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The spa volume math
square: L × W × depth × 7.5 × seat factor round: D² × depth × 5.9 × seat factor
The seat factor is what generic calculators skip: molded shells displace 15–35% of the box with seats, loungers, and footwell contours. Skipping it overdoses every chemical by the same margin — the classic reason new spa owners chronically run hot on chlorine.
A worked example
A 7×7 ft spa with 2.75 ft of water: raw box 1,010 gallons — obviously wrong for a hot tub, which is the seat factor's point. At the typical 0.75 factor... still high; molded 7-footers usually land 380–420 real gallons because depth to the waterline is closer to 2.25 ft than the shell suggests. Measure the water, not the cabinet, and the estimate lands.
Once you have the number
Enter it once in the spa chlorine calculator and it's remembered site-wide. The other spa habit worth stealing from pool owners: the wait-time check after dosing — small water settles fast, but shock still needs its window.
Frequently asked questions
How many gallons is my hot tub?
Typical ranges: 2–3 person spas 200–300 gallons, 4–5 person 300–400, 6–7 person 400–500. But seats overstate size — the honest number comes from measuring, or from the refill-with-a-flow-meter trick.
How do I calculate hot tub volume from measurements?
Square/rectangular: length × width × water depth (feet) × 7.5, then multiply by ~0.75 because seats and footwells displace a quarter of the box. Round: diameter² × depth × 5.9 × 0.75.
Why does spa volume matter so much?
Doses are tiny and swings are violent: being 100 gallons wrong in a 350-gallon spa is a 30% dosing error on every chemical, every time. Pools forgive estimation; spas punish it.
What's the most accurate way to know?
Time your next refill: hose into a 5-gallon bucket first to get your fill rate (say 100 seconds per 5 gal), then time the full fill. 20 minutes at that rate = 60 gallons per 20 min... the math writes itself and it's accurate to a few percent.