How Do You Size a Steam Shower Generator?

Published September 27, 2026

Summary

Steam-generator sizing starts with the finished enclosure dimensions, but different manufacturers use different adjustment methods. Ceiling height, wall and ceiling materials, glass, exterior exposure, and other factors may change the recommended model. Use the sizing tool or table from the brand you plan to buy. One cubic-foot rule does not carry across product lines.

Measure the finished enclosure

Use the interior length, width, and ceiling height after the wall finish is installed. Those dimensions define the space the generator has to heat and fill with steam.

ThermaSol's PRO Series III manual starts from length x width x height in cubic feet. MrSteam asks for the same three measurements, floor to ceiling, and has you enter them in its VirtualSpa design assistant.

Materials can matter depending on the brand

MrSteam says each enclosure material holds heat differently and that this is a factor in the generator you choose. Steamist, now part of Delta Faucet Company, turns that into multipliers: 2.0 for natural stone or marble, 1.35 for ceramic or porcelain tile or glass block, and 1.1 or 1.2 for one or two exterior walls.

On Steamist's table, a 5 by 8 foot shower with an 8-foot ceiling (320 cubic feet) comes to 432 adjusted cubic feet in ceramic tile, which calls for a 10 kW generator, and 640 in marble, which calls for 15 kW.

Ceiling height can change the model you need

ThermaSol sizes up one generator for every foot over 8 feet, to a maximum of 10 feet. Steamist multiplies by 1.15 for a 9-foot ceiling and 1.3 for a 10-foot ceiling.

If the room is still being designed, settle the ceiling height before the generator and electrical work are finalized.

Choose the generator before electrical rough-in

The selected generator determines the electrical requirements. Changing generator size after the circuit has already been roughed in can mean changing cable, breaker, disconnect, or routing.

Sources

Specifications, prices and installation requirements are as the makers published them on these pages on the date above and can change. Check the current manual and the retailer before you buy.