How to Calculate Age from Date of Birth (Manual Method)

The manual method in 3 steps

1. Subtract the years: current year − birth year.
2. Subtract the months: current month − birth month. If negative, borrow 12 months (subtract 1 from years).
3. Subtract the days: current day − birth day. If negative, borrow the number of days in the previous month (subtract 1 from months).

That's it — the only tricky part is the borrowing, which works exactly like subtraction in school arithmetic. Let's see it in action.

Worked example 1: no borrowing needed

Birth: 14 March 1990  ·  Today: 1 October 2026

• Years: 2026 − 1990 = 36
• Months: October (10) − March (3) = 7
• Days: 1 − 14 = negative → borrow. September has 30 days, so days = 1 + 30 − 14 = 17, and months drop to 6.

Result: 36 years, 6 months, 17 days. Even “simple” dates often need day-borrowing — that's normal.

Worked example 2: borrowing months too

Birth: 25 November 2015  ·  Today: 1 October 2026

• Years: 2026 − 2015 = 11
• Months: 10 − 11 = negative → borrow 12 months: months = 10 + 12 − 11 = 11, years drop to 10.
• Days: 1 − 25 = negative → borrow September's 30 days: days = 1 + 30 − 25 = 6, months drop to 10.

Result: 10 years, 10 months, 6 days. When borrowing days, always use the length of the month before the current date's month.

How leap years fit in

Leap years (like 2024, 2028) add 29 February, and the manual method handles them automatically because you borrow real calendar month lengths. If the previous month is February in a leap year, borrow 29 days; otherwise 28.

People born on 29 February legally celebrate on 28 February or 1 March in non-leap years depending on jurisdiction — but for age math, just use the actual calendar dates and the method works.

Expressing age in total days, weeks or months

Sometimes you need more than years-months-days:

• Total days: count days between the dates (a date-difference calculation). Roughly: years × 365 + leap days + remaining days.
• Total weeks: total days ÷ 7.
• Total months: years × 12 + extra months.

Doing this by hand across leap years is error-prone — that's exactly what the age calculator and date difference calculator automate.

Common uses for exact age

• School admissions and exam eligibility cut-offs.
• Legal milestones: voting, driving, drinking age.
• Retirement planning and pension calculations.
• Medical dosing for children, which is often age-and-weight based.
• Milestones: “10,000 days old” birthdays and anniversaries.

Mental shortcuts for common age questions

For everyday situations you rarely need the full borrow-everything method:

• “How old will they turn this year?” Just subtract birth year from the current year. Someone born in 1990 turns 36 at some point in 2026.
• “Are they over 18 yet?” Compare month-by-month: if the birth month has passed this year, use (current year − birth year); if not, subtract one more.
• Half-birthdays: add 6 months to the birth date — a fun milestone for kids (and kid-like adults).
• Decade counting: for rough ages, round to the nearest 5 or 10 years — fine for small talk, never for forms.

And remember the golden rule of borrowing: when days go negative, borrow from the previous month's length — 31, 30, 28 or 29 days depending on the actual calendar. Write the two dates vertically (year-month-day) and work right to left, exactly like column subtraction. With a little practice you'll do it in under 30 seconds — or let the age calculator do it in one tap.

Try it now: Age Calculator

Enter any birth date and get the exact age in years, months and days — including age on any past or future date.

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How to Calculate Age — FAQs