How old are you exactly — in years, months, and days? It sounds simple, but manual calculation often goes wrong because months have different lengths and leap years exist. This guide teaches you how to calculate age correctly: the manual step-by-step method, the Excel formula, and the tricky edge cases — plus an automatic calculator if you want an instant answer.

How to Use the Age Calculator

1. Open the Age Calculator.
2. Enter your date of birth using the date picker.
3. Click Calculate Age.
4. Read your exact age in years, months, and days — plus total months, total days, total weeks, and a countdown to your next birthday.

Everything runs in your browser; your birth date is never sent to or stored on any server.

How to Calculate Age Manually (Step by Step)

The method works like column subtraction, with a “borrow” rule when days or months run short:

1. Subtract the years: current year − birth year.
2. Subtract the months: if the current month is smaller than the birth month, borrow one year (subtract 1 from years, add 12 to months).
3. Subtract the days: if today’s date is smaller than the birth date, borrow one month (subtract 1 from months, add the number of days in the previous month).

Worked Example: Born May 15, 1990 — Today Is October 3, 2026

• Years: 2026 − 1990 = 36
• Months: October (10) − May (5) = 5 → tentatively 36 years, 5 months
• Days: 3 < 15, so borrow one month (September = 30 days): months 5 − 1 = 4; days = 3 + 30 − 15 = 18

✅ Result: 36 years, 4 months, 18 days.

Note: the borrowed days always follow the previous month’s length (February has 28 or 29 days — never just assume 30). This is where most manual calculations go wrong.

Special Case: Born on February 29

• In non-leap years, the birthday is celebrated on February 28 or March 1 depending on family custom and local convention.
• Official age calculation still uses the borrowing logic above — a good calculator handles it automatically.
• Example: born February 29, 2000; today October 3, 2026 → 26 years, 7 months, 4 days (computed as if born February 28, 2000).

Age in Different Time Units

Sometimes you need age expressed differently — for insurance quotes or just curiosity. For our example age (36 years, 4 months, 18 days ≈ 13,290 days):

Unit	Value (example)	How It’s Computed
Total months	436 months	36 × 12 + 4
Total weeks	1,898 weeks	Total days ÷ 7
Total days	13,290 days	Direct date difference
Total hours	318,960 hours	Total days × 24

Calculating Age in Excel & Google Sheets

The DATEDIF function (date difference) is the fastest spreadsheet method. Syntax: =DATEDIF(birth_date, TODAY(), "unit")

Formula	Returns
=DATEDIF(A2,TODAY(),"Y")	Full years
=DATEDIF(A2,TODAY(),"YM")	Remaining months
=DATEDIF(A2,TODAY(),"MD")	Remaining days
=DATEDIF(A2,TODAY(),"D")	Total days

Full “X years Y months Z days” format:
=DATEDIF(A2,TODAY(),"Y")&" years "&DATEDIF(A2,TODAY(),"YM")&" months "&DATEDIF(A2,TODAY(),"MD")&" days"

Note: DATEDIF doesn’t appear in Excel’s autocomplete — type it manually and it still works.

Baby Age in Months

For children under 5, pediatricians track age in months, not years, because development changes so fast.

Example: baby born June 10, 2025; today October 3, 2026 → 15 months, 23 days (about 16 months for vaccination schedules).

Common Mistakes When Calculating Age

• Subtracting only the years (2026 − 1990 = 36) without checking whether this year’s birthday has passed — if not, subtract one year.
• Always borrowing 30 days — the borrowed days follow the previous month (28, 29, 30, or 31).
• Forgetting leap years in total-day counts over multi-decade spans (the error can reach several days).
• Typing dates as text in Excel — make sure the cell holds a real date (Date format), not text.
• Using “decimal years” (e.g., 36.37 years) for official purposes — institutions always want full years-months-days.

FAQs

How accurate is an online age calculator?
Very accurate when it uses proper calendar arithmetic — including leap years and varying month lengths — like the Age Calculator here. Far more precise than just subtracting birth year from the current year.

How do I quickly tell if this year’s birthday has passed?
Compare today’s month/day with the birth month/day. If today is earlier, the birthday hasn’t happened yet — subtract 1 from the year difference.

Can I calculate age for a future date?
Yes — for example, a child’s age when starting school in 2030. Enter the target date instead of today.

What if I only know the birth year?
Subtract it from the current year, then subtract 1 more if this year’s birthday hasn’t occurred yet. Without month and day, the answer is approximate within ±1 year.

Is my date of birth stored anywhere?
No. A proper age calculator does everything in your browser without sending data to any server.

Why does the age on my ID differ from my calculation?
Usually a recorded-vs-actual birth date discrepancy, or the institution rounding to whole years. Your birth certificate is the official reference.

Related Tools

For the gap between any two dates (not from birth), use the Date Duration Calculator. For everyday arithmetic, there’s the Online Calculator.