Using a screen as a ruler sounds convenient, but phones, laptops, and monitors all have different pixel densities and physical sizes. How accurate can an online ruler become after proper calibration, and what usually causes the biggest measurement errors? The tool supports calibration using either the screen diagonal or a standard-size plastic card.

The biggest error usually comes from skipping calibration or using a guessed screen size. Phones and laptops rarely match the default diagonal the browser assumes, so the ruler ends up off by a few percent. I use the online ruler tool at this ruler when I need quick on-screen measures, and it handles the calculation from pixels to inches well. Just pick the manual diagonal entry over the auto-detect option.