the years. 2026 − 1990 = 36. The birthday, 15 May, has already happened in 2026, because September comes after May, so the full 36 years count. The years part of the age is 36 years.
So the age is 36 years, 4 months, and 13 days. You can check it by adding it back: 15 May 1990 plus 36 years is 15 May 2026, plus 4 months is 15 September 2026, plus 13 days is 28 September 2026, which is the current date. If adding the age back to the birth date does not land on the current date, one of the subtractions has gone wrong.
If you want to check any date quickly, use the age calculator. Enter the birth date and the date you want the age on, and it will give you the years, months, and days, and the total days as well.
Borrowing: When the Day or Month Is Too Small
Borrowing is the part of the method that needs the most care, because the number you borrow depends on the calendar. Here is a second example where borrowing is needed in both the days and the months.
Suppose someone was born on 20 December 1995, and the current date is 10 March 2026. Work through it step by step.
Step 1: Write the dates. Current date: 2026, month 3, day 10. Birth date: 1995, month 12, day 20.
Step 2: Subtract the days. 10 − 20 = −10, which is not allowed, so borrow from the months. The previous month is February 2026, which has 28 days, because 2026 is not a leap year. Add 28 to the current day: 10 + 28 = 38. Subtract the birth day: 38 − 20 = 18. Reduce the months by one, from 3 to 2. The days part is 18 days.
Step 3: Subtract the months. The months are now 2, and the birth month is 12, so 2 − 12 is not allowed either. Borrow 12 months from the years: 2 + 12 = 14, and 14 − 12 = 2. Reduce the years by one, from 2026 to 2025. The months part is 2 months.
Step 4: Subtract the years. 2025 − 1995 = 30. The years part is 30 years.
So the age is 30 years, 2 months, and 18 days. Check it by adding back: 20 December 1995 plus 30 years is 20 December 2025, plus 2 months is 20 February 2026, plus 18 days is 10 March 2026, because February 2026 has 28 days and the remaining 8 days of February plus 10 days of March make 18. The check works.
| Birth date | Date checked | Age |
|---|---|---|
| 15 May 1990 | 28 Sep 2026 | 36 years, 4 months, 13 days |
| 20 Dec 1995 | 10 Mar 2026 | 30 years, 2 months, 18 days |
| 1 Jan 2000 | 1 Jan 2026 | 26 years, 0 months, 0 days |
Leap Years and 29 February Birthdays
A leap year has 366 days instead of 365, with an extra day, 29 February. A year is a leap year if it is divisible by 4, except for years divisible by 100, which are leap years only if they are also divisible by 400. So 2024 was a leap year, 2026 is not, 2000 was a leap year, and 2100 will not be.
Leap years matter in two ways. First, they change the number of days you borrow when the previous month is February. If you are calculating an age in March of a leap year, and you need to borrow from February, you borrow 29 days, not 28. In the example above, the calculation was done in March 2026, which is not a leap year, so 28 days was correct. In March 2024, the same borrowing would have used 29 days. To check the days between two dates, use the date calculator.
Common Mistakes to Avoid
The most common mistake is to subtract only the years. If you were born in 1990 and the current year is 2026, you are not automatically 36. If your birthday is in December and the current month is September, you are still 35 until December arrives. Always check whether the birthday has happened yet this year, and if it has not, subtract one year from the simple year difference.
Try it: Work out any age in years, months, and days with the Age Calculator — enter the date of birth and it does the borrowing for you.
Frequently Asked Questions
How do I calculate age from date of birth? Subtract the birth date from the current date, starting with days, then months, then years, borrowing when the top number is smaller. For example, born 15 May 1990 on 28 September 2026 gives 36 years, 4 months, and 13 days.
What is the age of someone born on 15 May 1990 on 28 September 2026? 36 years, 4 months, and 13 days. The calculation is 28 − 15 = 13 days, 9 − 5 = 4 months, and 2026 − 1990 = 36 years, because the birthday on 15 May has already happened in 2026.
How do leap years affect age calculation? Leap years add 29 February, so they change the number of days you borrow when the previous month is February in a leap year. People born on 29 February usually celebrate on 28 February or 1 March in non-leap years, and the legal birthday rule varies by country.
Why is my age not just the current year minus my birth year? Because your birthday may not have happened yet this year. If you were born in December 1990 and it is now September 2026, you are still 35, not 36. Always check the month and day, not just the year.
