Age Calculator

Your age is the most fundamental number in your legal and financial life. It determines when you can drive, vote, drink, sign a contract, receive Medicare, claim Social Security, and make penalty-free retirement withdrawals. It affects your car insurance rate, your health insurance plan, your loan terms, and your eligibility for dozens of government programs.

Most people know their age in years. Far fewer know it precisely in years, months, and days and almost nobody knows what their exact age means for the specific decisions they're making right now.

This calculator gives you the precise number. The content below tells you what that number means legally, financially, medically, and in a few ways that might genuinely surprise you.

There is a number that productivity researchers, life coaches, and writers have been quietly referencing for years: 10,000 days.

You cross 10,000 days of life at approximately age 27 years and 107 days—the equivalent of one-quarter of a century plus a little change. For most Americans, 10,000 days is the age when formal education is complete, the first career is taking shape, and the decisions that compound over a lifetime are beginning to be made.

Malcolm Gladwell's famous 10,000-hour rule of deliberate practice overlaps interestingly here—10,000 hours of focused practice at 3 hours per day takes approximately 9.1 years. Start at 18, reach mastery-level practice hours at approximately age 27. At the same age you cross 10,000 days.

Approximately 1 in 1,461 people are born on February 29, a leap day. In the United States, roughly 212,000 people have this birthday. They are sometimes called "leaplings" or "leap year babies," and their birthday presents a genuinely interesting legal question: when do they turn a legally significant age in a non-leap year?

Source: American Bar Association Journal, "The Leap Year Birthday Problem in US Law," 2020.

If you have Korean heritage or work with Korean communities, understanding the traditional Korean age system matters because it was officially unified with the Western age system on June 28, 2023.

Under the traditional Korean system, a person was considered 1 year old at birth (not 0), and gained an additional year on January 1 each year rather than on their birthday. This meant a baby born on December 31 would be considered 2 years old by the following January 2—just two days after birth. The difference between Korean traditional age and Western chronological age could be 1 or 2 years depending on whether the person's birthday had occurred yet in the current year.

South Korea's National Assembly passed legislation in December 2022 that standardized the Western (Gregorian) age system as the official legal standard for all government documents, laws, and administrative purposes, effective June 28, 2023. The traditional and "year age" systems (common in informal and cultural contexts) persist in social usage but are no longer used in official Korean government documents.

This calculator uses the Western chronological age system—completed years from date of birth to today.

For parents of premature infants and pediatric healthcare providers, chronological age and corrected age (also called adjusted age) are two different measurements with two different clinical purposes.

The American Academy of Pediatrics (AAP) recommends using corrected age for developmental assessments until approximately 24 to 36 months of chronological age. Using chronological age for a premature baby's developmental milestones will make the child appear behind on every metric when they are actually developing normally.

One of the most common practical uses of an age calculator is determining whether a child meets the kindergarten enrollment cutoff for their state. All 50 states have an age cutoff typically between September 1 and December 1 by which a child must turn 5 to enroll in kindergarten for that school year.

The most common cutoff date is September 1 used by 32 states including California, Texas, New York, Florida, and Illinois. Children who turn 5 on or before September 1 may enroll; those who turn 5 after that date must wait until the following school year.

Use the "calculate age on a specific date" feature in the calculator—enter your child's birth date and your state's cutoff date to instantly determine eligibility.