While screen protractors naturally measure angular relationships independent of scale, measuring physical distances requires establishing a pixel-to-unit calibration factor. Free Online Protractor includes a built-in linear scale calibration engine.
1. Setting a Known Reference Distance
Identify a feature of known real-world dimension in your image—such as a 100mm scale bar on a blueprint, a standard doorway (30 inches), or a ruler placed in a photograph.
2. Dragging the Scale Calibration Line
Activate the "Scale" tool from the toolbar. Position handle A and handle B across the known reference feature, then enter the known physical length and select your unit (mm, cm, m, in, ft).
3. Automatic Ray Vector Distance Calculation
Once calibrated, the protractor displays the calculated length of Ray VA and Ray VB alongside the measured angle, enabling simultaneous distance and angle analysis.
Key Technical Takeaways
- Scale calibration converts screen pixels to millimeters, centimeters, meters, inches, or feet.
- Linear distances of Ray VA and Ray VB are calculated automatically.
- Calibration settings are saved per session for multi-measurement workflows.