Shower Tile Estimation
Shower tile has two components: wall tile and floor tile. They're estimated separately because they often use different tile sizes (and sometimes different materials). Wall tile is calculated from the wall area minus any windows or niches. Floor tile is the shower base area.
A standard 3x4-foot shower with 8-foot ceilings has about 120 sq ft of wall area. A 3x5 walk-in shower: about 140 sq ft of walls. The floor is simple: 12 sq ft for a 3x4 and 15 sq ft for a 3x5.
Waste factor for showers should be 15% -- higher than floors -- because corners, niches, bench seats, and fixture cutouts generate more waste. Also account for the shower niche (if any) -- a typical niche adds 3-4 sq ft of tiled surface.