Free Calculator API

v1 — free, no API key, CORS enabled

Every Tallytopia calculator — all 73 of them — is available as a JSON API. Results come from the same deterministic engine as the website (see methodology), and every response includes the full step-by-step working, not just the answer.

Endpoints

• GET /api/v1/calculators — list every calculator with its links
• GET /api/v1/calculators/{slug} — a calculator's inputs (keys, defaults, ranges, options) and steps
• GET /api/v1/calculate/{slug} — run it; pass inputs as query parameters, omit any to use its default

Example

curl "https://tallytopia.com/api/v1/calculate/mortgage-payment?principal=300000&annualRate=7&years=30"

{
  "calculator": "mortgage-payment",
  "title": "Mortgage Payment Calculator",
  "inputs": { "principal": 300000, "annualRate": 7, "years": 30 },
  "steps": [
    { "key": "monthlyRate", "label": "Monthly interest rate",
      "value": 0.00583, "formatted": "0.58%" },
    { "key": "numPayments", "label": "Number of payments",
      "value": 360, "formatted": "360" },
    { "key": "payment", "label": "Monthly payment",
      "value": 1995.9075, "formatted": "$1,995.91" },
    { "key": "totalPaid", "label": "Total paid over term",
      "value": 718526.69, "formatted": "$718,526.69" },
    { "key": "totalInterest", "label": "Total interest",
      "value": 418526.69, "formatted": "$418,526.69" }
  ],
  "result": { "key": "payment", "label": "Monthly payment",
    "value": 1995.9075, "formatted": "$1,995.91" },
  "disclaimer": "Results are informational only — not financial, medical or legal advice.",
  "docs": "https://tallytopia.com/api-docs"
}

OpenAPI spec & interactive explorer

A machine-readable OpenAPI 3.1 spec is generated from the calculator definitions themselves, so every endpoint's parameters, defaults, ranges and option values are always exact: /api/v1/openapi.json. Import it into Postman, Insomnia or a code generator — or try every endpoint live in the interactive API explorer.

Inputs

Pass each input as a query parameter using the keys from the details endpoint. Number inputs take plain numbers; date inputs take ISO dates (startDate=2026-07-18); dropdown inputs accept only their listed option values. Calculators without a pinned currency accept currency=EUR-style display overrides. Invalid input returns HTTP 400 with a plain-English error; an unknown slug returns 404.

Limits and terms

Free for any use, no key needed, CORS open to all origins. Rate limit: 60 requests per minute per IP (HTTP 429 beyond that). If you build something public with it, a link back to the calculator's page on Tallytopia is appreciated but not required. No uptime guarantee — this is a free service; for embedded UI instead of raw JSON, see the embed snippet on any calculator page. All results carry the standard disclaimer: informational only, not financial, medical or legal advice.

Questions

Email hello@tallytopia.com— including if you'd like higher limits or new calculators.