Calipers looks like a nice utility — but allow me suggest a very similar freeware alternative that I’ve been using for many years called Cool Ruler. Other than being a ruler vs. calipers, it’s very similar: Cool Ruler floats over your other apps, and lets you measure in inches, cm, pixels, or any custom unit you can define. You can also specify a scaling factor (preset or user-defined) — perfect when you’re working with an on-screen object that’s already scaled down by a specific percentage. Clicking on the ruler adds red marks to make it easy to transfer a precise measurement somewhere else on the screen. It even has a handy built-in calculator. It’s one of those utilities I couldn’t live without when creating PowerPoint slides, doing desktop publishing layouts, etc.

Best of all, it’s free and can be downloaded from the authors here.