Online Ruler (Actual Size) — Screen Ruler in cm & Inches | Cruse
Free calibrated online ruler actual size on screen. Measure in cm, mm, and inches with credit card or screen size calibration. 100% private, no app needed.
How to Use This Tool
Calibrate Your Display
Click the "Calibrate" button or the status pill in the center card. Place a standard credit card or debit card against the screen and adjust the slider until the on-screen card matches your card perfectly.
Choose Any Screen Edge
Select your preferred edge (Left, Right, Top, or Bottom) from the center shortcuts or top toolbar. The ruler dynamically stretches across the entire screen height or width.
Place Real Object & Measure
Align your real-world item directly against the screen edge. Click anywhere on the ruler surface to drop a precision dashed guide line with exact distance readouts in cm, mm, inches, or pixels.
Technical Reference & Specifications
| Parameter / Standard | Specification |
|---|---|
| Screen Coverage | Full Viewport (100vh / 100dvh) with dynamic window resize adaptation |
| Edge Placements | Left (Vertical), Right (Vertical), Top (Horizontal), Bottom (Horizontal) |
| Supported Units | Centimeters (cm), Millimeters (mm), Inches (in), Pixels (px) |
| Calibration Standard | ISO/IEC 7810 ID-1 Credit Card (85.60 × 53.98 mm) & Hardware Presets |
| Imperial Precision | 1/16 inch subdivisions with fractional labels |
| Metric Precision | 1 millimeter subdivisions with centimeter numbering |
| Display Technology | Hardware-accelerated HTML5 Canvas with sub-pixel DPR scaling |
| Client Privacy | 100% in-browser calculation. No camera, cookies, or tracking. |
Frequently Asked Questions
How does an online ruler measure actual physical size?
Computer screens vary from 80 PPI on large monitors to over 460 PPI on modern mobile phones. An uncalibrated online ruler merely guesses at 96 PPI. This tool allows you to calibrate against standard physical objects (like an ISO/IEC 7810 credit card) to calculate your exact screen Pixel Per Inch (PPI), ensuring 1:1 true-to-life physical accuracy.
Can I use a credit card or debit card for calibration?
Yes! All credit cards, debit cards, driving licenses, and national ID cards globally adhere to the ISO/IEC 7810 ID-1 standard measuring exactly 85.60 mm wide by 53.98 mm tall. Holding your physical card to the screen card preview gives near-micron calibration accuracy.
Why does the ruler cover the entire screen?
Unlike standard rulers that stop in the middle of the screen, this online ruler spans the entire height (over 25 cm on typical desktop monitors) and width of your display. You can measure longer items without running out of screen space. You can also hide the center controls for a completely blank measurement canvas.
How do I add and delete measurement guide lines?
Click anywhere on the active ruler or screen surface to drop a vertical or horizontal dashed guide line. A live digital readout shows the exact distance from the zero edge. Press the Escape key or click "Clear Guides" to remove all active guide lines.
Can I use this ruler on mobile phones or tablets?
Yes. The ruler automatically detects mobile devices and supports touchscreen gestures, orientation changes, and fullscreen measurement.
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