What this tool does
The Screen Ruler lets you measure distances and areas directly on your screen. Click and drag inside the measurement area to draw a rectangle, and the tool instantly reports its width, height, diagonal length, and total area. Pixel rulers along the top and left edges give you a reference grid, while live crosshairs track your cursor position in real time. You can switch between pixels and approximate centimeters depending on your needs.
How it calculates
**Formulas:** \`\`\` Width = |endX - startX| Height = |endY - startY| Diagonal = sqrt(Width^2 + Height^2) Area = Width x Height \`\`\`
For centimeter conversion the tool uses the standard assumption of 96 CSS pixels per inch, adjusted by your browser's device pixel ratio:
\`\`\` cm = pixels / (96 x devicePixelRatio) x 2.54 \`\`\`
This gives a reasonable approximation on most monitors, though the exact result depends on your display's actual pixel density.
Who should use this
- **Web designers** checking spacing, padding, and element sizes against their design specs - **Front-end developers** verifying layout measurements during development - **UI/UX professionals** auditing visual consistency across components - **QA testers** confirming that rendered dimensions match requirements - **Students** learning about screen resolution, pixel density, and coordinate systems
How to use
1. Move your cursor into the measurement area. The crosshair guides and coordinate readout track your position. 2. Click and drag to draw a selection rectangle. A dashed outline appears showing the region you are measuring. 3. Release the mouse button. The width, height, diagonal, and area appear in the results grid below. 4. To switch units, use the Pixels / Centimeters toggle above the measurement area. 5. Click Reset to clear the current measurement and start over.
Pixel density and physical accuracy
Screens vary widely in pixel density. A 4K laptop display packs far more pixels per inch than a standard office monitor, so the same pixel count represents a smaller physical distance on the higher-density screen. The centimeter mode uses your browser's reported device pixel ratio together with the industry-standard assumption of 96 DPI for CSS pixels. This is accurate enough for quick estimates but should not replace a physical ruler when exact real-world measurements matter.
FAQs
Q: Why are the centimeter values only approximate? A: Browsers report a device pixel ratio but do not expose the monitor's actual physical DPI. The tool assumes 96 CSS pixels per inch, which is the web standard but may differ from your screen's true density.
Q: Can I measure elements outside the measurement area? A: The ruler measures within its own canvas. To measure other page elements, take a screenshot and use an image editor, or use browser DevTools to inspect element dimensions directly.
Q: Does the tool work on touch screens? A: The tool relies on mouse events (mousedown, mousemove, mouseup). Touch support may vary by device and browser.
Q: What does the diagonal measurement represent? A: It is the straight-line distance between opposite corners of the rectangle you drew, calculated with the Pythagorean theorem.
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