Screen Calibration Guide

Last updated: 14 August 2026

Calibration adjusts Online Free Ruler to the physical size of your own display. Complete it before measuring so the markings on screen correspond as closely as possible to real centimeters, millimeters, and inches.

Before You Start

Open the ruler on the display you want to measure with, set your browser zoom to 100%, and use the same browser and display settings you plan to use afterward. If you use multiple monitors, calibrate each one separately.

Method 1: Match a Standard Card

Matching a standard card against the on-screen calibration outline to calibrate the ruler — Match the on-screen outline to the width of a standard card to calibrate your display.

Open Calibrate and select the card method. Hold a bank card, credit card, or ID card flat against the screen, then move the slider until the on-screen outline matches the card width exactly. This is usually the best method because it measures the actual display you are using. Do not use a card if its physical size differs from the standard card size shown by the tool.

Method 2: Detect or Select Your Device

Auto-detect your device or pick it from the list to calibrate the ruler quickly.

Use auto-detect or choose a listed device profile when it matches your device and display configuration. This method is quick, but device settings, browser zoom, and display scaling can still affect the result. Check the ruler against a known-size object afterward and fine-tune with the card method if needed.

Method 3: Enter the Screen Diagonal

Entering the screen diagonal size to calibrate the ruler — Enter your screen diagonal in inches to estimate the pixel density.

Enter the physical diagonal size of your display in inches when you know it and the screen resolution is correctly reported. The tool uses these values to estimate pixel density. This can be useful for monitors and laptops, but a card comparison is a better final check whenever one is available.

Check and Recalibrate

After calibration, compare a few marks with a physical ruler or a known-size object. Recalibrate after changing browser zoom, operating-system display scaling, screen resolution, monitor, browser, or device. You may also need to recalibrate after an operating-system update that changes display settings.

If the Ruler Still Looks Wrong

Confirm that browser zoom is 100%, the page is not being scaled by an accessibility or display setting, and the correct display is active. Reset calibration and try the card method again. If the issue continues, contact us with your device model, browser and version, screen size or resolution, and the calibration method you used.

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