Calculating exact age sounds simple but gets surprisingly tricky when you account for month lengths and leap years. Here is the step-by-step method used by our calculator.
Step 1 — Count Complete Years
Start by finding how many complete years have passed since the birth date. If the birthday in the current year has already occurred, the age in years is the current year minus the birth year. If the birthday hasn't yet occurred this year, subtract one additional year.
Example: Born 15 March 1990, today is 20 May 2025. The birthday has occurred this year (15 March < 20 May), so years = 2025 − 1990 = 35 years.
Step 2 — Count Remaining Months
After accounting for complete years, count the remaining months from the last birthday. If the current day is after the birth day-of-month, count complete months. If the current day is before the birth day-of-month, the current month is not yet complete.
Step 3 — Count Remaining Days
Finally, count the days from the last complete month to today. If the current day (20) is after the birth day (15), the difference is 20 − 15 = 5 days remaining.
Leap Year Birthdays
People born on 29 February celebrate their exact birthday only in leap years. In non-leap years, most legal systems use either 28 February or 1 March as the birthday. In the UK, the legal birthday is 28 February. In some US states and New Zealand, it is 1 March. Our calculator handles this automatically.
Why Months Make Age Calculation Complex
Months have different lengths (28–31 days), so counting months isn't the same as counting blocks of 30 days. Our calculator counts complete calendar months, which means a person born on 31 January is one month old on 28 February (or 29 February in a leap year) — not on 2 or 3 March.
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