PPI, pixels per centimeter and pixel pitch describe the same idealized rectangular grid with different units. Once PPI is known, the other two values follow without another device measurement.
Convert PPI to pixels per centimeter
One inch is exactly 2.54 centimeters, so px/cm = PPI ÷ 2.54. A density of 108.785539 PPI is approximately 42.828952 px/cm.
Convert PPI to millimeter pitch
One inch is 25.4 millimeters, so pitch = 25.4 ÷ PPI. At 108.785539 PPI the idealized center-to-center spacing is approximately 0.233487 mm.
Interpret pitch as grid spacing
The arithmetic returns ideal pixel-grid spacing. It does not measure subpixel layout, gaps, aperture ratio, panel construction or optical performance.