Question 1

How do I calculate "X percent of Y"?

Accepted Answer

Convert the percent to a decimal (divide by 100), then multiply by Y. So 18% of 250 = (18 ÷ 100) × 250 = 0.18 × 250 = 45. The formula is `result = (percent / 100) × value`. Our calculator does this automatically when you pick the "What is X% of Y?" mode and shows the breakdown so you can copy the working into your homework or invoice.

Question 2

How do I figure out "X is what percent of Y"?

Accepted Answer

Divide X by Y, then multiply by 100. So if 45 is what percent of 250: (45 ÷ 250) × 100 = 18%. The formula is `result = (part / whole) × 100`. This is the inverse of "X% of Y" and is the standard way to express a quantity as a fraction of a total — useful for test scores, market share, and conversion rates.

Question 3

How do I calculate percent change between two values?

Accepted Answer

Take the difference (new − old), divide by the absolute value of the old, then multiply by 100. Going from 200 to 230: ((230 − 200) ÷ 200) × 100 = 15% increase. The formula is `result = ((newValue − oldValue) / |oldValue|) × 100`. Positive means increase, negative means decrease, zero means unchanged. We use the absolute value of the old number so the sign of the result correctly tracks direction even when working with negative values.

Question 4

Is my input data sent to any server?

Accepted Answer

No. All calculations run in your browser via JavaScript. Salary figures, sale prices, exam scores, financial deltas — none of it reaches our servers. We make no network requests and ship no analytics tied to the numbers you enter.

Question 5

What's the difference between "percent off" and "percent change"?

Accepted Answer

"Percent off" is a one-directional discount: 20% off $80 means subtract 20% × $80 = $16, giving you $64. "Percent change" is two-directional: it can be a positive change (increase) or negative (decrease). Going from $80 to $64 is a -20% change, exactly matching the 20% off. They're different perspectives on the same math: percent-off starts with the discount and asks for the new price; percent-change starts with both prices and asks for the percentage.

Question 6

How do I add sales tax to a price?

Accepted Answer

Use the "Add X% to Y" mode. Enter the tax rate (e.g. 8.875% for New York City) and the pre-tax price. The calculator adds the tax to the subtotal. Math: `result = value + (percent / 100) × value = value × (1 + percent / 100)`. For an $100 subtotal at 8.875% tax: $100 + $8.88 = $108.88. To reverse-calculate (find the pre-tax price from a tax-inclusive total), divide by `1 + percent / 100` — for instance, $108.88 ÷ 1.08875 = $100.

Question 7

Why does division by zero produce an error?

Accepted Answer

Two of our calculations involve division: "X is what % of Y" divides by Y, and "% change from X to Y" divides by X. When the divisor is zero, the math is undefined — there's no meaningful answer. We catch this explicitly and surface a clear error message rather than displaying `Infinity` or `NaN` which would confuse users. If you're computing growth from zero (e.g. a new product's first month of sales), there's no percent answer; report the absolute change instead.

Question 8

How is percent change different from percent points?

Accepted Answer

A trap that catches even economists. If interest rates rise from 5% to 7%, that's a 2 percentage-point increase BUT a 40% increase ((7-5)/5 × 100). Reporters and analysts routinely conflate the two. Our calculator computes percent CHANGE on the values you give it; if you want percent-point change, subtract the two values directly (no formula needed). For example, an unemployment rate going from 4.0% to 4.4% is a 0.4 percentage-point rise or a 10% rise — both true, very different magnitudes.

Question 9

Can I use this for tax-bracket and salary-raise math?

Accepted Answer

Yes. For a salary raise: use "% change from X to Y" with old and new salaries to confirm the announced raise. For a hypothetical "what would $X be after a 4% raise": use "Add X% to Y" with 4 and $X. For tax bracket calculations: each bracket applies to the income inside it, so a marginal-rate bump only affects new dollars — use "Subtract X% from Y" with the bracket rate on the portion of income in that bracket. Stack the results across brackets for total tax.

Question	Formula	Worked Example
What is X% of Y?	(X / 100) × Y	18% of 250 = 45
X is what % of Y?	(X / Y) × 100	45 is 18% of 250
% change from X to Y	((Y − X) / \|X\|) × 100	200 → 230 is +15%
Add X% to Y	Y × (1 + X / 100)	$100 + 8.875% tax = $108.88
Subtract X% from Y	Y × (1 − X / 100)	$80 − 20% off = $64

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