Screen size
A browser can't read the physical size of a display directly, so we match your screen's resolution and pixel density against a table of common phones, laptops, and monitors. If the match is wrong (or missing), just pick your device above. On desktop the default is 14.2″ MacBook Pro; on phones it's 6.1″.
Fine calibration (most accurate)
Hold a real credit card, ID, or bank card against the outline and drag until the box matches it exactly. Cards are a standard 85.6 × 54 mm worldwide.
How accurate is this?
Auto-detection and the size list are estimates based on your reported screen resolution; real-world error is usually a few percent. For a genuinely reliable measurement, use the credit-card calibration — it typically lands within a millimetre. Keep browser zoom at 100% for the ruler edges to be exact.