Choose an edge from the toolbar above, or use one of the quick controls below.
Turn your browser into a fast, accurate screen ruler. Measure small objects in centimeters, millimeters, inches, or pixels directly on your computer monitor, laptop, tablet, or phone display. Calibrate with a physical reference card for actual size measurements.
Physical calibration remains the source of truth. Use the verification panel to confirm your display before measuring an object.
Hold a real physical ruler or tape measure against the four test bars below to verify true 1:1 scale fidelity.
Measure the on-screen 100.0 mm bar with your physical ruler. Enter the observed length to calculate error and auto-correct.
Calibrate the display against a real physical reference once, then use the same measured scale without depending on a phone model catalog.
The interface stays quiet because the measurement is the point. The same tool can move between design checks, code work, study, and practical making without becoming a dashboard.
Check spacing, components, mockups and print sizes before shipping.
Compare CSS pixels with physical dimensions and inspect responsive layouts.
Check small parts, fasteners, PCB spacing and workshop dimensions.
Measure diagrams, geometry figures and physical references without installing an app.
Most measurements are straightforward. When real-world size matters, physical calibration provides a reliable 1:1 scale on your display.
An online ruler is a browser-based measurement utility that renders a real-world dimensional scale in centimeters, millimeters, inches, or pixels directly on your computer monitor, tablet, or smartphone screen.
Accuracy depends on your display hardware and calibration. By default, web browsers operate in virtual 96 CSS PPI (approx. 3.78 px/mm) and cannot query the physical dimensions of external monitors. Calibrating against a standard physical card or ruler provides a reliable, verified 1:1 scale on that display.
Click Calibrate in the toolbar. Place a standard ISO/IEC 7810 ID-1 card (like a credit card, library card, or transit pass measuring exactly 85.60 mm × 53.98 mm) or a physical ruler against your screen, and adjust the on-screen slider until the dimensions match exactly.
If the uncalibrated screen scale is larger than physical reality (e.g., displaying 1.20 cm for a 1.00 cm object), the active pixel density is overestimated and must shrink (correction factor 0.8333). If it displays smaller (e.g., 0.90 cm for a 1.00 cm object), the scale expands (factor 1.1111). Calibration calculates this exact ratio.
Yes. Browser zoom directly scales CSS viewport pixels. For accurate physical measurements, always ensure your browser zoom level is set to 100% (Ctrl+0 or Cmd+0). If you zoom in or out, measurements will scale proportionally.
Yes. Modern smartphones and tablets are ideal portable measuring surfaces. The responsive interface adapts to portrait and landscape modes with edge-to-edge rulers, drag-and-drop measurement guides, and a crosshair tool.
Yes. Laptops and desktop monitors provide larger measuring surfaces for papers, packages, and hardware components. If using multiple monitors with different pixel densities, calibrate each screen individually.
Select "cm" or "mm" in the unit toolbar. Align your object with the zero mark at the ruler origin, then read the scale or click to place a draggable measurement guide at the opposite edge.
Select "in" in the unit toolbar. Major numbered marks represent full inches, with subdivided ticks for 1/2", 1/4", 1/8", and 1/16" increments. Click anywhere on the ruler to inspect decimal inches and live pixel dimensions.
No. The tool runs 100% client-side inside your browser. All calibration preferences and guide positions are saved strictly in your local device storage (localStorage) and never transmitted to external servers.
Use cm, mm, inches or pixels, calibrate when the physical size matters, and keep the rest of the interface out of your way.