TIP: How to Use the macOS ScreenShot Tool as a Screen Ruler

We often need to get the offset distance on the screen from a window corner, or a found image corner, to a button or other object we want to click on. This shows you how to get that offset.

From Use macOS screenshot tool as a screen ruler

You can select a region of the screen by pressing Command–Shift–4 and then holding the left mouse button pressed while dragging the mouse cursor around on the screen. You can see the pixel size of your selection and use it to measure things on the screen as long as you keep the mouse button pressed.

Release the mouse button to take a screenshot. You can dismiss the screenshot/measuring tool by pressing the Escape key.

Screen ruler demonstration video.

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Well that’s clever. One of those things that, once I saw it, I thought “Well I should have thought of that ages ago…”

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I've used Cmd+Shift+4 too, however, depending on the background it is sometimes difficult to read the coordinates. Does anyone know if there's a way to change the font size?

As an alternative, I now use PixelSnap an app included in the Setapp bundle. With PixelSnap, once an area is selected (using click and drag), the region remains highlighted with the W x H clearly displayed until esc is pressed.

Thanks for sharing.
There are definitely some better tools available, if you are willing to pay for them.
There is also PixelStick – Mac Onscreen Measuring Tools, available free/$10, which I have.
I tend to use the free macOS tool unless I need something more powerful.