Percentage Calculator

Calculate percentages in three different modes. All math runs locally in your browser.

Mode

X% of Y X is ?% of Y % change

What is

% of

Result

Answer —

Use this tool from AI agents

This tool's engine is available to AI agents over MCP as percentage. Add the Clean.tools server, then your agent can call it directly.

Claude Code

claude mcp add --transport http clean-tools https://mcp.clean.tools/mcp

Claude Desktop (mcpServers)

{"mcpServers":{"clean-tools":{"command":"npx","args":["-y","@clean-tools/mcp"]}}}

Cursor

https://mcp.clean.tools/mcp

Or call it over plain HTTP from any script. Same engine, JSON in and JSON out, no key:

curl

curl -s https://api.clean.tools/v1/percentage \
  -H 'content-type: application/json' \
  -d '{"mode":"of","percent":15,"value":200}'

JavaScript

const res = await fetch("https://api.clean.tools/v1/percentage", {
  method: "POST",
  headers: { "content-type": "application/json" },
  body: JSON.stringify({"mode":"of","percent":15,"value":200})
});
const data = await res.json();

OpenAPI spec npm: @clean-tools/mcp Full developer guide

How it works

This percentage calculator supports three common modes: finding X% of a number, determining what percentage one number is of another, and computing the percent change between two values. Results update automatically as you type.

All calculations run entirely in your browser. No data is sent to a server. No signup, no tracking. Works on mobile and desktop. If you would rather compute percentages from a script or an AI agent, the same engine is available through the Clean.tools percentage API, or you can install the @clean-tools/mcp package from npm.

Frequently asked questions

What is the difference between percent change and percentage points?

The "% change" mode measures the relative difference between two numbers, so a move from 10 to 15 is a 50% change. Percentage points measure the plain gap between two percentages, so a rate rising from 10% to 15% is 5 percentage points. This tool reports percent change, not percentage points.

How does percent change handle negative or zero starting values?

Percent change is measured against the absolute value of the starting number, so going from -50 to -25 shows as a positive change. If the starting value is 0 the result is undefined, because it would require dividing by zero, and the calculator shows a dash instead of a number.

Can I enter decimals and negative numbers?

Yes. Every field accepts decimals and negative values, and the result recalculates as you type. Answers are rounded to 10 decimal places so long floating-point tails do not clutter the output.

Do my numbers get sent anywhere?

No. Every calculation runs locally in your browser with JavaScript. Nothing is uploaded, stored, or tracked, and the tool keeps working offline once the page has loaded.

Can I use this from the command line or an AI agent?

Yes. The same percentage engine is exposed as a REST endpoint at https://api.clean.tools/v1/percentage, so you can POST a mode and your numbers as JSON and read the answer back from any script. It is also an MCP server you can add to Claude, Claude Code, or Cursor, which lets an agent run the calculation on your behalf. The @clean-tools/mcp package on npm runs that same server locally over stdio.

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Use Clean.tools from your AI agent

This tool is also percentage on the Clean.tools MCP server — the same engine, callable by your AI agent directly. No key, no signup: requests are processed in memory and request contents are never stored.

Claude Code

claude mcp add --scope user --transport http clean-tools https://mcp.clean.tools/mcp

Any MCP client (streamable HTTP, authless)

https://mcp.clean.tools/mcp

Every tool, the REST API, and setup for Cursor, Claude Desktop, VS Code & more →