Calculating age is simple in most cases: subtract the birth year from the current year, then subtract one if the birthday has not yet occurred this year. A person born on March 15, 1990, turns 36 on March 15, 2026. Before that date in 2026, they are still 35. After it, they are 36.

The Basic Formula

age = current year − birth year
(subtract 1 if today is before the birthday this year)

This gives you age in whole years, which is the most common way age is expressed. For most purposes — filling out a form, checking eligibility for something age-based, telling someone how old you are — this is all you need.

Calculating Exact Age in Years, Months, and Days

When you need a more precise figure — how many years, months, and days old someone is on a specific date — the calculation has a few more steps:

  1. Start with the birth date and the target date (usually today).
  2. Calculate the difference in years: how many complete years have passed since the birth date.
  3. From the anniversary of the last birthday, calculate how many complete months have passed.
  4. From the start of the current incomplete month, count the remaining days.

For example, if someone was born on October 5, 1995, and today is June 7, 2026: they have completed 30 full years (last birthday: October 5, 2025). From October 5 to June 7 is 8 complete months (Oct→Nov→Dec→Jan→Feb→Mar→Apr→May = 8 months, with June 7 being 2 days into the next month). So their exact age is 30 years, 8 months, and 2 days.

The Leap Year Complication

Leap years add a subtle wrinkle. February 29 only exists in leap years — a person born on February 29 technically only has a calendar birthday once every four years. Different countries and legal systems handle this differently:

  • Most systems use February 28 in non-leap years as the official birthday for legal purposes (e.g., when you turn 18 or reach retirement age).
  • Some systems use March 1 in non-leap years, arguing that the day "after February 28" is March 1 when February 29 does not exist.
  • Japan uses March 1 by law for leap-day birthdays in non-leap years.

For most practical purposes, people born on February 29 celebrate their birthday on February 28 in non-leap years, and there is no real legal ambiguity unless a specific age milestone is on the line.

Age Calculation Across Time Zones

For most purposes, the time zone consideration is irrelevant — age is counted in calendar days, not hours. But technically, someone born on June 7 in New Zealand is born on June 6 in the United States. Which date is their "official" birthday? Legal documents use the date in the country of birth, which is the standard approach worldwide.

Why Age Calculation Is Used

Accurate age calculation matters in a surprising number of contexts:

  • Medical and pharmaceutical dosing: Many drug dosages are weight- or age-adjusted, particularly for children. The difference of a few months can affect which dose range applies.
  • Legal milestones: Driving, voting, drinking, and retirement ages are typically enforced to the day, not the year.
  • Sports eligibility: Youth sports leagues often use age cutoff dates — a child born one day after the cutoff may be in a different division than peers born the same week.
  • School enrollment: Most school systems have a birthday cutoff date for the school year; children born on either side of the cutoff can end up a full grade apart.
  • Insurance and financial products: Some insurance premiums and pension calculations use age at last birthday or age at next birthday, which can differ by a day.

Calculating Age Automatically

When you need to know someone's exact age without doing the arithmetic manually, the age calculator at SoftEdit Tools takes a date of birth and returns the exact age in years, months, and days as of today — plus a countdown to the next birthday. Enter the birth date and the result is instant.

The Bottom Line

For a simple age in years: current year minus birth year, minus 1 if the birthday has not happened yet this year. For an exact age in years, months, and days, you need to track the most recent birthday anniversary and count forward from there. For anything involving a legal age threshold or a precise calculation, use an age calculator to avoid off-by-one errors.