How to Calculate Age in Years, Months and Days (Formula + 3 Worked Examples)

“How old am I, exactly?” It sounds like the simplest question in the world — until a form asks for your age in years, months AND days, and you realise that just subtracting the birth year doesn’t cut it. If you want to know how to calculate age precisely, you’re in good company. Parents filling out school admission forms, HR teams checking eligibility cut-offs, and anyone who’s ever insisted they’re “almost 30” all need the exact number, not a guess.

Here is the good news: there is a simple method — the borrowing method — that gives you the precise answer with nothing more than subtraction. In this guide you will learn how to calculate age in years, months and days by hand, see three fully worked examples (including the tricky leap-year baby), get the 30-second Excel shortcut, and learn the mistakes that trip almost everyone up. And if you just want the answer now, our free age calculator does it instantly.

Why “Years, Months and Days” Is Trickier Than It Looks

Your age in whole years is easy — but the moment you need months and days, three quirks of the calendar gang up on you.

First, months are uneven. Seven of them have 31 days, four have 30, and February can’t make up its mind between 28 and 29. Any calculation that treats every month as “about 30 days” will drift by days or even weeks.

Second, your birthday might not have happened yet this year. Someone born in November is still their “old” age in October, even though the year number has moved on. Forgetting this is the single most common error in age math — more on that below.

Third, leap years. Every fourth year (with a few century exceptions) February grows an extra day, which quietly shifts every calculation that crosses it. People born on 29 February get the weirdest deal of all: their actual birthday only exists every four years.

The borrowing method handles all three quirks. Think of it like subtraction with carrying — the same technique you learned in primary school, applied to dates.

How to Calculate Age by Hand: The Borrowing Method

Grab two dates: the date of birth and the “as of” date (usually today). Then work through these four steps in order — days first, then months, then years.

Step 1 — Subtract the days. Take the day of the “as of” date minus the day of the birth date. If the result is zero or positive, great. If it is negative, borrow one month: add the number of days in the PREVIOUS month to your day number, then subtract the birth day. Remember to reduce your month count by one later.

Step 2 — Subtract the months. Take the month of the “as of” date minus the month of the birth date (minus one more if you borrowed in Step 1). If the result is negative, add 12 and remember to reduce the year count by one.

Step 3 — Subtract the years. Take the year of the “as of” date minus the birth year (minus one more if you borrowed in Step 2). This is your completed years.

Step 4 — Read the answer as years, months, days.

You will need this reference for Step 1 — the actual length of each month:

The key detail: when you borrow days, you borrow from the month BEFORE the “as of” month — and you borrow that month’s actual length, not a flat 30. Borrowing in October means borrowing September’s 30 days. Borrowing in March means borrowing February’s 28 (or 29 in a leap year). This is where rough calculators go wrong.

Sounds fiddly in the abstract, so let’s do it three times with real dates. By the third one, you’ll be doing it in your head.

Worked Example 1: How to Calculate Age When the Birthday Already Passed

Let’s say someone was born on 15 March 1990, and today is 7 October 2026.

Step 1 — Days: 7 − 15 = −8. Negative, so borrow one month. The month before October is September, which has 30 days. Days become 7 + 30 − 15 = 22. (And we owe the months column one month.)

Step 2 — Months: 10 − 3 = 7, minus the 1 we borrowed = 6. Positive, no further borrowing.

Step 3 — Years: 2026 − 1990 = 36. The birthday (15 March) already happened this year, so no adjustment needed.

Answer: 36 years, 6 months, 22 days.

Sanity check it: 36 years after 15 March 1990 is 15 March 2026. Add 6 months → 15 September 2026. Add 22 days → 7 October 2026. It lands exactly on today. When your answer reconstructs the “as of” date like that, the math is right.

Worked Example 2: How to Calculate Age When the Birthday Is Still to Come

Now the case that catches everyone out. Born 20 November 1985, today is 7 October 2026.

Step 1 — Days: 7 − 20 = −13. Negative, so borrow. September has 30 days: 7 + 30 − 20 = 17 days. (Months owe one.)

Step 2 — Months: 10 − 11 = −1, minus the 1 borrowed = −2. Negative, so add 12: −2 + 12 = 10 months. (Years owe one.)

Step 3 — Years: 2026 − 1985 = 41, minus the 1 borrowed = 40. This is the step people skip — but the birthday (20 November) hasn’t happened yet in 2026, so this person is still 40, not 41.

Answer: 40 years, 10 months, 17 days.

Check: 40 years after 20 November 1985 → 20 November 2025. Add 10 months → 20 September 2026. Add 17 days → 7 October 2026. Exactly today. Notice how the year-borrow in Step 2 automatically handled the “birthday hasn’t happened yet” problem — that is the beauty of the method. You don’t need a separate rule; the arithmetic does it for you.

Worked Example 3: The Leap-Year Baby (Born 29 February)

Roughly 1 in 1,461 people are born on 29 February — and their age math comes with a genuine ambiguity, because their birthday doesn’t exist in most years. Here’s how to handle it.

First, the convention question. In non-leap years, do they turn a year older on 28 February or 1 March? Different calculators (and different countries’ legal rules) pick different answers. The two common conventions are 28 February or 1 March — just pick one and stay consistent. The worked example below uses 1 March, the more common choice in age calculators.

Our leap baby was born 29 February 2000. (Yes, 2000 really was a leap year — century years are leap years only when divisible by 400, and 2000 qualifies. 2026 is not a leap year: the last leap day was 29 February 2024 and the next is 29 February 2028 — see the leap year rules on timeanddate.com.) Today is 7 October 2026.

Step 1 — Days: 7 − 1 = 6. Positive, no borrowing. (We’re using 1 March as the reference birthday.)

