Data & Utilities

Unit ConverterConvert between units of measurement

The Helix Unit Converter API converts values between units of length, mass, volume, area, speed, temperature, digital storage, and time. Deterministic, server-side, and works without users needing to remember conversion factors.

Free tier with 100 requests/month
Fast, cached responses
99.9% uptime target
Standard REST + JSON

Get API Key (Free) →

Interactive Demo

Unit Converter

What is the Unit Converter API?

How it works

1GET /convert?value=10&from=km&to=mi&category=length returns the converted value.
2Supported categories: length, mass, volume, area, speed, temperature, digital, time.
3Temperature uses correct formulas for Celsius, Fahrenheit, and Kelvin (not naive multipliers).
4GET /units?category=length lists all supported units in a category.
5Pure compute — no external calls — so latency is essentially network round-trip time.

Endpoints

2 REST endpoints, all returning consistent JSON.

GET

/convert

Convert a value between two units

GET

/units

List supported units (optionally by category)

Code in 5 languages

Copy and paste into your project. Replace YOUR_API_KEY with your RapidAPI key.

// Unit Converter API
const response = await fetch(
  "https://unit-converter-by-helix-api.p.rapidapi.com/convert"
  {
    headers: {
      "X-RapidAPI-Key": "YOUR_API_KEY",
      "X-RapidAPI-Host": "unit-converter-by-helix-api.p.rapidapi.com"
    }
  }
);

const data = await response.json();
console.log(data);

What developers build with this API

Real use cases from production deployments.

Recipe & Cooking Apps

Convert American recipes to metric (cups → grams, °F → °C) and vice versa, so users from any country can cook them.

Engineering & Construction Tools

Convert between SI and US customary units in BOM exports, CAD imports, and field measurements.

Health & Fitness

Display weight, distance, and BMR in the user's preferred units. Don't force a global app into a single locale.

E-commerce Internationalization

Show product dimensions and weights in the local market's preferred units automatically.

Pricing

All tiers include this API. No credit card for free tier.

BASIC

Free

✓ 100 reqs/month
✓ Hard limit
✓ All endpoints

PRO

$9.99/mo

✓ 10,000/month
✓ +$0.005 each
✓ All endpoints

ULTRA

$29.99/mo

✓ 100,000/month
✓ +$0.002 each
✓ All endpoints

MEGA

$99.99/mo

✓ 1,000,000/month
✓ +$0.001 each
✓ All endpoints

View all 20 APIs included →

Frequently asked questions

Real answers to questions developers ask before integrating.

Which categories are supported?

Length, mass, volume, area, speed, temperature, digital storage, and time — covering the conversions developers actually need.

Are conversions exact?

Yes — factors come from BIPM/NIST definitions. Returned values are rounded to 8 decimals for predictable JSON.

What unit codes do you use?

Standard short codes — m, km, mi, kg, lb, gal, L, c, f, k, mb, gb, etc. Call /units for the full list per category.

Does this support currency conversion?

No — currency is volatile and lives in our Currency Exchange API. Unit Converter handles only physical/mathematical units.

Is there a rate limit?

Same as every Helix tier — Free 100/mo, Pro 10K/mo, Ultra 100K/mo, Mega 5M/mo.