Last reviewed October 2026
The short answer
A hot tub needs a flat, level, reinforced concrete pad, commonly about 4 inches thick, at least as big as the spa, with the electrical route planned before the pour. The spa's own installation manual is the final word on the base; read it before anything is formed.
Why the weight matters
Water weighs about 8.34 pounds per gallon. A spa holding a few hundred gallons carries well over a ton of water before the shell, the cover and the people, and all of it sits on the footprint of the cabinet.
| Capacity | Water alone | With 4 adults at about 180 lb each |
|---|---|---|
| 250 gal | about 2,090 lb | about 2,810 lb plus the shell |
| 350 gal | about 2,920 lb | about 3,640 lb plus the shell |
| 450 gal | about 3,750 lb | about 4,470 lb plus the shell |
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Water at 8.34 lb per gallon; adult weight is an illustrative round figure. Your spa's spec sheet lists its filled weight.
The checklist before the pour
- Read the spa manual's base requirement and footprint
- Plan the electrical: most spas need a dedicated circuit and a disconnect within sight of the tub, installed by a licensed electrician under permit
- Place conduit before the concrete so the slab is not cut later
- Size for the cover lifter, steps and service panel access
- Pour flat and level, slightly above the yard, on a compacted base
- Let the slab cure before the spa is set and filled