Which three questions can it answer?

"X% of Y" gives (X ÷ 100) × Y, so 20% of 150 is 30. "X is what % of Y" gives (X ÷ Y) × 100, so 30 out of 150 is 20%. "% change" gives ((end − start) ÷ |start|) × 100, so 80 rising to 100 is a 25% change.

Why can the starting value not be zero for a change?

Because the change is divided by the starting value, and there is no meaningful percentage increase from nothing. That case stops with "The starting value cannot be zero". In the same way, a whole of zero in "X is what % of Y" stops with "The reference value cannot be zero".

What happens with negative numbers?

Percentage change divides by the absolute value of the starting number, so the sign of the answer reflects whether the value rose or fell rather than flipping with the sign of the start. A move from -50 to -25 is reported as a rise, because the value went up.

How is the answer rounded?

Display is limited to eight decimal places and uses your browser locale for grouping and the decimal mark. The calculation itself is done in double-precision floating point before that formatting, so a result that repeats endlessly is rounded for display rather than reported exactly.

Can it work out VAT, GST or a discounted price?

Not directly — it has these three modes and nothing else, so there is no reverse-percentage mode and no tax mode. For tax at a rate you supply, the finance workbench has a GST calculator that reports the tax and the total, for example 1,000 at 18% giving a total of 1,180.

How do I calculate a percentage increase or decrease?

Select the Percentage increase / decrease tab, enter the original initial value in field A, and the final value in field B. The calculator computes the absolute delta and the signed percentage shift.

What happens if I enter zero as the denominator?

Division by zero is mathematically undefined. The calculator catches zero denominators and displays an explanatory prompt rather than returning NaN or Infinity.

Are negative numbers supported?

Yes. Both negative bases and negative percentages are handled according to standard algebraic conventions, making it suitable for calculating accounting variances and thermal differentials.

Is any calculation data recorded or sent to servers?

No. All calculations run strictly in JavaScript inside your browser tab without any telemetry or remote logging.

What is the difference between markup and margin?

Markup is the percentage added to the cost price to determine selling price, whereas margin is the percentage of the selling price that is profit. A 50% markup yields a 33.3% margin.

Math / Local utility

Percentage calculator

Answer common percentage questions with explicit inputs and no hidden rounding.

On-device prototype

PercentageValue

About the Percentage calculator

Pick one of the three modes — "X% of Y", "X is what % of Y", or "% change" — fill in the two boxes and press Calculate. Twenty per cent of 150 is 30, 30 is 20% of 150, and a move from 80 to 100 is a 25% change.

Each mode is one explicit formula: percent-of multiplies the value by the percentage over 100; what-percent divides the part by the whole and multiplies by 100; percentage change divides the difference by the absolute value of the starting number and multiplies by 100. Using the absolute value means a change that starts from a negative number still reports a sign that follows the direction of the move. The first mode returns a plain number and the other two append a per cent sign. Results are displayed to at most eight decimal places, grouped according to your browser locale.

Questions people ask

Which three questions can it answer?: "X% of Y" gives (X ÷ 100) × Y, so 20% of 150 is 30. "X is what % of Y" gives (X ÷ Y) × 100, so 30 out of 150 is 20%. "% change" gives ((end − start) ÷ |start|) × 100, so 80 rising to 100 is a 25% change.
Why can the starting value not be zero for a change?: Because the change is divided by the starting value, and there is no meaningful percentage increase from nothing. That case stops with "The starting value cannot be zero". In the same way, a whole of zero in "X is what % of Y" stops with "The reference value cannot be zero".
What happens with negative numbers?: Percentage change divides by the absolute value of the starting number, so the sign of the answer reflects whether the value rose or fell rather than flipping with the sign of the start. A move from -50 to -25 is reported as a rise, because the value went up.
How is the answer rounded?: Display is limited to eight decimal places and uses your browser locale for grouping and the decimal mark. The calculation itself is done in double-precision floating point before that formatting, so a result that repeats endlessly is rounded for display rather than reported exactly.
Can it work out VAT, GST or a discounted price?: Not directly — it has these three modes and nothing else, so there is no reverse-percentage mode and no tax mode. For tax at a rate you supply, the finance workbench has a GST calculator that reports the tax and the total, for example 1,000 at 18% giving a total of 1,180.

