Full-Screen Online Ruler in Inches and Centimeters
0.00"
Last reviewed on 2026-04-23.
InchRuler.com is a free virtual ruler that runs entirely in your browser. Pick your device from the dropdown at the top for a quick calibration, or open the ⚙️ dialog to fine-tune the ruler against a standard credit card. Once calibrated, you can drag the red line across the ruler to read off measurements in inches or centimeters.
Web browsers measure things in CSS pixels, and a CSS pixel is not a fixed physical size. The actual on-screen width of "one pixel" depends on your display's pixel density (PPI), on operating-system display scaling, and on the browser's zoom level. Without calibration, a ruler drawn at "96 pixels per inch" will usually be off by several millimeters on a typical laptop or phone. Calibration tells the ruler how many CSS pixels correspond to one real inch on your screen, so the tick marks line up with real-world distances.
If you know your display's diagonal size — most laptops, monitors, tablets, and phones have a well-known diagonal in inches — the ruler can combine that with the reported resolution to estimate the display's PPI. It's fast and usually accurate to within a millimeter or two.
Standard ID-1 bank cards (credit cards, debit cards, most national ID cards) are manufactured to ISO/IEC 7810 and measure 85.60 mm × 53.98 mm. Because every card is the same physical width, holding one against the on-screen reference and dragging the slider until the widths match gives the most reliable calibration, independent of what model or scaling your device is using.
MacBook Air and Pro users can jump straight to a precise PPI by selecting the exact model from the dropdown:
A full inch-to-centimeter reference table is available on the inch ↔ centimeter page.
An on-screen ruler is useful for everyday tasks: checking the width of a bolt or bead, measuring the dimensions of a photograph, comparing screen-print sizes, or giving a child a ruler for homework when the physical one has gone missing. It is not a replacement for a certified instrument. For engineering tolerances, medical or legal work, safety-critical dimensions, or anything where a fraction of a millimeter matters, use a calibrated physical tool. See the measurement accuracy page for a fuller discussion of where on-screen rulers do and don't belong.
Once you calibrate on a given device, the ruler stores the value in your browser's local storage and re-uses it on future visits. Re-calibrate if you change browser zoom, change operating-system display scaling, or move to a different screen. Installing the site as a PWA keeps the calibration tied to the installed app.