Guides / Hot tubs, saunas and plunges · Updated
How much a hot tub weighs
Water is 8.34 lb a gallon, and we count 185 lb an adult. Dry weight varies by model, so get yours off the spec sheet.
Short answer
Most 6-seat tubs hold 350 to 450 gallons. That's 2,919 to 3,753 lb of water alone. Add an 850 lb shell and 6 adults, and a 7 by 7 ft tub hits 5,296 lb. That's 108 psf. A normal deck is built for 50.
Weight by gallons
The spec sheet lists gallons. Find yours, then add the dry weight from the same sheet.
| Tub holds | Water | Water + 4 adults | Water + 6 adults |
|---|---|---|---|
| 250 gal | 2,085 lb | 2,825 lb | 3,195 lb |
| 300 gal | 2,502 lb | 3,242 lb | 3,612 lb |
| 350 gal | 2,919 lb | 3,659 lb | 4,029 lb |
| 400 gal | 3,336 lb | 4,076 lb | 4,446 lb |
| 450 gal | 3,753 lb | 4,493 lb | 4,863 lb |
| 500 gal | 4,170 lb | 4,910 lb | 5,280 lb |
Water = gallons × 8.34 lb. People at 185 lb each. None of this counts the tub itself.
One tub, empty to full
Here's the calculator's example. It's a 6-seat tub, 84 in by 84 in, 400 gallons, 850 lb dry. That's 49 sq ft on the deck.
| Tub | Weight | Per sq ft | vs a 50 psf deck |
|---|---|---|---|
| Empty | 850 lb | 17 psf | Under |
| Full of water | 4,186 lb | 85 psf | Over |
| Full, 6 adults in it | 5,296 lb | 108 psf | Over |
Per sq ft = weight ÷ the tub's footprint. Use the outside of the cabinet.
Water is most of it. The 850 lb shell is only 17 psf. Fill it and you're at 85, already over before anyone gets in.
Why 50 psf
The IRC builds a deck for 40 psf of people and furniture, plus 10 psf for the deck itself. That's the 50. A full tub puts all its weight on one small patch, so it blows past that on a normal deck.
Extra joists alone won't fix it. The posts and footings under the tub carry that weight too. Either an engineer designs the deck for the tub, or the tub goes on the ground.
On the ground
A level concrete pad, 4 to 6 in thick, carries any of these. Gravel or pavers work if the tub maker allows it. Whatever it sits on has to be flat. A soft corner can crack the shell, and that voids most warranties.
Before you buy
Get the dry weight, gallons and footprint off the spec sheet. Put them in the calculator with your deck and panel. You'll know before the truck shows up.
Size, gallons and dry weight for 16 popular models are in hot tub dimensions.