How to Calculate & Formula Details
Default scale uses the CSS specification of 96px = 1 logical inch. Calibration mode recalculates px-per-inch from a known 3.370in credit card width.This tool turns your phone, tablet, or computer screen into a ruler you can hold up to a real object, showing accurate inch and centimeter markings.
Why Calibration Matters
Browsers don't expose your screen's true physical size, so by default this ruler uses the CSS specification's assumption of 96 pixels per inch, which is reasonably accurate on many laptop and desktop monitors at 100% browser zoom, but can be noticeably off on phones, tablets, and high-resolution displays.
One-Tap Credit Card Calibration
For an exact match on your specific device, tap Calibrate and hold any standard credit or debit card (which are a standardized 3.370 inches wide worldwide) up to the on-screen rectangle. Use the +/- buttons to nudge it until it matches your card exactly, then save - your calibration is remembered on this device for next time.