GrandPerspective shows the blocks. Disk Forecast says what they are.
GrandPerspective is a free treemap: every file a rectangle, sized by the space it takes. It's a great way to spot one giant file. It won't tell you what that file is, whether it's safe to remove, or when your disk will fill.
The short version
Use GrandPerspective for a quick picture of everything on a drive. Use Disk Forecast to keep watch over your Mac's own disk, with a forecast in the menu bar, System Data explained, and a cleanup list sorted by risk. Both are free.
What GrandPerspective does well
GrandPerspective draws a folder or a whole drive as a treemap. Every file is a block, sized by the space it uses, so the giant movie export or the forgotten VM disk jumps out at a glance. It's free and has been around a long time.
Where it stops
Blocks, not answers. A treemap shows size. It can't tell you that the big block is Xcode's build cache, that it rebuilds itself, and that clearing it is safe.
You have to remember to look. You open it when the disk is already full. Disk Forecast watches every day and tells you weeks ahead.
System Data stays a mystery. Snapshots, purgeable space, and simulator runtimes don't show up as tidy blocks. Disk Forecast breaks them out and explains each one.
Disk Forecast vs GrandPerspective
| Disk Forecast | GrandPerspective | |
|---|---|---|
| Price | Free | Free |
| Tells you when the disk will be full | Yes, in the menu bar | — |
| How it shows your disk | Lists sorted by size, each item explained | A treemap of every file |
| Explains what each item is | Yes, in plain English | — |
| System Data broken down | Every part, with Apple's fixes | — |
| Cleanup sorted by risk | Yes | — |
| Source available | Yes, on GitHub | — |
“—” means we couldn't confirm it from the vendor's own materials as of October 2026.
What GrandPerspective does better
A whole-drive picture in one window. For finding the single biggest thing on a disk, a treemap is hard to beat.
What Disk Forecast does better
Everything after you spot the big block: what it is, whether it's safe, how to clear it without regret. Plus the forecast, so you're not doing this in a hurry.
Using them together
- Install Disk Forecast.
brew install --cask coreyhaines31/tap/diskforecast, or download the app from GitHub. - Start with Safe to clear. Caches, logs, and build folders in old projects, each explained, sent to the Trash after one confirm.
- Reach for the treemap when you're hunting. If you're after one mystery file, GrandPerspective is still a great flashlight.
Questions
Is GrandPerspective free?
Yes. GrandPerspective is a free disk visualizer for the Mac.
What is a treemap?
A picture of a folder where every file is a rectangle sized by how much space it takes. Big files jump out, but the rectangles don't say what anything is.
How is Disk Forecast different?
It lives in the menu bar, tracks free space every day, and tells you when you'll run out. When you need room, it lists what it found with a plain-English line for each item, sorted by how safe it is to clear.
Is Disk Forecast free?
Yes, at home or at work, with no account. The source is on GitHub and the app collects no telemetry.
Other Mac storage apps, compared
Know before it's full.
Free. No account, no subscription.
macOS 14 or later · Apple Silicon and Intelbrew install --cask coreyhaines31/tap/diskforecast