Hot tub water volume calculator
Dosing depends on gallons, so this is the number to nail down first. Enter the shape and a few measurements and get an estimate, with an allowance for the seats and footwell that keep a tub from holding its full geometric volume.
Measure to where the water actually sits, not the top rim. Seats make this less than the cabinet height.
Enter your dimensions to estimate gallons.
Typical volumes for common sizes
If you cannot measure, start here, then confirm against your model's spec sheet. These are typical ranges, not exact.
| Type or size | Typical gallons |
|---|---|
| Inflatable, 2 to 4 person (Intex, Coleman, Lay-Z-Spa) | 200 to 290 |
| Plug-and-play hard tub, 2 to 3 person | 170 to 220 |
| Mid-size hard tub, 4 to 5 person | 300 to 400 |
| Large hard tub, 6 to 7 person | 400 to 500 |
| Swim spa | 1,300 and up |
Typical residential ranges from manufacturer specifications. Your exact fill depends on the model, the seats, and how high you fill it.
How the estimate works
The geometric volume is length times width times average depth (or pi times radius squared times depth for a round tub), at 7.48 gallons per cubic foot. A real hot tub never holds that, because seats, the footwell, and contoured walls displace water, so we apply an allowance: about 25 percent off for a hard shell with seats, about 15 percent for an inflatable. The manufacturer spec or a metered fill is always more accurate than any dimensional estimate, so use this to get close, then confirm.
Common questions
Why is the answer less than length times width times depth?
Because the seats and footwell take up space that would otherwise be water. A contoured hard tub holds roughly a quarter less than its bounding box, which is why we apply a displacement allowance rather than reporting the raw geometric volume.
What is the most accurate way to know my gallons?
Two ways. Check the manufacturer spec for your exact model, or meter the fill: run the hose through a flow meter, or time a known flow rate, while filling from empty. A metered fill beats any calculation.
Does the dosing calculator use this number?
Yes. We remember your volume on this device, so once you work it out here it is already filled in on the dosing calculator.