STEM Calc MCP

When an AI guesses chemistry and physics homework, the numbers are often wrong. STEM Calc MCP fixes that: it is a free, open source MCP server that does the arithmetic with tested code, so you get the same answer every time. It works out molar mass, balances chemical equations, converts between grams, moles and particles, converts units and rounds to the right number of significant figures.

Install with one line:

npx -y stem-calc-student-mcp

Try these prompts:

1. "What is the molar mass of copper sulfate pentahydrate, CuSO4.5H2O?"
2. "Balance this equation: C3H8 + O2 -> CO2 + H2O"
3. "How many moles are in 36 grams of water?"

Use it with Claude Desktop, Claude Code, Cursor, VS Code, Windsurf and any other app that runs a local MCP server. It runs on your computer and needs no internet, no account and no API key.

Claude Desktop

Add this to claude_desktop_config.json, then restart the app:

{
  "mcpServers": {
    "stem-calc": {
      "command": "npx",
      "args": ["-y", "stem-calc-student-mcp"]
    }
  }
}

Claude Code

claude mcp add stem-calc -- npx -y stem-calc-student-mcp

Cursor

Add the same block as for Claude Desktop to .cursor/mcp.json.

VS Code

VS Code uses a servers key, not mcpServers. Add this to .vscode/mcp.json:

{
  "servers": {
    "stem-calc": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "stem-calc-student-mcp"]
    }
  }
}

Tools

All five tools are read only. They calculate an answer and never change anything on your computer.

Tool	What it does	Inputs	Example prompt
molar_mass	Molar mass of a formula in g/mol, with the mass each element adds	formula, like H2O, Ca(OH)2 or CuSO4.5H2O	"What is the molar mass of Ca(OH)2?"
balance_equation	Balances a chemical equation	equation, like H2 + O2 -> H2O	"Balance C3H8 + O2 -> CO2 + H2O."
stoichiometry	Converts between grams, moles and particles	value, from, to (grams, moles or particles) and formula, which grams need	"How many moles are in 36 grams of water?"
convert_units	Converts a value from one unit to another	value, from, to	"Convert 2.5 km to feet."
round_sig_figs	Rounds a number to a number of significant figures	value, sigFigs	"Round 123456 to 3 significant figures."

Every tool also returns its answer as fields next to the text, so an app that reads JSON gets the numbers without picking them out of a sentence.

What it knows

• Formulas. Write each element with a capital first letter. Round brackets and square brackets work, so Fe2(SO4)3 and K3[Fe(CN)6] are fine, and so are hydrates written with a dot, like CuSO4.5H2O. It knows 56 common elements, from hydrogen to uranium. A symbol it does not know gets a clear error, never a guess.
  • Fe2(SO4)3 has 2 iron, 3 sulfur and 12 oxygen atoms, and a molar mass of 399.858 g/mol.
  • K3[Fe(CN)6] has 3 potassium, 1 iron, 6 carbon and 6 nitrogen atoms, and a molar mass of 329.247 g/mol.
  • CuSO4.5H2O counts the five waters, so it has 1 copper, 1 sulfur, 9 oxygen and 10 hydrogen atoms, and a molar mass of 249.677 g/mol.
• Units. Length: m, km, cm, mm, in, ft. Mass: g, kg, mg, lb. Time: s, min, h. Volume: L, mL, m^3. Energy: J, kJ, cal, kcal. Temperature: K, C, F. It refuses to convert one kind into another, so metres to seconds is an error.
• What it does not do. It does not find a limiting reagent, work with charges or isotopes, or balance reactions that have no solution. When it cannot answer, it says so.

How it compares

Some MCP servers connect an assistant to a learning management system, so it can read your courses, grades and due dates. Those need an account, a login and a network connection. This one does none of that. It only does homework math, and it runs on your own computer with no sign in, no network calls and no files read or written. The same input gives the same answer every time.

Use an LMS server to find out what is due. Use this one to check the chemistry and unit conversions once you sit down to do it. They work fine side by side.

Node version

There are two floors here, and they are not the same number.

What you are doing	Node.js
Running the published server	20 or newer
Building and testing a clone	22.12 or newer, on the 22 and 24 release lines or 26 and later

The server itself is happy on Node.js 20, and that is what engines in package.json says. The test runner wants a newer Node.js, so the development floor lives in devEngines in the same file. npm checks devEngines before install, run and test, so a clone on Node.js 20 stops with a clear message instead of a strange failure part way through the test suite. The 23 and 25 release lines are left out on purpose, because the test runner does not run on them.

Both numbers live in package.json, and a test in tests/node-version.test.ts keeps them in step with the README and with the range the test runner asks for.

Development

npm install
npm run build
npm test

The tests call the same functions the tools use, and they check the README, llms.txt, glama.json and the package files against the tools in src/tools.ts.

License

MIT