Steam Generator Sizing: What kW Do You Need?

Short answer: Multiply your steam shower's length x width x height in feet to get cubic feet. Go one size up for every foot of ceiling over 8 feet, and for stone or porcelain walls. Then pick the generator from the chart below. Never go over the size's maximum room rating.

Step 1: Measure the cubic feet

Measure the inside of the enclosure in feet. Multiply length x width x height. Always round your measurements up, as SteamSpa's spec sheets direct.

Example: a 4 ft x 3 ft shower with an 8 ft ceiling is 4 x 3 x 8 = 96 cubic feet.

Step 2: Adjust for ceiling height and walls

• Ceiling over 8 feet: SteamSpa's spec sheets say to use the next larger size for every foot of ceiling over 8 ft.
• Wall material and outside walls: SteamSpa's online sizing tool asks for your wall material (ceramic tile, glass tile, porcelain tile or natural stone) and your number of exterior walls. If your shower is stone or porcelain, or sits on an outside wall, choose one size up. SteamSpa's manual says a generator one step larger than its chart is safe and even recommended.

Step 3: Match your room to a generator size

The middle column is what SteamSpa's online sizing tool recommends for each room size. The right column is the most room each size can handle, from SteamSpa's spec sheets. Do not go over it.

Need more than 12 kW? The Black Series packages in our SteamSpa steam generator packages go up to 24 kW, but SteamSpa doesn't publish room ratings for those sizes. Send us your measurements before you order.

Worked example

A 5 ft x 4 ft shower with an 8 ft ceiling:

1. 5 x 4 x 8 = 160 cubic feet.
2. The ceiling is 8 ft, so no height adjustment.
3. 160 falls in the 141 to 168 row: 10.5 kW. That is well under its 400 cu ft maximum.
4. If the walls are natural stone, go one size up to 12 kW.

Same idea with a taller ceiling: a 4 ft x 3 ft shower with a 9 ft ceiling is 108 cubic feet, which matches 9 kW. The ceiling is 1 ft over 8, so go one size up to 10.5 kW.

Plan the power: a dedicated 240V circuit

Every size needs its own dedicated circuit, per SteamSpa's spec sheets. SteamSpa's manual lists these for its 240V single-phase generators:

The manual also calls for an independent circuit breaker, a power disconnect within sight of the generator, and grounding. A licensed electrician must do the wiring to local and national code. Check your panel has room for the circuit before you pick a size.

Plan the water, drain and location

• Water supply: 1/2 in. copper line.
• Drain line: 1/2 in. copper line.
• Steam line: 5/8 in. copper for 4.5 kW, 3/4 in. copper for 6 to 12 kW. Each steam head needs its own steam line, with no valve in it.
• Where it goes: indoors, in a dry spot that won't freeze, within 25 ft of the steam room but not inside it. Leave at least 12 in. of space on both sides and the top. Mount it level.
• Warranty items: SteamSpa's manual requires an inline water filter, a quick release and pressure reducing valve, and installation by a licensed professional. Our SteamSpa in-line water filter fits the first item.
• A sealed enclosure: the manual says the steam room must be sealed so steam can't escape into the wall cavity, and recommends 100% silicone caulk around pipes and fittings.
• An exhaust fan outside the steam room is strongly recommended to clear leftover steam. See our exhaust fan guide.
• Acrylic or fiberglass showers: keep the steam pipe centered so it never touches material not rated for 240°F.

FAQ

What size steam generator do I need for a 4 x 3 shower?

With an 8 ft ceiling, it's 96 cubic feet. A 4.5 kW unit is rated to 90 cubic feet, so it's too small. The 6 kW is the smallest size rated for this room (up to 150 cubic feet), and SteamSpa's sizing tool recommends 7.5 kW for it.

Can I buy a bigger generator than the chart says?

Yes. SteamSpa's manual says one size larger than its chart is safe and even recommended. Just make sure your electrical service can handle the larger size's current.

Does tile or stone change the size I need?

SteamSpa's own sources differ here. Its spec sheets size by volume only, while its online sizing tool asks about wall material and outside walls. For stone, porcelain or an outside wall, choosing one size up is the simple, safe choice.

How many amps does a steam generator need?

From 19 A for 4.5 kW to 50 A for 12 kW, at 240V on a dedicated circuit. See the table above, and have a licensed electrician size the breaker and wire to code.

Does the generator go inside the shower?

No. SteamSpa says it should not be installed in the steam room. Put it within 25 ft, indoors, in a dry spot with good ventilation that won't freeze. SteamSpa's diagram shows the space under a vanity as one possible spot.

Shop by size

Every SteamSpa steam generator package has a Wattage option. Indulgence, Royal and Oasis come in 4.5 to 12 kW, and the Black Series in 4.5 to 24 kW. For example, the SteamSpa OAT450GD Oasis Series package lets you pick any size from 4.5 to 12 kW. Browse all SteamSpa steam generators, or send us your measurements and we'll help you check the size.

Related guides: steam generator packages compared and steam shower vs. traditional shower.

Sources: SteamSpa spec sheets for the 4.5 to 12 kW generators (maximum room ratings, sizing rule, water lines, dedicated circuit); SteamSpa Installation & User Guide (electrical table, location rules, sizing chart); SteamSpa's online sizing tool; our live listings, checked November 5, 2026. Always check the current listing and spec sheet before you order.