Calibrate
Place a real credit, debit, or ID card flat on the screen on the dotted strip below, then drag the right-edge handle until the card overlay matches the real card width exactly.
KBT Measure
CARD 85.6 MM
Calibration card
54 mm tall
Width: 324 px
= 85.6 mm
= 3.785 px/mm
= 96.2 DPI
Status: Using default 96 DPI (uncalibrated)
Tip: if the ruler shows 1 cm but a real ruler shows different, recalibrate using your credit card. Browser zoom of anything other than 100% will throw the calibration off.