🔌 Free Calculator API

Overview

The API runs the exact same math the website uses. Send a calculator's field values as query parameters or JSON and get the computed results back as JSON — including any amortization schedule or chart data the calculator produces.

• 198+ calculators, each with its own endpoint
• No API key — anonymous access, rate-limited by IP
• CORS — Access-Control-Allow-Origin: * (usable client-side)
• Rate limit: 60 requests/hour per IP (HTTP 429 when exceeded)
• Errors are free — only a successful (2xx) response uses one call from your monthly quota. Every 400, 401, 404, 429 and 500 costs you nothing.

Base URL

https://calculator.free/api/v1/

Index — list every calculator

GET https://calculator.free/api/v1/

Returns a machine-readable list of all calculators with each one's fields (key, label, type, default, unit) and result keys — enough to build a client dynamically. Each field also carries a required flag — true for the few fields that have no default (dates, mostly). Send those: a couple of calculators can infer one, the rest answer 400 without it.

Compute — run one calculator

GET  https://calculator.free/api/v1/<slug>/?field=value&field=value
POST https://calculator.free/api/v1/<slug>/     (JSON or form body)

Response shape:

{
  "ok": true,
  "slug": "mortgage",
  "country": "us",
  "inputs":   { ... the values actually computed with ... },
  "results":  { ... computed result keys ... },
  "schedule": { "columns": [...], "rows": [...] } | null,
  "chart":    { "type": "pie", "slices": [...] } | null,
  "defaults_applied": ["tax", "insurance"]
}

Country-aware calculators (mortgage, loan, income-tax, sales-tax…) accept ?country=us|uk|ca|au|in|ie|nz|za. Unknown slug returns 404; unsupported country or bad input returns 400 with a helpful message and the field spec.

Parameters, defaults and errors

Send only the fields you care about. Anything you leave out is filled with that field's published default — the same value the calculator page pre-fills, so the API and the website always return the same numbers for the same inputs. Each response lists what it filled in under defaults_applied.

Anything the API cannot resolve honestly is an error, never a zero: a misspelled parameter, a value of the wrong type, an unknown dropdown option, or a missing field that has no default (the date fields — a birth date cannot be guessed). Every 400 names the offending parameter and echoes the full field spec so a client can correct itself.

$ curl "https://calculator.free/api/v1/mortgage/?price=350000&rate=6.5&term=30"
{
  "ok": false,
  "error": "unknown_parameter",
  "message": "Unknown parameter 'term' for calculator 'mortgage'. Did you mean 'years'? ...",
  "unknown_parameters": ["term"],
  "did_you_mean": { "term": "years" },
  "valid_parameters": ["down", "extra", "extra_onetime", "extra_yearly", "hoa",
                       "insurance", "other", "pmi", "price", "rate", "tax", "years"],
  "fields": [ ... ]
}

The /api/v1/ index publishes every field's key, type, unit, default and whether it is required — enough to build a client that never guesses.

Authentication & plans

You can call the API with no key at all — anonymous requests are rate-limited by IP. For a higher, predictable monthly quota (and commercial use), create a key on your account page and send it one of three ways:

GET  https://calculator.free/api/v1/mortgage/?price=350000&key=YOUR_KEY
curl -H "Authorization: Bearer YOUR_KEY" "https://calculator.free/api/v1/bmi/?units=metric&height=180&weight=80"
curl -H "X-Api-Key: YOUR_KEY"           "https://calculator.free/api/v1/bmi/?units=metric&height=180&weight=80"

• Free — anonymous (IP rate-limited) or a free key with a small monthly quota. Non-commercial.
• Developer — $9/mo — 10,000 calls/month, commercial use.
• Business — $49/mo — 100,000 calls/month, priority throughput.

What counts against your quota

Only a successful response does. One 2xx = one call. Every error is free: a 400 for a misspelled parameter, a bad value or a missing required field, a 404 for an unknown slug, a 401 for a bad key, a 429, and anything that goes wrong on our side. Explore the contract and fix your request as many times as you need — you are billed for answers, not for corrections.

Because errors are free they are rate-limited instead: if one key provokes more than 200 rejected responses in an hour, it gets HTTP 429 "too_many_invalid_requests" until the hour rolls over. Successful calls never count towards that, so a working integration will never see it — it only catches a client stuck in a retry loop.

When a key's monthly quota is used up the API returns HTTP 429 with error "quota_exceeded"; an unknown or inactive key returns HTTP 401. See full plans and sign up on the Pricing page.

Examples

curl

curl "https://calculator.free/api/v1/mortgage/?price=350000&down=70000&rate=6.9&years=30"

JavaScript fetch()

const r = await fetch(
  "https://calculator.free/api/v1/bmi/?" + new URLSearchParams({
    units: "metric", height: "180", weight: "80"
  }));
const { results } = await r.json();
console.log(results.bmi, results.category);

Python

import requests

r = requests.get("https://calculator.free/api/v1/mortgage/",
                 params={"price": 350000, "down": 70000,
                         "rate": 6.9, "years": 30},
                 headers={"Authorization": "Bearer YOUR_KEY"})
print(r.json()["results"])

POST JSON

curl -X POST "https://calculator.free/api/v1/income-tax/?country=uk" \
  -H "Content-Type: application/json" \
  -d '{"income": 60000}'

All endpoints

One endpoint per calculator. Field keys and result keys for each are in the /api/v1/ index.