Measurement toolsEnglish
How to Use Guidelines and Selection
Use Guidelines to compare gaps and align edges. Use Selection to measure one box, including its width, height, diagonal, and area.
Read guideScreen measurement guides
Get better measurements from your screen
Start with calibration, learn which measuring mode to use, or fix a ruler that does not match a real object.
Start with calibration
This guide is available in every site language.
More guides in English
These guides are currently available in English. We will add translations only after each version has been reviewed.
Accuracy and troubleshootingEnglish
Why Is My Online Ruler Inaccurate?
Fix an online ruler that does not match a real object. Check browser zoom, calibration, display changes, and your reference object in the right order.
Read guide Pixels and displaysEnglish
CSS Pixels vs Device Pixels
Learn what CSS pixels and device pixels mean, how devicePixelRatio connects them, and why neither value alone gives a physical screen measurement.
Read guide Practical usesEnglish
How to Measure UI Spacing in a Browser
Measure padding, margin, gap, width, and height in CSS pixels with browser DevTools, then use Online Ruler for a visual or calibrated size check.
Read guide Practical usesEnglish
How to Measure an Image’s Size on Screen
Separate image file pixels from displayed CSS size and physical screen size. Use Inspect, a short browser check, and calibrated PPI correctly.
Read guide Practical usesEnglish
How to Use a Screen Ruler on a Phone
Set up and calibrate a screen ruler on a phone, choose a reference that fits, and use Guidelines or Selection without losing scale after rotation.
Read guide Pixels and displaysEnglish
PPI vs DPI: What’s the Difference?
Learn the difference between PPI and DPI, when to use pixels per inch or dots per inch, and why screen calibration needs effective PPI rather than printer DPI.
Read guide Accuracy and troubleshootingEnglish
Reference Object Sizes for Screen Calibration
Use the correct credit card, A4 paper, US Letter, or custom reference size for screen calibration, with exact dimensions, fit advice, and error checks.
Read guide