← Unit converter

Convert units with curl

Convert between 104 units of length, mass, volume, area, temperature, speed, time, data, pressure, energy, power and angle. Here is the whole call with curl, with the output it produced when we ran it.

Before you start

• An API key. Create a free account; it comes with free tokens every month. Put the key in an environment variable so it stays out of your code: export BLOODWEB_KEY=your_key
• curl 7.76 or later (for --fail-with-body, which makes curl stop on an error and still show the reason).

The code

Save it as unit-convert.sh and run bash unit-convert.sh.

curl -sS --fail-with-body \
  -H "Authorization: Bearer $BLOODWEB_KEY" \
  "https://bloodweb.net/api/v1/unit-convert?value=5&from=lb&to=kg"

What you get

Tested 19 October 2026: this exact code ran against our test copy of the API, which runs the same service code as bloodweb.net, and produced this.

The API answers with JSON:

{
    "value": 5,
    "from": "lb",
    "to": "kg",
    "result": 2.26796185,
    "category": "mass"
}

Change the call

Every option the Unit converter takes. Add or change them the same way as the ones in the code.

• value required: The amount to convert.
• from required: The unit it is in: a symbol (kg, mph, GiB) or a name (pounds, fahrenheit).
• to: The unit to convert to. Leave out for every unit in the same category.
• precision: Round results to this many decimal places. Left out, results keep 12 significant figures.

If it fails

A failed call is not charged. The answer is JSON with an error object; the code above prints its message. The usual ones:

• 401 missing_key or invalid_key: BLOODWEB_KEY is not set in the shell that runs the code, or the key was revoked.
• 400 bad_param: a value is wrong or missing. The message names the parameter.
• 402 insufficient_tokens: the balance does not cover the call. Top up in the dashboard.
• 429 rate_limited: too many calls at once. Wait the seconds in the Retry-After header and send it again.

Every code and its fix is in the docs.

Next

• The Unit converter page: every option, the limits, and Try it now in your browser.
• Getting started: keys, errors and rate limits for all the services.