Age Formula Explained

This reference explains the Age formula used by our free Age Calculator. Use it to understand variables, units, and common mistakes before you calculate.

The formula

Age Calculator formula

Variables and units

• birth date — Date of birth (calendar date)
• as-of date — Date to calculate age at (calendar date (default: today))

Worked example

Inputs: Born March 15, 1990; calculate age on July 31, 2026

1. Years: 2026 − 1990 = 36, but birthday hasn't occurred yet in 2026
2. Adjust: 36 − 1 = 35 years
3. Months: March to July = 4 months, minus adjustment = 4 months
4. Days: 15 to 31 = 16 days

Result: Age = 35 years, 4 months, 16 days

When to use this formula

Use this formula when you need a quick age estimate with values you already know. Our Age Calculator applies the same relationship automatically so you can verify handwritten work.

Common mistakes

• Forgetting to subtract a year when birthday hasn't occurred yet
• Confusing age at last birthday with running age in some cultures
• Timezone issues for babies born near midnight

Frequently asked questions