Age Calculator

Introduction

How old are you, exactly? Not just "I'm 28" — but precisely 28 years, 4 months, and 17 days. Or 10,398 days old. Or 13.5 days until your next birthday.

An age calculator doesn't just subtract birth year from the current year. It handles month and day differences correctly, accounts for leap years, and shows you your age across multiple units so you can pick the one that's most meaningful for your context.

This tool takes your date of birth and the current date, and gives you your exact age down to the day — plus a countdown to your next birthday. Everything runs in your browser; no data is stored or sent anywhere.

Technical & Concept Breakdown

The naive approach (subtracting years) gets you close but produces wrong results near birthdays.

Example: If today is February 15, 2026, and your birthday is March 3, 1997:

• Naive: 2026 - 1997 = 29 years ← wrong! Your 29th birthday hasn't happened yet this year
• Correct: 28 years, 11 months, 12 days

Correct age calculation algorithm:

1. Start with years = currentYear - birthYear
2. If the current month/day is before birth month/day, subtract 1 from years (the birthday hasn't happened yet this year)
3. Calculate remaining months: if current month ≥ birth month, months = currentMonth - birthMonth, else months = (12 - birthMonth) + currentMonth and adjust the day calculation accordingly
4. Calculate remaining days using the days-in-month of the relevant month, accounting for leap years

Leap year rule: A year is a leap year if:

• It's divisible by 4, AND
• Either NOT divisible by 100, OR divisible by 400
• So: 2000 ✓ (divisible by 400), 1900 ✗ (divisible by 100 but not 400), 2024 ✓

February 29 birthdays are rare (1 in ~1,461 births). The convention is to celebrate on February 28 or March 1 in non-leap years.

Total days calculation: Sum the days in each full year (accounting for leap years in that range), plus the days in partial months at the start and end.

Real-World Use Cases

Legal & Administrative: Age verification for legal contracts, elections, age-restricted services, and retirement eligibility often requires exact age on a specific date.

Medical & Healthcare: Pediatric medication dosing, developmental milestone tracking, and medical trial eligibility are often based on exact age in months, not just years.

HR & Payroll: Retirement plan eligibility, employment law (rules around hiring minors), and seniority calculations may depen...