How age is calculated
This tool counts full years between the date of birth and the reference date, then full months, then the remaining days — borrowing a month's worth of days whenever the day count would otherwise go negative. This mirrors how people naturally describe age ("35 years, 4 months, 12 days") rather than just a total day count.
Why the reference date matters
Age is always relative to a specific day. By default this calculator uses today, but many situations need a different reference date: checking eligibility for a program on its start date, verifying legal age at the time of a contract, or confirming a school cutoff date.
Total days and the next birthday
Alongside the years/months/days breakdown, the result also shows the total number of days lived and a countdown to the next birthday — both computed from the same two dates, so they stay in sync if you change either one.
Common uses for an age calculator
- Legal and administrative: confirming someone meets a minimum age requirement on a specific date.
- Health: pediatric dosing and growth charts that are calculated by exact age, not just birth year.
- Personal milestones: finding out exactly how many days until a big birthday.
Frequently asked questions
Why does the day count "borrow" from the month?
Months have different lengths, so if the birth day is later in the month than the reference day, one full month is converted into its number of days before subtracting — same logic as borrowing when subtracting by hand.
Can I calculate someone's age in the past or future?
Yes — set "Age as of" to any date on or after the date of birth to see what the age was, or will be, on that day.