Percentage Calculator

What this calculator does

This is a free percentage calculator that handles the four percentage questions people actually run into in real life, each on its own row. You can ask what is X% of Y, what percentage one number is of another, what the percent change is between two values, or what a number becomes after a percent increase or decrease. There is no calculate button. Every result updates as you type, the math runs locally in your browser, and nothing you enter is sent to a server.

The four modes look short, but together they cover most of the everyday percentage math you'd otherwise reach for a phone calculator for: tips, discounts, sales tax, grades, commission, tip-outs, body weight changes, stock and crypto moves, raises, markups, markdowns, budget cuts, survey results, and so on.

Who actually needs this

If you do percentages once a year, the formulas are easy to forget which way around they go. The most common mix-up is between X is what percent of Y and percent change from X to Y. They sound similar and they're not the same. Having one row per question means you can't accidentally use the wrong formula.

Common users:

• Shoppers checking a sale price or splitting a discount.
• Students checking homework or sanity-checking their own work.
• Servers and bartenders working out tip-outs.
• Anyone with a salary review trying to verify the math on a raise.
• Investors and traders looking at gains and losses on a position.
• Small business owners reasoning about margin, markup, and price changes.
• Anyone tracking weight, body composition, savings, or any number that changed between two moments and wants the percent move.

How to use it

Find the row that matches your question and fill in its two boxes. The result appears next to them.

Row 1: What is X% of Y?

Enter the percentage first, then the value. 20 and 80 gives 16. Read it as: twenty percent of eighty equals sixteen.

Row 2: X is what % of Y?

Enter the part first, then the whole. 16 and 80 gives 20%. Read it as: sixteen is twenty percent of eighty.

Row 3: Percentage change from X to Y

Enter the starting value (From) and the ending value (To). 80 to 100 gives +25%. An increase shows in green with a leading plus sign, a decrease shows in red. The sign reflects the direction the number moved, not the sign of the starting number. A flat zero change also shows green with +0%, since the up/down split treats zero as non-negative.

Row 4: Increase / decrease by %

Enter a value and a percentage. The calculator shows both the increased and the decreased result at the same time. 50 with 10% gives 55 increased (green) and 45 decreased (red).

How it actually works

Each row applies one fixed formula. Nothing more, nothing hidden.

• What is X% of Y?  (X / 100) × Y. So 20% of 80 is 16.
• X is what % of Y?  (X / Y) × 100. So 16 out of 80 is 20%.
• Percent change from X to Y:  ((Y − X) / |X|) × 100. The change divided by the absolute value of the starting number, then scaled to a percentage. 80 to 100 is +25%.
• Increase / decrease by %:  Value × (1 + P/100) for the increased result and Value × (1 − P/100) for the decreased one.

Results round to four decimal places, drop trailing zeros, and add thousands separators for readability (so 1234.5 shows as 1,234.5). If a calculation can't produce a finite number, most often because you divided by zero, the field shows a dash instead of a value.

A worked example: a 15% sale

Suppose a jacket is listed at $128 and there's a 15% off sale.

1. How much is the discount in dollars? Use Row 1: 15% of 128 = 19.2. You'll save $19.20.
2. What's the sale price? Use Row 4: 128 with 15%. The Decreased result is 108.8. You'll pay $108.80.
3. Sanity check the percent off: Use Row 3 going from 128 to 108.80. You should get about -15%. Confirmed.

Now suppose later you decide to put the price back up by 15% from the sale price. Use Row 4 again with 108.80 and 15%. The Increased result is 125.12, not 128. That's not a bug — it's the headline point of the next section.

Why a +25% move and a -25% move don't cancel out

This trips up almost everyone the first time. The reason is that the percentage is always measured against the starting number, and after a move the starting number for the next calculation has changed.

Going from 80 to 100: the +20 difference divided by 80 is +25%.

Coming back from 100 to 80: the -20 difference divided by 100 is -20%.

A percent gain and the percent loss that perfectly reverses it are never equal unless the change is zero. If you ever lose 50% on an investment, you need a +100% return on what's left to break even, not another +50%. The percent-change row makes this visible by always re-anchoring to the new starting value.

