Hot tubs can weigh anywhere from 600 pounds to more than 1,000 pounds when dry. Even a small, two-person spa weighs in at more than 3,000 pounds once filled. A six- to a seven-seat unit can top 6,000 pounds once filled. Installation of your spa should always be on a strong, durable surface such as a concrete slab or patio. If you choose to install the spa on your deck, speak with an expert to ensure it can hold that kind of weight.