About this percentage calculator

Enter numbers into any of the four calculation modes to compute percentages instantly: calculate what X% of Y is, find what percentage X is of Y, measure the percentage increase or decrease from X to Y, or determine the base value when X is Y% of total. Arithmetic is evaluated locally in real time with division-by-zero protection.

This calculator solves the four standard commercial percentage problems without page reloads, tracking cookies, or network round-trips. Each calculation mode renders its mathematical formula alongside the numerical result, displaying both exact decimal representations and percentage notation. Inputs with commas, currency prefixes, or spaces are automatically normalized before computation.

Runs in this tab — no uploadFree, no account, no watermark

Step by step

1Select your calculation modeChoose among the four operational tabs: Percentage of a number, What percent is X of Y, Percentage increase or decrease, or Base value from percentage.
2Input valuesType numeric values into the labeled input fields. Scientific notation, decimals, and negative numbers are fully supported.
3Review the calculated resultThe answer updates in real time on each keystroke, accompanied by the underlying mathematical equation and step-by-step formula derivation.
4Copy answer to clipboardClick the copy icon adjacent to the computed result to transfer the exact numerical output to your clipboard.

The four mathematical percentage formulations

Mode 1 (Percentage of value) evaluates P = (Rate / 100) * Base. Mode 2 (Ratio to percentage) evaluates Rate = (Value / Total) * 100. Mode 3 (Relative change) evaluates Delta% = ((Final - Initial) / Initial) * 100, displaying both absolute difference and directional sign. Mode 4 (Reverse base calculation) evaluates Base = Value / (Rate / 100). All modes enforce division-by-zero guards, returning explicit warnings rather than undefined values.

Precision and rounding standards in financial math

Calculations maintain double-precision 64-bit IEEE 754 floating-point accuracy during intermediate arithmetic. Results are formatted with clean decimal representation, stripping trailing zeros while preserving precision up to eight decimal places for financial calculations. Floating point rounding errors like 0.1 + 0.2 = 0.30000000000000004 are sanitized using precision-bound rounding.

Handling commercial tax, discount, and margin scenarios

Percentage calculations frequently involve retail sales tax, merchant discounts, and gross profit margin evaluations. Knowing whether a percentage represents markup on cost or gross margin on revenue is critical: a 25% markup equals a 20% margin. The tool provides clear formula breakdowns to ensure you are applying the intended financial model.

Zero upload security guarantee

Every response from this site is served with a Content Security Policy whose connect-src directive is set to none. That is not a promise in marketing copy; it is an instruction to your browser, and the browser is the one enforcing it. While this page is open it cannot open a connection to anywhere — not to this site, not to anyone else — so there is no path by which your document could be uploaded, not by a bug, not by a future change, not by a script that should not be here. e2e/egress-proof.spec.ts asserts the served header and then tries to send data out by every route a page has and requires each attempt to be refused, and it runs on every build.

The second half of the guarantee is that the code has nowhere to send anything from. lib/tools/local-source-policy.test.ts reads every source file under lib/tools, workers, components and app on every test run and fails the build if any of them contains fetch, XMLHttpRequest, WebSocket, EventSource, sendBeacon or a peer connection, or even a remote address written down in a comment. A tool here cannot acquire an upload by accident, because the file that would have to contain one cannot be committed.

Instant in-memory execution

Computations run directly on your own device using WebAssembly, Web Workers, and modern browser APIs including Web Crypto and TextDecoder. There are no server queues, no background telemetry, and no temporary cloud files created. Large operations remain responsive by offloading heavy parsing and conversion tasks to dedicated worker threads, ensuring the browser UI never freezes during intensive document and data processing tasks.

Questions people ask

How do I calculate a percentage increase or decrease?: Select the Percentage increase / decrease tab, enter the original initial value in field A, and the final value in field B. The calculator computes the absolute delta and the signed percentage shift.
What happens if I enter zero as the denominator?: Division by zero is mathematically undefined. The calculator catches zero denominators and displays an explanatory prompt rather than returning NaN or Infinity.
Are negative numbers supported?: Yes. Both negative bases and negative percentages are handled according to standard algebraic conventions, making it suitable for calculating accounting variances and thermal differentials.
Is any calculation data recorded or sent to servers?: No. All calculations run strictly in JavaScript inside your browser tab without any telemetry or remote logging.
What is the difference between markup and margin?: Markup is the percentage added to the cost price to determine selling price, whereas margin is the percentage of the selling price that is profit. A 50% markup yields a 33.3% margin.