How many gallons is a bathtub?
Last updated November 2026.
Quick answer
- Standard alcove tub
- about 45 gallons at 80% of its depth
- Freestanding soaking tub
- about 70 gallons for a 60 in oval
- Water weight
- 8.34 lb per gallon
- Hot water for that soak
- about 50 gallons from a 120°F tank
- Water heater sizing
- first-hour rating at least your peak-hour demand (energy.gov)
A standard alcove bathtub holds about 45 gallons when filled to a comfortable level, and a freestanding soaking tub about 70 gallons. Deeper soaking tubs hold more. Use the calculator below with your tub's inside length, width and depth to the overflow to get gallons, weight and hot water.
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How do you work out a bathtub's capacity?
Measure the inside of the basin, not the outside: length and width at the rim, and depth from the floor of the basin to the overflow. Multiply them, allow for curved sides (oval and round basins hold about 79% of a box of the same size), and divide cubic inches by 231 to get US gallons. Nobody fills a tub to the overflow with a bather in it, so the calculator assumes 80% of the depth unless you change it.
Bathtub gallons and hot water calculator
- Water in the bath
- 68 gal
- 257 liters
- Water weight
- 566 lb
- Hot water needed
- 49 gal
- at 120°F, mixed with 55°F cold
- Time to fill
- 8.5 min
- at 8 gal/min
An estimate from the basin's inside dimensions. Use the maker's published capacity for the actual tub, and size the water heater with a plumber.
Planning a stone bathtub?
Tell us about the room. We reply within one business day with options, and every quote gives the exact dimensions and weight of the piece.
How much does the water in a bathtub weigh?
Water weighs about 8.34 lb per US gallon, so a 70-gallon soak adds roughly 570 lb before the bather. In a solid stone tub that sits on top of 1,100 to 2,500 lb of stone; see the stone bathtub weight and floor-load calculator.
How much hot water does a bath need?
Less than the bath holds, because hot water from the tank is mixed with cold. Filling a 70-gallon soak to 102°F with a 120°F tank and 55°F incoming water takes about 50 gallons of hot water; colder mains water or a lower tank setting raises that.
What size water heater do I need for a soaking tub?
For a tank heater, the US Department of Energy says to choose a model whose first-hour rating at least matches your home's peak-hour demand. Add the soaking tub's hot water to the showers and appliances that run in the same hour; a plumber can confirm the size.
How long does a bathtub take to fill?
Divide the gallons by the filler's flow rate in gallons per minute, which is listed on its spec sheet. Water pressure in the house and pipe size change the real flow, so time a test fill once the filler is in.
Typical capacities by tub type
| Tub | Inside size used | Water at 80% depth |
|---|---|---|
| Standard alcove | 48 × 22 in inside, 13 in deep | about 45 gal |
| Freestanding oval soaking | 60 × 26 in, 16 in deep | about 70 gal |
| Published stone tubs | 60–78 in long | about 50–100 gal (Stone Forest, measured 4 in below the rim) |
For tub lengths and depths, see bathtub sizes and how deep a soaking tub should be.
Sources
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