Calibrated virtual ruler for measuring small objects on your screen. Supports centimeters, millimeters, and inches — calibrate once with a credit card, measure anything.
Place a real credit card against your screen and adjust slider until the box below matches your card size:
This tool renders a calibrated ruler directly on your screen, converting your display's physical pixel dimensions into real-world measurements. It uses a DPI calibration system— you set your screen's true pixel-per-inch density by placing a physical object of known size (like a standard credit card) on screen and adjusting a slider until it matches. Once calibrated, every ruler marking accurately represents millimeters or inches in physical space. The calibration is saved in your browser, so you only do it once per device.
Craftspeople and DIY makersuse it to quickly check whether a small hardware part — a bolt, hinge, bracket, or connector — matches a required size when a physical ruler isn't at hand. Placing the part directly on the screen next to the ruler gives an immediate size reference.
Online shoppers use it to verify product dimensions shown in listing photos. Placing a phone next to the screen and comparing listed dimensions against the ruler helps avoid returns from size surprises on jewelry, accessories, or small electronics.
Graphic designers and print professionals use it to verify that a design element at a specific print size (e.g., a business card at 85mm × 55mm) actually renders correctly on their design software canvas before sending to print.
Educators and students use it to measure diagrams, scale drawings, and images in textbooks or PDFs displayed on screen — especially when physical measurements of map scales or technical drawings are needed for calculation.
Jewelers and watchmakers use the high-resolution display of a modern phone as an improvised magnification and measurement aid for small components — ring sizes, watch strap widths, gemstone dimensions.
Web browsers expose the screen resolution (screen.width, screen.height) and the device pixel ratio (window.devicePixelRatio), but not the physical dimensions of the screen itself. Without knowing the actual screen width in inches, you cannot derive true PPI from the browser alone.
The calibration step bridges this gap. By asking you to match an on-screen element to a physical object of known real-world size, the tool back-calculates your display's actual PPI: PPI = rendered_width_in_pixels / known_physical_width_in_inches. Once PPI is known, every pixel measurement can be converted to physical inches or millimeters with standard arithmetic.
The calculated PPI is stored in localStorage and retrieved on subsequent visits. No server communication occurs — all computation runs entirely in the browser.
Curved monitors: PPI varies across a curved screen — the center calibration may not perfectly match the edges. Accuracy degrades toward the screen edges.
High-DPI / Retina displays: These displays report a devicePixelRatio of 2× or 3×. The calibration accounts for this, but ensure your browser zoom is at 100% before calibrating.
Operating system scaling: Windows display scale settings (125%, 150%) affect pixel reporting. Calibrate with the credit card method rather than entering PPI manually, as this bypasses scaling issues.
Maximum measurement range: The ruler length is limited to your screen width. For objects larger than your screen, this tool is not appropriate.
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