Contact area size
The size of the contact that touches the screen varies depending on the size of the finger, posture of the finger contacting the screen, and how hard the finger is pressed against the screen. Microsoft collected large samples in the lab to measure the contact area size.
The average size of the contact area using an index finger was 9.4 mm x 8.8 mm (bounding box width x height). One of the smallest that was observed in the lab was 5 mm x 6 mm, and the largest was 17 mm x 20 mm.
For “normal tap“ gestures, the average size observed in the lab was 10.8 mm x 10.5 mm.
Combining this information with various other data, including non-index finger usage such as side of a thumb hitting a space bar in the software keyboard, the sizes to use are as follows:
- Average – 9 mm x 9 mm
- Smallest – 7.5 mm x 7.5 mm
- Largest – 30 mm x 30 mm