How to Calculate Age (With Examples That Actually Make Sense)

Age in completed years

Exact years, months, and days

After finding completed years, add that many calendar years to the birth date, clamping to the final valid day when necessary. From that anchor, add the greatest number of whole calendar months without passing the target. The ordinal-day difference from the month anchor to the target is the remaining days.

Leap years and month ends

Gregorian leap years are divisible by 4, except century years unless divisible by 400. Month-end calculations also need a clamping policy. Calzivo treats February 28 as the non-leap-year anniversary for a February 29 birth date; official rules can differ.

Common mistakes

• Stopping after subtracting the years.
• Borrowing a fixed 30 days even when the preceding month has a different length.
• Using days divided by 365 as exact calendar age.
• Rounding up because the next birthday is close.
• Using today instead of the form's stated reference date.
• Assuming the calculator determines legal age or eligibility.

Scope and verification

Write dates unambiguously, preferably as YYYY-MM-DD, and use the exact cutoff date required. For legal, school, benefits, insurance, employment, or contract decisions, verify the governing rule and local-date convention. This guide explains arithmetic and does not determine eligibility.