The Dock keeps changing size on a Mac: set it once and lock it
· 6 min read
The Dock changes size for three reasons: someone dragged the thin divider line in it by accident, it ran out of room and shrank its icons to fit, or it moved to a display with a different resolution. The first is the most common and has a permanent fix: set the size you want, then lock it with one Terminal command so a stray drag does nothing. The other two need a different approach, covered below.
Work out which kind of change it is
- It jumps to a clearly different size and stays there. The divider was dragged, usually while reaching for the Trash or a folder next to it.
- Icons get smaller as you open more apps and grow back when you quit them. The Dock is full and is shrinking to fit.
- It looks different after you plug in a monitor or change resolution. The setting has not changed; the screen has.
- Icons swell under the pointer and settle back. That is Magnification, a setting, not a fault.
Set the size properly
Open System Settings, then Desktop & Dock. The Size slider sets the base size. The Magnification slider sets how large icons grow under the pointer; slide it to Off to disable it.
You can also resize the Dock directly. Find the thin divider line between your apps and the Downloads and Trash end. When the pointer is over it, it turns into a double-headed arrow. Drag up to enlarge the Dock and down to shrink it. Holding Option while you drag snaps to standard icon sizes. Control-click the divider for a shortcut menu with Turn Hiding On, Turn Magnification On, Position on Screen and Dock Settings.
For an exact, repeatable size, use Terminal:
defaults write com.apple.dock tilesize -int 48
killall Dock
The number is the icon size. Around 36 is compact, 48 is a comfortable middle, 64 is large. killall Dock restarts the Dock so it reads the new value; your open windows are not affected. To see the current value, run defaults read com.apple.dock tilesize.
If magnification is what bothers you, you do not need to switch it off for good. Hold Shift and Control while moving the pointer along the Dock and magnification flips to the opposite of its setting for as long as you hold the keys.
Lock the size so it cannot be dragged
This is the fix for the accidental drag. Once the size is right:
defaults write com.apple.dock size-immutable -bool true
killall Dock
Dragging the divider now does nothing. To change the size later, remove the lock first:
defaults delete com.apple.dock size-immutable
killall Dock
Two related keys work the same way and are useful on a shared Mac, or one set up for someone who finds a rearranged Dock confusing:
position-immutablestops the Dock being moved to another edge of the screen.contents-immutablestops icons being added, removed or reordered.
Write each one with -bool true and delete it to undo, exactly as above. What defaults write does, and how to undo one safely explains what these commands change and where the values are kept.
When the Dock shrinks because it is full
No setting stops this. When the icons no longer fit along the edge of the screen, the Dock scales everything down so that they do. The Size setting is a maximum, not a guarantee. The fix is to give it room:
- Remove apps you do not launch from the Dock. Drag an icon up and out until Remove appears, then let go. The app is not deleted.
- Turn off Show suggested and recent apps in Dock in Desktop & Dock. It adds up to three recently used apps.
- Turn on Minimize windows into application icon. Minimized windows stop piling up at the right-hand end.
- Remove folders you do not use from the section beside the Trash.
- Keep the Dock at the bottom on a wide display. On the left or right edge it has only the height of the screen to work with, so it fills sooner.
When it changes with a display
The Dock has one size setting for every screen. The same value looks larger on a low-resolution monitor and smaller on a dense laptop panel, and choosing More Space in System Settings, Displays makes everything smaller, the Dock included.
The Dock can also move. When displays have separate Spaces, moving the pointer to the bottom edge of another display and continuing to push down brings the Dock to that screen. If it then looks a different size, that is the display, not the setting.
macOS has no per-display Dock size. You can pick a compromise, or keep two versions of the Terminal command above and run the one that suits where you are sitting.
If you would rather not keep commands around, Dock size and magnification is one of the power-ups in Mainspring, which shows hidden macOS settings as switches and snapshots each value before it changes so one click puts it back. Its scenes bundle several power-ups together and can apply themselves on a trigger such as connecting a display, so a desk scene with a larger Dock can switch on when the monitor is plugged in, and a laptop scene is one click away when you leave. It is $29 once for up to three Macs, with a free one-day trial. Change Mac settings automatically when you unplug the charger describes the same idea with a different trigger.
If the size resets by itself
Two less common causes are worth ruling out.
A major macOS upgrade occasionally returns Dock settings to their defaults. Set the size and the lock again; Mac settings that revert after a macOS update lists the others to check at the same time.
On a Mac issued by an employer, a management profile can enforce Dock settings and put them back at each login. If your changes never stick on a work Mac, that is the likely reason, and the person to ask is whoever manages it.
Questions
Why did my Dock suddenly get huge?
Almost always the divider. Open Desktop & Dock and move the Size slider back, or run the tilesize command, then lock it.
Does locking the size stop the Dock shrinking when it is full? No. The lock stops manual resizing. A Dock with too many icons still scales down to fit, so remove a few.
Is there a way to make the Dock a different size on each monitor? Not built in. The size is one value shared by every display, so the choice is a compromise size or switching the value when you change desks.
Does killall Dock close my apps? No. It restarts only the Dock process, which comes back in a second or two. Apps and windows stay as they were.