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Free Tool Arena

Developer API

Public JSON API for our calculators.

Call Free Tool Arena's calculation logic directly from your code. Free, no API key required, CORS-enabled, edge-cached. Same math as the web tools, with primary-source citations in every response.

Why this exists

Most free-tool sites lock their math behind a UI. Build a finance dashboard, a Slack bot, a script — and you’re re-implementing mortgage formulas yourself, hoping you got the rate-conversion right. We thought that was silly. Here’s a stable, cited, free public API for the calculators we already publish.

Conventions

  • HTTP GET with query parameters. No body. No POST.
  • JSON in / JSON out. All numbers; no strings unless documented.
  • CORS open (Access-Control-Allow-Origin: *). Call from any domain, browser or server.
  • Edge-cached at 24h. Same params return cached responses; you don’t pay our compute on repeat calls.
  • Every response includes _meta with the formula, sources, and lastVerified date. No black boxes.
  • No API key, no auth, no rate limits documented. Be reasonable.

Endpoints

Mortgage payment

GET /api/tools/mortgage

Example:

https://freetoolarena.com/api/tools/mortgage?principal=400000&annualRate=6.5&years=30

Sample response:

{
  "monthlyPayment": 2528.27,
  "totalInterest": 510178.46,
  "totalPaid": 910178.46,
  "payments": 360,
  "input": { "principal": 400000, "annualRate": 6.5, "years": 30 },
  "_meta": {
    "tool": "mortgage-calculator",
    "lastVerified": "2026-04-30",
    "formula": "M = P × [r(1+r)^n] / [(1+r)^n − 1]",
    "sources": [...]
  }
}

Compound interest

GET /api/tools/compound-interest

Example:

https://freetoolarena.com/api/tools/compound-interest?principal=10000&annualRate=7&years=30&monthlyContribution=500

Sample response:

{
  "futureValue": 685693.82,
  "totalContributed": 190000,
  "totalInterest": 495693.82,
  "input": { "principal": 10000, "annualRate": 7, "years": 30, "monthlyContribution": 500, "compoundingPerYear": 12 },
  "_meta": { ... }
}

Tip calculator

GET /api/tools/tip

Example:

https://freetoolarena.com/api/tools/tip?bill=84.50&tipPercent=18&people=3

Sample response:

{
  "tipAmount": 15.21,
  "total": 99.71,
  "perPerson": 33.24,
  "input": { "bill": 84.50, "tipPercent": 18, "people": 3 },
  "_meta": { ... }
}

BMI calculator

GET /api/tools/bmi

Example:

https://freetoolarena.com/api/tools/bmi?weightKg=72&heightM=1.78

Sample response:

{
  "bmi": 22.72,
  "category": "healthy",
  "input": { "weightKg": 72, "heightM": 1.78 },
  "_meta": { ... }
}

Errors

Invalid or missing inputs return HTTP 400 with a helpful { error, message, example } body. Server errors return 500. Successful calls always return 200.

Roadmap

More endpoints are coming for: paycheck, ROI, retirement, macros, BMR/TDEE, currency conversion, password strength, embeddings cost, AI agent loop cost, and document tooling where it makes sense (mail-merge, address-format).

Want a specific tool exposed on the API? Contact us with the use case — we’ll prioritize.

Stability

Endpoints documented here are v1 stable. We’ll only break them with a deprecation window and a versioned URL. Methodology changes (e.g. updated tax brackets) happen in place but are reflected in _meta.lastVerified.

Last reviewed 2026-05-04. See also: source & transparency, tool methodology.