Disk & Storage Explainer
By A treemap shows a hierarchy as nested rectangles. In a disk space analyzer each rectangle is a file or folder and its area is proportional to that item's size. Blocks sit inside the outline of the folder that holds them. The largest blocks are the largest space users. Colour usually marks file type and clicking a block zooms into that folder. It was invented by Ben Shneiderman at the University of Maryland in 1990 to see who was filling a shared 80 MB disk. Its weakness is that thousands of tiny files shrink to an unreadable smear, which is why analyzers pair it with a sorted list.
Open any disk space analyzer and the screen fills with a mosaic of coloured blocks, some huge, some slivers. That mosaic is a treemap, and once you know how to read it, it answers the question that sent you to the tool in a single glance: what is using the space.
This explainer covers where the chart came from, how the blocks map to your files and why disk tools chose it over a pie or bar chart. It ends with the one thing it does badly, so you know when to switch to the list beside it.
Where Did the Treemap Come From?
In 1990 Ben Shneiderman, a professor at the University of Maryland's Human-Computer Interaction Lab, had an everyday problem. The lab shared a single 80 MB hard disk and it kept filling up. He wanted to see who was using how much, and a folder tree drawn as an outline wasted most of the screen on indentation while saying nothing about size.
His answer was to give the whole screen to the root folder and slice it up: each subfolder gets a strip proportional to its size, and each strip is sliced again for its own contents. That repeats down to single files. He called it a treemap. That first slice-and-dice layout produced long thin strips that were hard to compare, so in 2000 Bruls, Huizing and van Wijk published the squarified treemap, which keeps every block close to a square. Most disk tools today use a squarified layout, often with the shaded cushion look that the SequoiaView project from Eindhoven made popular in the early 2000s.
How Do You Read a Treemap of a Drive?
Four rules cover it. First, every block is one file or one folder. Second, the area of a block is its size, so a block twice as big holds twice the data. Third, blocks inside a larger outline belong to that folder. The outline's area is the folder's total. Fourth, colour is a label, usually file type, so all video files share one colour and all images another.
In practice you scan for the biggest blocks and read their labels. Hover over one and the tool shows the full path and exact size. Click and the treemap redraws with that folder as the root, so you can drill from the drive to a folder to the file inside it that is causing the trouble. Then go back up and check the next biggest block.
Why Do Disk Space Analyzers Use a Treemap?
Because nothing else fits a whole drive on one screen. A pie chart can show the top ten folders and then runs out of slices. A bar chart can rank a few hundred items before it needs scrolling. A drive holds millions of files in tens of thousands of folders, and a treemap shows every one of them at once, with the folder structure visible as nesting rather than hidden in a list.
It also matches how people think about the problem. Nobody wants a precise percentage for every folder. They want to know which three things are big enough to bother with, and a treemap makes those three the largest shapes on the screen. That is why every serious disk tool since the 1990s, from WinDirStat and WizTree to TreeSize and SpaceSniffer, has a treemap at its centre.
What Can a Treemap Not Show?
Small things. Area is proportional to size, so a folder holding twenty thousand 50 KB documents becomes a block full of dots too small to label or click, even though together they might take 1 GB. A treemap also flattens depth: a file eight folders down and a file one folder down look the same, and only the hover tells you where it lives.
That is why a treemap alone is a poor disk analyzer. It needs a sorted folder list beside it for exact numbers and deep paths, and a biggest-files view for the single giants that hide inside otherwise ordinary folders. Univik Disk Usage Reporter puts all three on one screen, with a file type summary as the fourth, so the picture and the numbers cover each other's gaps. On our test drive, 952 GB across 12.65 million files, the treemap was on screen in roughly 53 seconds.
The treemap is the fastest way to see the problem and the slowest way to prove it. It shows you the shape in a second. The list beside it gives you the number and the path you can act on. We never ship one without the other.
Which Disk Tools Use a Treemap?
Almost all of them. WinDirStat, the open-source classic, draws a cushion-shaded treemap under its folder tree and colours blocks by extension. WizTree pairs a treemap with a very fast file list read from the NTFS table. TreeSize from JAM Software includes one in its free and paid editions. SpaceSniffer is built almost entirely around an animated treemap you drill into. Univik Disk Usage Reporter shows a zoomable treemap next to a folder ranking, a biggest-files view and a file type summary. The whole set exports as an HTML or CSV report.
They differ in speed, licence and what sits around the treemap rather than in the chart itself. The roundup of disk space analyzers compares them on those points.
Treemap and the Other Views Compared
Each view in a disk analyzer answers a different question. This is what to use when.
| View | What it shows | Best for | Misses |
|---|---|---|---|
| Treemap | Every file and folder as nested blocks | Which two or three things are eating the drive | Thousands of tiny files |
| Sorted folder list | Folders ranked by size with share | Exact numbers and the full tree | The picture |
| Biggest files view | Top files with full paths | The single giants, wherever they hide | Folder context |
| File type summary | Space per extension | Whether video, images or archives dominate | Which specific files |
| Pie chart | One level as slices | A handful of top folders | Anything nested, anything past ten items |
Univik Disk Usage Reporter draws a zoomable treemap of any drive or folder and pairs it with a ranked folder list, a biggest-files view and a file type summary on one screen. Free edition, everything on your own PC, with the report written out as HTML or CSV.
Windows 7 to 11 · a 1 TB drive in about a minute · nothing uploaded
The short version
- A treemap draws a folder tree as nested rectangles. Area is size, nesting is folder structure, colour is file type.
- Ben Shneiderman invented it in 1990 at the University of Maryland to see what was filling a shared 80 MB disk.
- Read it by finding the biggest blocks, hovering for the path and clicking to zoom into a folder.
- It beats pie and bar charts because it shows a whole drive, every level, on one screen.
- It cannot show thousands of tiny files, so a good analyzer pairs it with a sorted list and a biggest-files view.
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Treemap Questions
About This ExplainerLeena Taylor Paul wrote this explainer for Univik, which has made Windows utilities since 2013 and builds its disk analyzer around this chart. The history was checked against Ben Shneiderman's own account of the treemap's origin and the 2000 squarified treemap paper, and the timing figure comes from that same 952 GB scan. 22 September 2026. Questions about a treemap that looks wrong? Support is below.
Reference: A brief history of treemaps (Observable) · Treemapping (Wikipedia) · Univik Disk Usage Reporter