Hounsfield unit converter
On the Hounsfield scale water is 0 HU and air about -1000, so 1000 HU means twice the attenuation of water, a relative density of 2. Enter HU, attenuation, relative density or percent of water and see every equivalent at once. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Value
- 0 hu
- Value in
- 0
- Unit in
- hu
- Mu water
- 0.206
- Hu
- 0
Computed tomography reports tissue contrast on the Hounsfield unit scale, where distilled water is defined as zero and air as about minus one thousand. Radiologists, medical physicists, radiation therapists, and students constantly need the matching relative linear attenuation coefficient, water-relative density, absolute attenuation in inverse centimetres, or percent of water density for the same reading. This Hounsfield unit converter accepts one finite value, names the unit you already hold, and optionally a positive water attenuation coefficient for absolute μ, then returns every equivalent form with the exact CT definition factors.
Why Hounsfield units sit next to attenuation and relative density
People search for a Hounsfield unit converter when a CT report, phantom certificate, or treatment-planning table is stated in HU and they need the matching relative attenuation or density, or when a physics model is built in μ and they must report the same number on the clinical scale.
The Hounsfield scale is a linear transform of the linear attenuation coefficient of a voxel relative to water: HU equals one thousand times (μ of the material minus μ of water) divided by μ of water, with air near zero attenuation mapping to about minus one thousand. That definition makes water exactly zero HU and gives soft tissue positive tens of HU, fat negative tens to low hundreds, and compact bone large positive values.
Relative linear attenuation is simply μ over μ of water and equals (HU plus one thousand) divided by one thousand. In the classic soft-tissue CT model the same ratio is used as relative density with water equal to one, so a reading of zero HU is density one, air at minus one thousand HU is density zero, and one thousand HU is density two.
Absolute attenuation in inverse centimetres multiplies that ratio by a water coefficient appropriate to the effective beam energy; this tool defaults to zero point two zero six per centimetre as a common diagnostic-energy reference and lets you override it. Percent density is one hundred times relative density so water is one hundred percent. Mixing those representations without a converter shifts calibration offsets, breaks electron-density tables, and mislabels bone versus soft-tissue thresholds.
This capability forces an explicit unit on the input value and always returns the full set of equivalents so clients can display HU next to μ and density without a second call. The math is pure and local: no network lookup, no scanner vendor database, and no silent clamp of impossible negative relative densities into a fake zero.
Where teams use it and what it costs
Radiology trainees convert window-centre HU ranges into relative density language when learning soft-tissue versus bone windows. Medical physics groups map phantom inserts certified in HU onto relative electron-density or attenuation tables used by treatment planning systems, keeping the plus-one-thousand offset explicit. Radiation oncology planners reconcile scanner HU calibrations with density overrides when commissioning new protocols or comparing vendors.
Research pipelines that store μ images need a single deterministic step to publish companion HU series for clinical review. Quality-assurance scripts assert that water phantom means stay near zero HU and that derived relative density stays near one within tolerance. Product and education teams embed the same conversion in lab tools so students never hand-write the factors under exam pressure.
Map your UI labels to the allowed unit names, treat unknown-unit errors as user-fixable feedback, and keep the system of record in whichever form your domain prefers while still exposing every field in the response.
What you can do with it
CT report and teaching demos
Convert clinical HU readings to relative density and attenuation ratios when explaining water, fat, soft tissue and bone windows to trainees.
Treatment planning density tables
Map phantom or scanner HU values to relative density and absolute μ for commissioning tables, with an explicit water attenuation coefficient.
FAQ
What happens if I send an unrecognized unit?
The error message includes Unknown unit and lists the supported families so you can correct the label.
How are water and air defined on this scale?
Air with μ ≈ 0 maps to −1000 HU (relative density 0).
Are relative density and μ/μ_water the same number?
Under the classic soft-tissue CT model both equal (HU + 1000) / 1000. This tool returns both fields for naming clarity; they share that exact factor.
What does it cost?
The browser tool is free and runs locally.
Which water attenuation value does it use?
It uses 0.206 per centimetre by default, a common reference for diagnostic CT energies, and you can enter your own value. It only matters when converting to or from absolute attenuation per centimetre.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/conv/hounsfield \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"value":0,"unit":"hu"}'const res = await fetch("https://api.kit.forhosting.com/conv/hounsfield", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"value": 0,
"unit": "hu"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/conv/hounsfield",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"value": 0,
"unit": "hu"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/conv/hounsfield", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"value":0,"unit":"hu"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"value":0,"unit":"hu"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/conv/hounsfield", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"value": 0,
"unit": "hu"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "conv.hounsfield",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Unknown unit names fail with a clear invalid_input error instead of a silent wrong scale. Use the tool free on this page, or call the same deterministic logic by API at $0.002 per request for dose-planning notebooks, phantom QC scripts, and teaching demos that must never invent the plus-one-thousand offset by hand.
How to call the converter and read the response
Hounsfield values may be negative (air and fat) or positive (soft tissue and bone). Send value as a finite number, unit naming what that number already is, and optionally mu_water as a positive water linear attenuation in inverse centimetres. The default mu_water is zero point two zero six, which is only used when converting to or from absolute mu_cm; pure relative conversions among HU, mu_ratio, relative_density, and percent_density ignore energy-dependent scaling beyond the exact algebraic factors. Allowed units are hu (aliases hounsfield, ct_number), mu_ratio (mu_rel, relative_attenuation, linear_attenuation_ratio), relative_density (rel_density, rd, rho_rel), mu_cm (mu, attenuation, linear_attenuation, per_cm, cm_1), and percent_density (pct_density, percent_water). When unit is mu_ratio, relative_density, mu_cm, or percent_density the value must be non-negative; negative relative quantities raise invalid_input because they are not physical under the standard non-negative attenuation model. The engine reduces every input to HU with exact rearrangements of the definition, then emits hu, mu_ratio, relative_density, mu_cm, and percent_density, plus unit_in, value_in, the mu_water used, and a short formula string for audit trails. Missing fields, non-finite numbers, non-positive mu_water, and unknown unit names all fail with invalid_input naming what was wrong, so form validation and API error handling stay aligned. Classic checks include zero HU becoming mu_ratio one and relative density one, minus one thousand HU becoming zero relative density, and one thousand HU becoming density two.
Where teams use it and what it costs
Prefer this endpoint when search intent is specifically Hounsfield or CT number conversion to attenuation and density; use general density converters for mass density among kg per cubic metre and g per cubic centimetre without CT calibration, and use absorbed-dose unit converters when the question is gray versus rad rather than CT numbers.
Integration use cases
Phantom QC and automation
Assert water near 0 HU and air near −1000 HU in scripts, converting the same measures to mu_ratio and percent density for logs.
Technical questions
Which units does this Hounsfield converter support?
Hounsfield units (hu), relative linear attenuation μ/μ_water (mu_ratio), relative density with water = 1 (relative_density), absolute linear attenuation in cm⁻¹ (mu_cm), and percent water density (percent_density). Common aliases such as hounsfield, ct_number, rd, and per_cm are accepted.