Online Ruler
Measure anything on your screen in real centimeters or inches. Select your device to get started.
Select your device
Search for your device or enter the screen size manually.
About the Online Ruler
The Online Ruler turns your screen into a measuring stick marked in real centimeters or inches. It solves a small, common problem: you need to measure something and no ruler is nearby. Because every screen has a different pixel density, a ruler drawn in raw pixels is wrong on most devices. This tool calibrates to your screen first, then draws ticks at true physical spacing, so a centimeter on screen is a real centimeter.
You start by telling the tool your screen size. Pick your device from the searchable list (iPhones, Galaxy phones, Pixels, iPads, MacBooks, Windows laptops, monitors and more) or type a diagonal in inches and press Go. The tool reads your screen resolution, works out how many pixels equal one millimeter, and draws the ticks. Phones get a vertical ruler, desktops a horizontal one, and a CM / IN toggle switches units.
Everything runs in your browser using your screen's own dimensions, so nothing you enter is uploaded or stored. It is free, needs no signup, and works the moment the page loads. If you do not know your screen size, a second tab lets you calibrate by holding any bank or ID card against the display and dragging a slider until an on-screen box matches the card's width.
How to use the Online Ruler
- 1On the "I know my device" tab, search the device dropdown or type your screen diagonal in inches and press Go.
- 2If you do not know it, open the "I don't know my screen size" tab, drag the slider until the box matches a real card on your display, then press Done - Show Ruler.
- 3On desktop, set browser zoom to 100% with Ctrl/Cmd + 0 before calibrating.
- 4Hold the object against your screen, aligned with the 0 mark at the left edge on desktop or the top edge on a phone, then read where it ends.
- 5Use the CM / IN toggle to switch units, and press Recalculate if a measurement looks wrong.
What you can use it for
Sizing a package or part
You need to know if a screw, cable, or small package fits a slot but the tape measure is in another room. Hold the item against the screen, line it up with the 0 mark, and read the length in millimeters or inches.
Checking a phone screen spec
You are comparing two phones and want to feel how big a 6.7 inch display really is. Select that device from the preset list, and the ruler shows the actual span, with its length in centimeters and inches printed above the scale.
Measuring without any ruler
You are traveling with only a laptop and a wallet. Use the card calibration tab, match the on-screen box to your credit card's 85.6 mm width, and you have a working ruler for receipts, jewelry, or fabric swatches.
Frequently asked questions
Does this send my measurements or screen info anywhere?
No. The whole tool runs in your browser, reading your screen's resolution and the diagonal you select to calculate pixel spacing locally. Nothing you type is uploaded or saved, and closing the tab discards everything. Once the page loads you can even use it offline.
Why do I have to set browser zoom to 100%?
Calibration assumes one CSS pixel maps to a fixed physical size. Zooming changes that mapping, so the ruler stretches or shrinks. On desktop the tool watches for zoom changes and shows a red warning if it detects one. Press Ctrl/Cmd + 0 to reset to 100%.
What if I don't know my exact screen size?
Use the "I don't know my screen size" tab. Place any credit, debit, or ID card flat on your screen and drag the slider until the on-screen box matches the card's width. Every such card is 85.6 mm wide by ISO standard, so this gives a correct scale.
Why does the ruler look different on my phone?
On screens narrower than 600 pixels the tool draws a vertical ruler with numbers to the right, since a tall phone gives more usable length. Hold the phone upright in portrait. Wider screens get a horizontal ruler with numbers below. Both layouts use the same toggle and calibration.
Can I measure something longer than the ruler?
The ruler only spans the visible area, and the tool prints that span above the scale. It does not scroll to extend the measurement. For objects longer than your screen, measure in sections and add them together, or use a larger display.
How does the tool know the real size from my screen?
From the device diagonal you give and your screen resolution. It reads screen width and height in pixels, finds the pixel diagonal with the Pythagorean theorem, then divides the physical diagonal (one inch is 25.4 mm) into it to get pixels per millimeter, and ticks are spaced by that value.