Developer API

Unit Conversion API

A free, versioned REST API for deterministic unit conversion — length, weight, speed, angle, data storage, temperature, and currency. Built for developers and AI agents alike.

Quickstart Interactive Docs (Swagger) ReDoc

Quickstart Authentication Endpoints Supported Converters Code Examples Error Codes Rate Limits Getting an API Key

Quickstart

Convert 25 kilograms to pounds in one request.

curl -X POST https://curepdf.com/api/v1/conversions/convert/ \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"category": "weight", "from": "kg", "to": "lb", "value": 25}'

Response

{
  "success": true,
  "result": {"input": {"value": 25.0, "unit": "kg"}, "output": {"value": 55.1156, "unit": "lb"}},
  "conversion": {"category": "weight", "from": "kg", "to": "lb"},
  "formula": "lb = kg × 2.2046226218487757",
  "formatted_result": "55.1156 lb",
  "precision": 6,
  "meta": {"api_version": "v1", "timestamp": "2026-01-01T00:00:00Z", "source": {"name": "CurePDF Unit Conversion API"}}
}

Authentication

The two discovery endpoints (category list, category detail) and the AI manifest are public — no key required. POST /conversions/convert/ requires an API key, sent as a bearer token:

Authorization: Bearer YOUR_API_KEY

An X-API-Key: YOUR_API_KEY header also works, for clients that can't set a custom Authorization scheme. Keys are never accepted via query string.

Keys are staff-issued, not self-serve yet — see Getting an API Key below.

Endpoints

Base URL: https://curepdf.com/api/v1/

Method	Path	Auth	Description
`GET`	`/conversions/`	None	List every supported category and its unit codes.
`GET`	`/conversions/{category}/`	None	Full unit metadata (code, name, symbol) for one category.
`POST`	`/conversions/convert/`	Required	Convert a value from one unit to another within a category.
`GET`	`/ai/manifest/`	None	Machine-readable discovery manifest for AI agents/tooling.

POST /conversions/convert/ accepts category, from, to, value (required), plus optional precision (integer, 0–12, default 6) and mode ("decimal" or "binary", only used by category="data_storage").

Supported Converters

Only categories actually implemented and tested are ever listed here — this table is generated live from the same registry the API itself reads, so it can't drift out of sync with what convert/ actually accepts.

Category	Slug	Unit codes
Length Converter	`length`	km, m, dm, cm, mm, um, nm, mi, yd, ft, in, nmi
Weight Converter	`weight`	t, kg, g, mg, ug, lb, oz, st, us_ton, uk_ton, ct
Speed Converter	`speed`	mps, kmh, mph, kn, fps, mach
Angle Converter	`angle`	deg, rad, grad, turn
Data Storage Converter	`data_storage`	bit, byte, k, m, g, t, p
Temperature Converter	`temperature`	c, f, k, r, re
Currency Converter	`currency`	USD, EUR, GBP, INR, JPY, CAD, AUD, CHF, CNY, SGD

Code Examples

The same request in three languages. Replace YOUR_API_KEY with a real key — never a placeholder committed to source control.

cURL

curl -X POST https://curepdf.com/api/v1/conversions/convert/ \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"category": "temperature", "from": "c", "to": "f", "value": 100}'

Python

import requests

resp = requests.post(
    "https://curepdf.com/api/v1/conversions/convert/",
    headers={"Authorization": "Bearer YOUR_API_KEY"},
    json={"category": "temperature", "from": "c", "to": "f", "value": 100},
)
print(resp.json()["result"]["output"])

JavaScript

const resp = await fetch("https://curepdf.com/api/v1/conversions/convert/", {
  method: "POST",
  headers: {
    "Authorization": "Bearer YOUR_API_KEY",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({ category: "temperature", from: "c", to: "f", value: 100 }),
});
const data = await resp.json();
console.log(data.result.output);

Error Codes

Every error uses the same shape: {"success": false, "error": {"code": "...", "message": "..."}}. No stack traces are ever returned.

HTTP	Code	Meaning
401	`MISSING_API_KEY`	No key was sent on a request that requires one.
401	`INVALID_API_KEY`	The key doesn't exist, was revoked, or has expired.
429	`RATE_LIMIT_EXCEEDED`	Too many requests in the last 60 seconds for this key.
429	`DAILY_LIMIT_EXCEEDED`	This key's daily request cap was reached.
429	`MONTHLY_LIMIT_EXCEEDED`	This key's monthly request cap was reached.
400	`UNSUPPORTED_CATEGORY`	The `category` isn't one of the values from `GET /conversions/`.
400	`UNSUPPORTED_UNIT`	The `from`/`to` unit isn't valid for the given category.
400	`INVALID_VALUE`	`value` is non-finite, too large, or (for temperature) below absolute zero.
400	`INVALID_PRECISION`	`precision` is outside the 0–12 range.
400	`PARSE_ERROR`	The request body isn't valid JSON.
404	`NOT_FOUND`	The category in `GET /conversions/{category}/` doesn't exist.
503	`SERVICE_UNAVAILABLE`	Currency conversion requested, but live rates aren't currently available.

Rate Limits

Every key has its own per-minute, daily, and monthly caps, set by CurePDF staff when the key is issued — there's no single global limit. A capped value of 0 means unlimited for that window. Limits are checked in that order (per-minute, then daily, then monthly), so the response tells you exactly which one was hit.

Getting an API Key

API keys are currently issued by CurePDF staff rather than through self-serve signup. Contact us with what you're building and your expected request volume, and we'll set you up with a key and reasonable limits.

Once issued, treat the key like a password: it's shown once, at creation, and only a hash is stored afterward — if it's lost, ask us to revoke it and issue a new one.

Also See

Interactive Swagger docs · ReDoc reference · AI discovery manifest (JSON) · Raw OpenAPI schema