The quick answer: most acrylic hot tubs hold roughly 200–500 US gallons (about 750–1,900 L). Most inflatables hold a few hundred to about 1,000 litres. Your tub's manual or spec sheet gives the exact number, and that's the figure to use for dosing.
If you don't have it, the calculator below estimates it from your tub's shape, or you can measure it exactly with the fill-time method.
Why the exact number matters in a hot tub
In a 20,000 gallon pool, being 500 gallons off barely matters. In a 300 gallon hot tub, being 150 gallons off means every dose is 50% wrong. That's the difference between perfect water and burning eyes. Volume also sets your drain interval: (gallons ÷ 3) ÷ bathers per day.
Typical volumes
These are typical ranges. Models vary, so check your spec sheet.
| Tub type | Typical volume |
|---|---|
| 2–3 seat acrylic | ~200–300 gal (750–1,150 L) |
| 4–6 seat acrylic | ~300–450 gal (1,150–1,700 L) |
| 6–8 seat acrylic | ~400–500+ gal (1,500–1,900+ L) |
| Inflatable, 2–6 people | a few hundred to ~1,000 L (~80–265 gal) |
| Swim spa | Thousands of gallons; use the spec sheet |
Where to find the real number
- The manual or spec sheet. Look for "water capacity", "fill volume" or "litres". Many manufacturers publish spec sheets online by model.
- A label on the tub, often near the equipment bay or on the skirt panel.
- The box, for inflatables. Capacity is usually printed in litres, sometimes at "80% fill" or at the max fill line. Note which.
Calculating it yourself
Square or rectangular tub (acrylic)
- Measure the inside length and width at the waterline, and the water depth from the waterline to the floor of the footwell.
- Volume in gallons ≈ length × width × depth × 7.48 (all in feet).
- Subtract for the seats and footwell shape. Molded seats displace a large share of a tub's space, so the raw box number overestimates. Treat it as an upper limit, and prefer the spec sheet or the fill-time method.
Round tub
- Gallons ≈ inside diameter × inside diameter × water depth × 5.9 (feet).
- Metric: litres ≈ 785 × diameter² × depth (metres).
- Example: an inflatable with a 1.5 m inside diameter filled to 0.55 m ≈ 970 L (≈257 gal).
Inflatables have soft, curved walls and sometimes inflatable seats, so use the box figure if you have it.
"7x7", "8x8" and outside dimensions
Outside dimensions include the cabinet, insulation and shell thickness. Two 7x7 tubs can differ by 100 gallons or more depending on seat design and depth. Use them only as a rough guide.
Measure it exactly: the fill-time method
This is the most accurate home method, and it takes one refill.
- Time a known container. With your hose fully on, time how long it takes to fill a 5 gallon (or 20 litre) bucket. Example: 30 seconds per 5 gallons is 10 gallons per minute.
- Time the full fill from empty to your normal water line, with the hose at the same setting. Example: 40 minutes.
- Multiply: 10 gal/min × 40 min = 400 gallons.
Do the bucket test twice and average it. If you fill through the filter housing, the flow rate is the same as long as the hose setting doesn't change. A hose-end water meter makes this even easier.
Check it with chemistry
If your doses keep overshooting or undershooting, your volume may be off. Add a known amount of a chemical you can test well, such as alkalinity increaser (sodium bicarbonate), and compare the rise with what the calculator predicted for your stated volume:
- About 17 g of sodium bicarbonate raises TA by 10 ppm in 1,000 litres.
- If the rise is much bigger than predicted, your real volume is smaller, and vice versa.
Use this as a sanity check rather than a precise measurement, since test kits have limited resolution.
Quick conversion table
| US gallons | Litres | Imperial (UK) gallons |
|---|---|---|
| 150 | 568 | 125 |
| 200 | 757 | 167 |
| 250 | 946 | 208 |
| 300 | 1,136 | 250 |
| 350 | 1,325 | 291 |
| 400 | 1,514 | 333 |
| 450 | 1,703 | 375 |
| 500 | 1,893 | 416 |
Swim spas and plunge tubs
Swim spas hold far more water than a hot tub (often several times as much), and most are run at lower temperatures. Use the spec sheet's figure, and expect dosing closer to a small pool's than a hot tub's. Cold plunge tubs and stock-tank tubs are usually simple shapes, so the round or rectangle formula works well. Just measure the water depth, not the container height.
Common mistakes
- Using the "people" rating as a volume. Six-person tubs vary enormously.
- Using an inflatable's "maximum" capacity when you fill to a lower line.
- Measuring outside dimensions.
- Forgetting the footwell is deeper than the seats.
- Switching between litres and gallons mid-calculation. 1 US gallon = 3.785 L; 1 imperial (UK) gallon = 4.546 L. UK owners: most product labels use litres; check before using a "gallons" figure.
Set it once in Spa Calc
Choose your inflatable model from Spa Calc's presets, or enter your tub's litres or gallons once. Every dose and your drain countdown use that number from then on. If you measure a more accurate volume later, update it and everything recalculates.
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Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.