Best Free Unit Converter

How it works

1. Pick a category. Choose length, weight, temperature, volume, area, speed, data or time.
2. Enter the value and units. Type the amount and choose the units to convert from and to.
3. Read the result. The converted value appears instantly, with no rounding surprises.

About this tool

Unit Converter handles the everyday conversions across eight categories: length, weight, temperature, volume, area, speed, data size and time. Pick a category, enter a value, and choose the units to go from and to — metric to imperial or the reverse. The factors are exact where a standard defines them (an inch is exactly 2.54 cm), and temperature is handled with the proper formulas rather than a single ratio. It runs instantly and stores nothing.

What people use it for

• Convert miles to kilometres or pounds to kilograms
• Switch a recipe between cups, millilitres and ounces
• Convert °F to °C correctly (not with a ratio)
• Work out file sizes between MB, MiB and GB

Developer API

Automate this tool from your own code. Send a POST request to the endpoint below and get the same result the web tool produces. It is rate-limited per IP and needs no signup — API keys for higher limits are coming.

• cURL
• Python
• JavaScript

curl -X POST https://best.free/api/tools/unit-converter/ \
  -H 'Content-Type: application/json' \
  -d '{"category": "length", "value": 10, "from": "mi", "to": "km"}'

import requests

r = requests.post(
    "https://best.free/api/tools/unit-converter/",
    json={"category": "length", "value": 10, "from": "mi", "to": "km"},
)
print(r.json())

const r = await fetch("https://best.free/api/tools/unit-converter/", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ category: "length", value: 10, from: "mi", to: "km" }),
});
const data = await r.json();
console.log(data);

Response: JSON with the converted value.

{"ok": true, "value": 16.09344, "formatted": "16.0934"}

Frequently asked questions