How accurate is this online ruler?
Accuracy depends on your screen's DPI (dots per inch). Use the "Auto Detect" button to get the most precise DPI for your display. For standard 96 DPI monitors the ruler is very accurate. High-DPI / Retina screens may need a manual adjustment.
What does DPI mean and why does it matter?
DPI (Dots Per Inch) is the number of pixels in one inch of your screen. The default is 96 DPI for most Windows monitors. MacBook Retina displays are typically 144–220 DPI. Setting the correct DPI is essential for accurate cm, mm and inch measurements.
Can I use this ruler to measure objects on screen?
Yes. Hold a physical object against your screen next to the ruler and read the measurement. Make sure your DPI is correctly set for the most accurate physical measurements.
How do I switch between horizontal and vertical?
Click the "Horizontal" or "Vertical" button in the controls bar. The ruler will rotate automatically. Vertical mode is useful for measuring heights and portrait-oriented objects.