Shottr (https://shottr.cc/) is a great screenshot and annotations app and also includes this feature.

Not a promotion, just a very satisfied user of the app; it's quite beautifully designed.

Another fan of Shottr. The tool is a pleasure to use.

Same, a fan aswell. And it's mostly free!

ShareX (https://getsharex.com/) doesn't have quite this nice UX but it's free. I often use it alongside browser dev tools.

Here's a screenshot of me measuring this comment box https://i.imgur.com/yoTHbzq.png

Windows Powertoys also has a screen ruler option.

Not something I need, but a really excellent sales pitch.

Has anyone used both this and xScope from The Iconfactory and could provide a comparison?

I have used xScope ever since it was released and was one of my must-have tool. In the last 5+ years, my need for design tools have reduced a lot. I subscribed to SetApp and PixelSnap is part of it. I just have it around for fun and a nostalgic feeling that I get out of doing “designs”.

If you already have xScope, it should serve the purpose pretty well and much more. PixelSnap is tad “modern” in a sense (I don’t know how to explain that one). If I remember correctly, xScope has other tools that should be handy for a dedicated designers.

I've been using xScope since late 2015. I haven't used PixelSnap, but looked at it and was considering buying as PixelSnap's magic "snappable" features are pretty cool. xScope is $50 and PixelSnap is $40 to install on 1 Mac, but $70 to install on 2 Macs and $160 to install on 5 Macs. I have two laptops and a desktop and appreciate licenses to install on my personal Macs for non-simultaneous use, and xScope has this through the App Store purchase.

xScope also offers other tools. I use Rulers, Loupe, Guides, and Crosshair and they are all nice and work as expected.

https://xscopeapp.com/features

Microsoft Powertoys has a measuring tool like that

https://learn.microsoft.com/en-us/windows/powertoys/screen-r...

A dedicated app should be good, but there is basic measurement with cmd+shift+4

... on macOS

What is the use case for this? I've never needed to measure something on my screen.

I've been a PixelSnap user for a couple of years. I mainly use it for frontend dev: measuring the width or height of an element in the browser without needing to open DevTools or measuring an Electron BrowserWindow's dimensions to determine how large I should make it by default to ensure everything fits on load. I probably use it around once a quarter though, so I'm glad it's not subscription-based.