How the sizing works
Start with length × width × height in feet. A 5 × 4 ft shower with an 8 ft ceiling is 160 cubic feet.
Hard surfaces soak up steam, so the volume goes up. Delta’s guide multiplies by 1.35 for ceramic or porcelain tile, glass or glass block, and by 2.0 for natural stone or marble. Each exterior wall adds 10%. Ceilings of 9 ft add 15% and 10 ft add 30%; Delta recommends 8 ft or lower and caps it at 10 ft. This calculator multiplies the factors together.
Mr.Steam uses the same idea with its own numbers: add 110% for natural stone, marble, glass block or concrete, add 40% for tile on cement board or a mortar bed, take off 15% for acrylic, and add 15% per foot above 8 ft. Generators from different makers have different ranges, so size with the maker’s own chart.
A skylight or outside window means the next size up, and it must be double-pane and sealed from inside the shower, per Delta. Every generator here runs on 240 V and needs its own double-pole breaker; a licensed electrician wires it.
Sources: Delta SimpleSteam residential sizing guide; Mr.Steam planning guide. The list matches Delta SimpleSteam kits; ask us to size Mr.Steam, ThermaSol or Kohler generators.