Reversing a percentage (finding the original)

This calculator goes forward: value to result. If you need to work backwards — find the original price before a discount, or the pre-tax amount on a receipt — you can still do it using Row 2 or with plain division.

• Item costs $80 after a 20% discount, what was the original? The $80 is 80% of the original. So divide: 80 ÷ 0.80 = $100.
• A receipt total of $54 includes 8% sales tax. What was the pre-tax price? The total is 108% of the pre-tax. Divide: 54 ÷ 1.08 ≈ $50.
• Your paycheck is $3,200 after a 4% raise. What was it before? Divide: 3,200 ÷ 1.04 ≈ $3,076.92.

You can verify any of these by putting the answer back into Row 4 with the same percentage and confirming you get the value you started with.

Common pitfalls

• Forgetting the order of inputs. Row 1 wants percentage first, then the value. Row 2 wants the part first, then the whole. If your answer looks way off, you probably swapped them.
• Confusing "of" with "change." "16 is what percent of 80" (Row 2) gives 20%. "80 changed to 16" (Row 3) gives -80%. Both involve the numbers 16 and 80. Different questions, very different answers.
• Treating percentage points and percent change as the same thing. If a mortgage rate goes from 4% to 5%, that's a 1 percentage point rise, but a 25% percent change. Row 3 will give you the second one. For the first, just subtract.
• Stacking discounts by adding. A 30% sale followed by a 20% off coupon is not 50% off. It's 1 - (0.70 × 0.80) = 44% off. Use Row 4 twice if you want to step through it.
• Negative starting values. Row 3 divides by the absolute value of the From number on purpose, so the sign of the result reflects direction (up or down), not the sign of the starting number. If your starting number is negative — for example a company moving from a -$10M loss to a -$5M loss — read the result as direction-of-change and double-check the dollar figures by hand.

What to do if the result looks wrong

• Check you're in the right row. The single biggest source of wrong answers is using Row 2 when you wanted Row 3, or vice versa.
• Check the input order inside the row. Row 1 is "% then value", Row 2 is "part then whole", Row 3 is "from then to."
• Estimate it in your head. 10% of a number just shifts the decimal one place. Half of that is 5%. Most everyday percentages are close to one of these. If the tool's answer is in a different order of magnitude, you typed something wrong.
• Check for a missing decimal point or a stray zero in either input box.
• If you see a dash, one of the inputs is empty, zero where it shouldn't be (Row 2 and Row 3 divide by Y and X respectively), or not a number.

When NOT to use this

• Compound growth over multiple periods. If something grows 5% per year for 10 years, you can't multiply 5% by 10 and add it. Use the Compound Interest calculator.
• Tax brackets and tiered rates. Marginal tax rates apply different percentages to different chunks of income. A flat percent calculator will mislead you.
• Percentiles in statistics. "85th percentile" is a ranking, not a percent of anything. Different tool entirely.
• Annualized returns or CAGR. Row 3 gives total percent change between two points. It doesn't annualize a multi-year return.
• Tip splitting on a bill. The Tip Calculator handles the per-person split and the rounding properly in one step.

Adjacent concepts worth knowing

Markup vs margin. If you buy something for $80 and sell it for $100, the markup is 25% (calculated on cost, the $20 over $80) and the margin is 20% (calculated on the selling price, the $20 over $100). Same dollars, different denominators, different percentages. Row 2 will give you either one. Just pick the right whole.

Percentage points vs percent. Already mentioned above, but worth repeating because news reporting confuses these constantly. "Unemployment rose from 4% to 5%" is +1 percentage point, or +25% in relative terms. They're both correct, but they mean very different things.

Effective vs nominal rates. When percentages compound, the headline rate is not the rate you actually earn or pay. Use the compound interest tool when periods matter.

Privacy and how it runs

All four calculations are plain JavaScript arithmetic that runs in your browser as you type. There is no calculate button, no server round-trip, no logging of your inputs, and no analytics on the values you enter. You can disconnect from the internet after the page loads and the calculator still works.