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On-screen Ruler

On-screen ruler to roughly measure object lengths.

Place an object against the screen and read the scale. Scroll sideways if needed.

Calibration

100%

Approximation notice: Browsers cannot know a screen's true physical size (DPI), so this ruler is a best-effort approximation for reference only — not for precise measurement.
What is this?

The on-screen ruler lets you measure the approximate length of a small object directly on your browser screen, in cm or inches, without a physical ruler. Calibrating against a standard-size object such as a credit card makes the scale closer to real life.

How to use it

First, in the Calibration section, pick a reference object you have on hand (credit card, business card, or A4 paper). Place the real object against the screen and move the slider until the displayed rectangle's width matches the object's actual width, then press Save. After that, use the cm/inch buttons above the ruler to switch units and measure objects against the screen.

Without calibration, the ruler shows an approximation based on a standard 96dpi assumption. Press Reset to clear any saved calibration and return to the default scale.

Real-world uses

Handy for a quick reference check on the rough size of small parts, cables, or cards when no physical ruler is nearby, or to get a rough sense of a product's listed dimensions while shopping online.

FAQ

Is it exactly accurate to real length?
No. Browsers cannot know a screen's true physical size (DPI), so this ruler calculates its scale from a calibration against a standard-size object like a credit card, making it an approximation. Use a real ruler or calipers for precise measurement.
How do I calibrate it?
In the calibration section below, choose a reference object such as a credit card or business card, place the real object against the screen, move the slider until the displayed rectangle's width matches it, then press Save.
Is the calibration saved?
Yes. The calibration value is stored in this browser's localStorage, so it persists on the same device and browser. You'll need to recalibrate on a different device or browser.
Do I need to recalibrate when switching cm/inch?
No. The calibration value is a pixels-per-millimeter ratio for the screen, so once calibrated it applies equally to both the cm and inch scales.

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