Drag the red (A) and blue (B) guides to position them. The horizontal ruler spans the top, the vertical ruler the left side — major ticks every 50 px, minor every 10 px. Enter your screen PPI (96 is typical desktop, mobile phones often 300+) to get a millimetre estimate. Hover the canvas to read cursor coordinates. Reset puts the guides back to defaults; Clear hides them outside the ruler. Works entirely in your browser; devicePixelRatio is used to keep ticks sharp on Retina displays.