How to Calculate Age from Date of Birth (Step by Step)

Calculating age sounds simple — subtract the birth year from the current year. But that quick formula often gives the wrong answer, especially around birthdays.

The accurate method works in three steps: first count full years between the two dates, then count the leftover months, then the leftover days. If the day-of-month hasn't been reached yet this month, borrow days from the previous month — just like long subtraction.

Most digital calculators (including Dob2Age) use this exact approach, which handles leap years automatically because the math is rooted in real calendar dates.

For most use cases — forms, school admissions, fitness apps — the years value alone is enough. But for milestones (a baby's first 100 days, a 10,000-day anniversary), the full breakdown into months, days, hours and seconds is far more meaningful.