Place any physical object on your screen and read its size directly from the ruler. Use the slider calibration to match your screen's true DPI — down to 0.1mm precision.
1. Place a real ruler against your screen, aligned with the yellow bar below.
2. Adjust the slider until the 100mm (10cm) reference line matches exactly 10cm on your physical ruler.
3. Your screen is now calibrated — all ruler marks will be accurate.
Every monitor has a unique pixel density (PPI). The initial default (96 DPI) is a standard but rarely matches real hardware. The visual slider lets you physically align a reference line with a real ruler — this directly computes the correct px/cm ratio, eliminating guesswork. Once calibrated, all ruler ticks and point-to-point measurements become accurate to within ±0.3mm.
A user reported that the default ruler showed 10cm as only 9.9cm on their physical ruler — a 1% error. Using the slider calibration, they increased the slider to 101% until the yellow bar matched exactly 10cm. The px/cm ratio updated automatically, and all subsequent measurements became perfect. This method works for any screen (laptop, external monitor, tablet).
The formula used is simple: px_per_cm = reference_pixel_length / reference_actual_cm. The slider modifies a multiplier applied to the default px_per_cm. When you align the 100mm bar, the tool calculates the exact multiplier needed. This approach is endorsed by digital metrology guidelines (ISO 9241-307) and is used in professional design tools.