How accurate is an on-screen ruler?

Pixel measurements are exact. Physical units are only accurate after calibration, because a browser cannot know your display's real size and assumes 96 pixels per inch by default.

How do I calibrate a screen ruler?

Hold a bank card against the screen and resize the rectangle to match it. Every ISO ID-1 card is exactly 85.60 mm wide, anywhere in the world, which makes it a reliable reference.

Why does the browser assume 96 pixels per inch?

It is a value fixed in the CSS specification as a reference, dating from typical monitors of the 1990s. It is true of almost no modern display, which is why physical measurements need calibrating.

Is the calibration saved?

Yes, in this browser, so it only needs doing once per device. It does not transfer to another machine, since every screen is different.

Can I measure something on a printed page?

Not reliably. This measures what is on the screen. Holding paper against a monitor introduces parallax and depends on the screen being flat, so a physical ruler is better for physical objects.

Screen Ruler

Measure anything on screen in pixels, and in real inches once calibrated.

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About the Screen Ruler

Pixel measurements are always exact. Inches and centimetres are not, until you calibrate — a browser has no way to know your display's physical size, so it assumes 96 pixels per inch, a figure fixed in the CSS specification and true of almost no modern screen.

A bank card fixes that in one drag, because ISO/IEC 7810 makes every ID-1 card exactly 85.60 by 53.98 millimetres, everywhere in the world. It is the one object almost everyone has that can calibrate a display.

Your calibration is saved in this browser, so it only has to be done once per device.

How it works

Drag on the canvas to measure — width, height and diagonal are shown.
Calibrate against a bank card to get real-world inches and centimetres.
Your calibration is remembered for next time.

Frequently asked questions

How accurate is an on-screen ruler?: Pixel measurements are exact. Physical units are only accurate after calibration, because a browser cannot know your display's real size and assumes 96 pixels per inch by default.
How do I calibrate a screen ruler?: Hold a bank card against the screen and resize the rectangle to match it. Every ISO ID-1 card is exactly 85.60 mm wide, anywhere in the world, which makes it a reliable reference.
Why does the browser assume 96 pixels per inch?: It is a value fixed in the CSS specification as a reference, dating from typical monitors of the 1990s. It is true of almost no modern display, which is why physical measurements need calibrating.
Is the calibration saved?: Yes, in this browser, so it only needs doing once per device. It does not transfer to another machine, since every screen is different.
Can I measure something on a printed page?: Not reliably. This measures what is on the screen. Holding paper against a monitor introduces parallax and depends on the screen being flat, so a physical ruler is better for physical objects.

Privacy

Everything happens locally. Your files are read by your own browser, processed on your device, and never uploaded — closing the tab is all it takes to erase them.

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