Step 2 — Months: 10 − 3 = 7. Positive.

Step 3 — Years: 2026 − 2000 = 26. The 1 March “birthday” has passed this year, so 26 full years.

Answer: 26 years, 7 months, 6 days.

If you used the 28 February convention instead, you’d get 26 years, 7 months, 9 days — one day’s difference. Neither is “wrong”; that’s why stating your convention matters whenever precision counts, like on legal documents.

Here’s all three examples side by side:

The 30-Second Method: Calculate Age in Excel and Google Sheets

Doing the borrowing method by hand is satisfying, but for a spreadsheet full of birthdates you want a formula. Both Excel and Google Sheets have a purpose-built function: DATEDIF.

Put the birthdate in cell A2, then:

=DATEDIF(A2, TODAY(), “Y”) & ” years, ” & DATEDIF(A2, TODAY(), “YM”) & ” months, ” & DATEDIF(A2, TODAY(), “MD”) & ” days”

The three unit codes do the heavy lifting: “Y” counts complete years, “YM” counts the leftover months ignoring years, and “MD” counts the leftover days ignoring months and years. Replace TODAY() with a specific date in another cell if you’re calculating age “as of” a past or future date — handy for backdated forms.

Two honest caveats. First, DATEDIF is a semi-hidden function — Excel won’t autocomplete it, but it works in every modern version including Excel for the web. Microsoft documents the full syntax on its DATEDIF function support page. Second, Microsoft itself warns that the “MD” unit has known quirks and doesn’t recommend relying on it. If you want a bulletproof version, compute the days separately with EDATE:

=DATEDIF(A2, TODAY(), “Y”) & ” years, ” & DATEDIF(A2, TODAY(), “YM”) & ” months, ” & DATEDIF(EDATE(A2, DATEDIF(A2, TODAY(), “Y”)*12 + DATEDIF(A2, TODAY(), “YM”)), TODAY(), “D”) & ” days”

It looks intimidating, but all it does is jump forward to the most recent “month-iversary” of the birthdate with EDATE, then count the remaining days with a plain “D” difference. Copy it once, reuse it forever.

Working Backwards: Find a Birthdate from an Age

Sometimes the question runs in reverse: you know someone’s age and need their birth year. The logic is the same subtraction, flipped around.

Say a job candidate tells you they’re 34 on 7 October 2026. Start with 2026 − 34 = 1992 — then ask the birthday question. If they’ve already celebrated their birthday this year, they were born in 1992. If their birthday is still coming (say, in December), they were born in 1991. That one question is the whole trick.

This comes up more than you’d think: recruiters sanity-checking CVs, event organisers verifying age-restricted entry, and anyone trying to remember which year a childhood friend was born. For the exact day (not just the year), reverse the borrowing method or — more sensibly — let our free age calculator do the forward math while you test candidate dates.

Where Your Exact Age Actually Matters

Most days, “34” is a good enough answer. But plenty of real situations demand the full years-months-days breakdown:

• School admissions. Many schools set cut-off ages like “must be 5 years and 6 months by 1 April.” A child born a week after the cut-off waits a whole year — parents calculate this to the day.
• Driving licences and voting. These flip on a single birthday; being even a day short means waiting.
• Retirement and pensions. Pension schemes often define eligibility as an exact age (say, 60 years), and the first eligible date is the birthday itself.
• Insurance premiums. Life and health insurance price bands shift at specific ages — and when your premium jumps, our percentage increase guide shows you exactly how to work out the size of the hike.
• Children’s medicine. Paediatric doses are frequently weight- AND age-based, calculated in months for infants.
• Loan eligibility. Lenders set minimum and maximum age limits (often 21 to 60); check yours with our loan EMI guide once you know you qualify.

Whenever a form says “age as on” a specific date, run the borrowing method with that date — not today. Backdating is the #1 reason official forms get rejected.

Common Mistakes to Avoid

1. Forgetting the birthday check. Subtracting the years and stopping — without asking whether this year’s birthday has happened — overstates the age by a full year for roughly half the population at any given moment. The borrowing method handles it automatically; mental math often doesn’t.
2. Dividing total days by 365.25. This gives you a decimal number of years, not years-months-days, and the .25 fudge drifts by a day around every leap year boundary. Fine for astronomy, wrong for forms.
3. Always borrowing 30 days. February has 28 (or 29); April, June, September and November have 30; the rest have 31. Borrow the previous month’s actual length or your day count will be off.
4. Mixing leap-day conventions. If you calculate with 28 February but the form’s system uses 1 March, your answer differs by a day. For legal paperwork, ask which convention applies.
5. Trusting DATEDIF’s “MD” unit blindly. Microsoft flags it as unreliable — use the EDATE version above for anything important.
6. Confusing “age in months” with “the months part.” A person who is 40 years and 10 months old is 490 months old in total (40 × 12 + 10). The “10 months” is just the remainder. Forms usually want the breakdown; statistics usually want the total.

Calculate Your Age Instantly (Free Tool)

You now know exactly how to calculate age in years, months and days — the borrowing method for pen and paper, three worked examples you can sanity-check, the Excel formula for bulk work, and the mistakes to dodge. For everyday use, though, skip the arithmetic: our free age calculator takes any birthdate and returns the exact years, months and days (plus totals in months, weeks and days) in one click. Bookmark it for the next form that asks.

Frequently Asked Questions

That’s how to calculate age properly: the borrowing method — days, then months, then years, borrowing real month lengths — gives you an exact years-months-days answer every time, no guesswork. You’ve seen it work on a passed birthday, a coming birthday, and the leap-year edge case; you have the Excel shortcut for bulk calculations; and you know the six mistakes to avoid. Next time a form asks for your age “as on” some date, you’ll have the answer in under a minute — or in one click with our free age calculator.