xScope

xScope is the oldest of the Mac measuring tools and still the deepest. Where newer apps do one trick well, this is a drawer of eight: rulers you place and lock, guides that snap to edges, a frames tool for checking layout boxes, a loupe, a crosshair reporting coordinates as you move, and an Overlay that pins a mockup image on top of the running interface at whatever opacity you like so you can flick between design and build.

That Overlay is the reason people who have xScope keep it. Nothing else does the compare step as directly, and it is also precisely the tool that the sandboxed Mac App Store build is worse at — which is the single most useful thing to know before you buy. The direct purchase emails you a serial, accepts upgrade discounts, and does not trade features for the convenience of Apple handling the receipt.

$49.99 is more than the newer measuring apps cost, and The Iconfactory has charged for major versions across nearly twenty years at 50% off for owners. Version 4 has been getting updates recently, so the next bill is not imminent, but plan for one.

If all you want is the size of a button and the gap next to it, this is more app than you need and PixelSnap is faster to learn. xScope pays off when alignment work is a regular part of the job.