EveryCalc

Free finance calculators for every part of life, exposed as a REST API and an MCP server so AI assistants can do the math for you.

Live site: https://everycalc.xyz API docs (OpenAPI): https://everycalc.xyz/api/v1/openapi.json MCP endpoint: POST https://everycalc.xyz/mcp (Streamable HTTP, no auth)

What it is

EveryCalc is a collection of interactive personal-finance calculators. This repo hosts the server code that powers two machine-friendly interfaces on top of those calculators:

1. REST API at everycalc.xyz/api/v1/* — plain GET endpoints, JSON in and out.
2. MCP server at POST everycalc.xyz/mcp — eight calculator tools for Model Context Protocol clients (Claude, Cursor, and friends).

No API key. No signup. No local install for the hosted version.

Every tool result includes a calculator_url deep link to the matching interactive calculator on everycalc.xyz, so users can explore the numbers visually after the assistant does the math.

The 8 tools

Tool	What it computes
calculate_mortgage	Monthly payment breakdown: principal and interest via the amortization formula, plus property tax, homeowner insurance, PMI, and HOA
calculate_refinance	Refinance break-even month and lifetime savings
calculate_affordability	Home affordability via the 28/36 rule
calculate_loan	Amortizing loan payments
calculate_compound_interest	Compound interest projections with monthly contributions
calculate_percentage	Percentage calculations
calculate_tip	Tip and bill splitting
calculate_salary	Salary conversions between hourly, weekly, biweekly, monthly, and annual pay

Use the hosted MCP server

Add this to your MCP client config:

{
  "mcpServers": {
    "everycalc": {
      "url": "https://everycalc.xyz/mcp"
    }
  }
}

That is the whole setup. The server is remote (Streamable HTTP), so there is nothing to install.

Use the REST API

All endpoints are GET with query parameters. Example:

GET https://everycalc.xyz/api/v1/calculate/mortgage?home_price=400000&down_payment_pct=20&annual_rate_pct=6.5&term_years=30

Response:

{
  "ok": true,
  "tool": "calculate_mortgage",
  "inputs": { "homePrice": 400000, "annualRatePct": 6.5, "termYears": 30, "...": "..." },
  "results": {
    "loan_amount": 320000,
    "down_payment_usd": 80000,
    "monthly_principal_interest": 2022.62,
    "monthly_total": 2022.62,
    "total_interest": 408142.36,
    "total_cost": 808142.36
  },
  "calculator_url": "https://everycalc.xyz/mortgage-calculator/"
}

Available endpoints (full spec in openapi.json):

• GET /api/v1/health
• GET /api/v1/calculate/mortgage
• GET /api/v1/calculate/refinance
• GET /api/v1/calculate/affordability
• GET /api/v1/calculate/loan
• GET /api/v1/calculate/compound-interest
• GET /api/v1/calculate/percentage
• GET /api/v1/calculate/tip
• GET /api/v1/calculate/salary
• GET /api/v1/openapi.json

Errors return {"ok": false, "error": {"code": "...", "message": "...", "field": "..."}} with an appropriate HTTP status.

Run it yourself

The server is a Cloudflare Worker. The calculation math lives in src/engine.js as pure functions (no I/O), shared by the REST handlers and the MCP handlers in src/index.js.

Prerequisites: Node 18+, wrangler.

npm install -g wrangler
wrangler login
# point [assets] directory at your static-site build, then:
wrangler deploy

The OpenAPI document is generated from the same handlers, so API docs never drift from behavior.

Tests

npm test

test/test.mjs exercises the REST endpoints against the pure math functions; test/test-mcp.mjs exercises the MCP tool handlers. All green as of the last deploy.

MCP registry

This repo includes server.json, the manifest for the official MCP registry.

License

MIT. See LICENSE.