A calculator MCP server for Claude

Written on 26 October 2026 by Ratio.

Language models are good with words and unreliable with sums, and they don't know today's exchange rates. Ratio's MCP server hands those sums to Ratio, so Claude answers with the real number.

What Claude can ask Ratio

Anything you can type in a Ratio sheet: currencies and crypto at today's rates, stocks and metals, units, time zones, working days, VAT, loans, take-home pay, equations and developer maths. A few lines Claude sent, with Ratio's answers:

Ratio also says how it got there, such as the exchange rate it used or a country's VAT rate, and Claude can pass that on. Currency and stock results change with the market.

Two tools

calculate works out one line. calculate_sheet works out several lines as one sheet, so later lines can use names (rent = 1200 eur), line references (L1) and prev, and it returns the sum of every line. Both take an optional region, a country code such as DE, for VAT rates, public holidays and salary rules.

Set it up in Claude Code

You need Ratio 0.13 or later from this website in your Applications folder. Then run:

claude mcp add --scope user ratio -- /Applications/Ratio.app/Contents/Helpers/ratio mcp

--scope user makes Ratio available in every project. claude mcp list shows whether it's connected.

Set it up in Claude Desktop

Open Settings › Developer › Edit Config and add Ratio to mcpServers in claude_desktop_config.json:

{
  "mcpServers": {
    "ratio": {
      "command": "/Applications/Ratio.app/Contents/Helpers/ratio",
      "args": ["mcp"]
    }
  }
}

Keep any servers that are already there. Quit and reopen Claude, and Ratio's tools show up under the tools button in the message box. Write the full path: Claude Desktop doesn't look in /usr/local/bin.

Cursor, VS Code and other apps

Any app that runs local MCP servers works, with the same command: /Applications/Ratio.app/Contents/Helpers/ratio and the argument mcp. In Cursor it goes in ~/.cursor/mcp.json under mcpServers, written as for Claude Desktop. In VS Code, add it to .vscode/mcp.json:

{
  "servers": {
    "ratio": {
      "type": "stdio",
      "command": "/Applications/Ratio.app/Contents/Helpers/ratio",
      "args": ["mcp"]
    }
  }
}

Things to ask

• Split 184 euros between 3 people, in dollars.
• What's 60,000 euros gross in Germany after tax?
• What date is 15 working days after 1 March in Romania?
• How much are 25 Apple shares worth in pounds?
• What do 1.2 million tokens cost at $3 per million?

Try Ratio free for 7 days · $14.99 once, updates included

Questions

Does Ratio see my conversation? No. Ratio runs on your Mac and only sees the lines Claude sends it, such as “184 eur / 3 in usd”. It goes online only for exchange rates and the prices of the stocks and metals a line names, as the app does.

Does it work with the Mac App Store version? No. The App Store version has no command line tool, so the MCP server comes with Ratio from this website.

Does it work on Windows and Linux? Not yet. The MCP server is part of the ratio command, which is on the Mac only for now.

Does it cost extra? No. It comes with Ratio, $14.99 once, with updates included.

Claude answers without using Ratio. What do I do? Ask for it: “use Ratio to work this out”. If Ratio's tools aren't listed, check that the server is connected with claude mcp list in Claude Code, or under the tools button in Claude Desktop.