How to calculate age accurately

Calculating someone's exact age means walking the calendar — not dividing total days by 365. This guide shows the right way to do it, by hand, in a spreadsheet, or with a live calculator.

Step 1 — Start with a complete birth date

You need day, month, and year. For second-level precision you also need the birth time and the timezone where the birth happened.

Step 2 — Count whole years first

Subtract the birth year from the current year, then subtract one more if today is before the birthday. On April 21, 2026, someone born October 5, 1990 is 35, not 36.

Step 3 — Count remaining months and days

Anchor the most recent past birthday and count whole months forward, then leftover days. From October 5, 2025 to April 21, 2026 is 6 months, 16 days.

Step 4 — Add seconds

Take the difference in milliseconds between the birth instant and now, divide by 1,000. The live calculator updates this number every second.

Leap years

Walk the calendar one year at a time so leap days are absorbed naturally. Naively dividing days by 365 accumulates error.

Timezones

Convert both birth instant and now to UTC before subtracting. The calculator does this automatically.

Try it

Open the live age calculator for instant, accurate results.

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