What is a normal Hounsfield value?

There is no single normal, because the scale measures material rather than health. Water is 0 by definition, air about −1000, fat below zero, most soft tissue between roughly 20 and 60, and bone several hundred or more. What matters is whether a measurement fits the tissue it is supposed to be.

Why does my report quote a number for a cyst?

Because the number is the proof. A round lesion measuring close to the density of water is fluid, which is what makes it a simple cyst rather than something solid. Quoting the value is how the radiologist demonstrates the conclusion rather than merely asserting it.

Do MRI reports use Hounsfield units?

No — the scale belongs to CT. MRI signal is described in relative terms instead, as hyperintense or hypointense compared with surrounding tissue, because MRI brightness depends on the sequence rather than on an absolute scale.

Hounsfield units

Read Your Scan Editorial Team·Last updated 21 August 2026

Written from published radiology references and reviewed for plain language. Not written or signed by a physician, and not a diagnosis. How we write these pages →

In plain language

CT's density scale, where water is zero, air is far below it and bone is far above. It lets a radiologist say what something is made of — a number in the range of fat, fluid or blood is often enough to identify a finding without any further test.

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Where you'll see this

CT findings, as a measurement written "HU" — for example "a 6 HU lesion", meaning simple fluid.

Your report may word it “HU”, “Attenuation” or “Density measurement”. These mean the same thing.

Questions worth asking your doctor

Did this scan use the right technique to answer my question?
Would contrast, or a different sequence, show something this one could not?
Do I need another scan, or is this one enough?

How often does this turn up?

Hounsfield units are the measuring scale of CT, and while radiologists use them constantly they surface in reports mainly when a number settles a question. The scale is anchored to two fixed points: water is 0 and air is about −1000. Fat sits below zero, soft tissue in the tens, and bone in the hundreds or above. Because the scale is absolute rather than relative, a density can be measured rather than merely described.

Does it mean something is wrong?

A number in the report is usually there because it was decisive rather than because it was worrying. The classic use is characterising something that would otherwise be ambiguous: a kidney or liver lesion measuring in the range of water is fluid and therefore a simple cyst, which is one of the most common ways a frightening-looking round shadow gets confidently dismissed. A measurement quoted alongside a finding is generally the radiologist showing their working.

What to do with it

Nothing follows from the units themselves. Where a number has been quoted, it has usually already done its job in the same sentence — classifying the finding as fluid, fat, soft tissue, blood or calcium. If a measurement is described as indeterminate, that means the density fell in a range that does not settle the question, and the next step is usually a different kind of scan rather than a repeat of the same one.

Frequently asked questions

What is a normal Hounsfield value?: There is no single normal, because the scale measures material rather than health. Water is 0 by definition, air about −1000, fat below zero, most soft tissue between roughly 20 and 60, and bone several hundred or more. What matters is whether a measurement fits the tissue it is supposed to be.
Why does my report quote a number for a cyst?: Because the number is the proof. A round lesion measuring close to the density of water is fluid, which is what makes it a simple cyst rather than something solid. Quoting the value is how the radiologist demonstrates the conclusion rather than merely asserting it.
Do MRI reports use Hounsfield units?: No — the scale belongs to CT. MRI signal is described in relative terms instead, as hyperintense or hypointense compared with surrounding tissue, because MRI brightness depends on the sequence rather than on an absolute scale.

How the scan was taken

Where this word comes up

What “lucency” means on an X-ray or CT reportLucency means an area let more X-rays through — it looks darker on the film. What that describes, why it is a shape and not a diagnosis, and what changes its meaning.What phleboliths are on a CT or X-ray reportSmall calcium stones inside pelvic veins. Why nearly half of adults have them, why they are harmless, and why they matter when someone is looking for a kidney stone.What an “adrenal incidentaloma” is on a CT reportA lump on an adrenal gland, found on a scan done for something else. Why most are harmless, what Hounsfield units have to do with it, and the two questions that decide what happens next.MRI vs CT scan: what each one showsMRI and CT answer different questions. What each physically detects, which tissues it sees well, how long it takes, and why your doctor chose one.How to read a brain CT scanWhat a radiologist checks on a head CT and in what order: windows, symmetry, grey-white contrast, ventricles, cisterns — and what an early scan misses.

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