Bathtub Volume Calculator: How Many Gallons?

How the bathtub volume calculator works

Measure the inside of the tub at the water line, multiply length by width by water depth, then take off roughly a fifth for the sloped sides and rounded base. That is the whole method. The calculator does the multiplication, applies the fill factor you choose (0.8 unless you change it) and converts the answer into the units you need.

In US units the three measurements are in inches, so the product is in cubic inches, and dividing by 231 turns it into US gallons, because a US gallon is defined as exactly 231 cubic inches (NIST's conversion tables give it as 3.785412 litres). The defaults are close to the inside of a 60-inch alcove tub: 51 × 23.5 × 11.5 in is 13,782.75 in³, which is 59.7 gallons as a box. Multiply by 0.8 and divide by 231 and you get 47.7, shown as 48 gal, or 181 L. On the metric side the inputs are millimetres and the product is divided by 1,000,000 to give litres. The metric defaults of 1,300 × 600 × 290 mm make a 226.2-litre box, and 0.8 of that is 181 L, which the result card also lists as 39.8 UK gallons and 47.8 US gallons.

Measure the inside, not the catalogue size

This is where most wrong answers start. A tub sold as 60 × 32 in measures that across the outside of the rim and apron. The water sits in a much smaller basin. Kohler's Bellwether K-876 specification sheet is a useful check because it prints both: the tub is 60 × 32 in overall, but the basin is 51.25 × 23.3 in across the top, the water depth to the overflow is 11.3 in, and the rated capacity is 44 gallons (166.6 L). Type the catalogue size into the calculator with a 12 in depth and it returns 80 gallons. That is 1.8 times the real figure, and anyone sizing a water heater from it would buy far more tank than the bath needs.

Run the tape across the top of the basin at the height the water will reach, down the centre for the length and across the middle for the width. Take the depth from the lowest point of the base up to the bottom lip of the overflow opening, since nothing above the overflow stays in the tub. If you plan to run shallower baths, use that depth instead. Where the ends curve, measure the full length at the water line and let the fill factor allow for the rounding.

Choosing a fill factor

The same Kohler tub shows how much the factor matters. Its basin as a box holds 58.5 gallons. At the default 0.8 the calculator gives 46.8 gallons, about 6% above the maker's 44. Drop the factor to 0.75 and it returns 43.9 gallons, or 166 L against the stated 166.6 L. That tub has a sloped back rest and a rounded base, so 0.75 is a reasonable setting for a standard tub with a sloped back.

Nearly straight-sided soaking tubs sit closer to 0.85. Roll-top, clawfoot and slipper tubs with deep curves at both ends are nearer 0.6, and heavily moulded spa or whirlpool shells can go down to 0.5. When you are unsure, erring high is the safer mistake for hot-water planning, because a heater sized for a slightly larger bath still fills the real one. If the tub is already installed, the exact answer costs one bucket: fill it with a container of known size and count.

US gallons, UK gallons and litres

There are two gallons in common use and they are not interchangeable. The US gallon is 3.785 litres. The imperial gallon, still seen in the UK and Canada, is 4.54609 litres by definition in Schedule 1 of the UK Weights and Measures Act 1985, about 20% larger. So the metric default of 181 litres reads as 47.8 US gallons but only 39.8 UK gallons. A capacity quoted in gallons on a British product sheet is almost certainly imperial, while North American sheets, Canadian editions included, normally quote US gallons: Kohler's US-CA sheet gives 44 gallons as 166.6 L, which is 3.786 litres to the gallon. For cubic feet, one cubic foot is 1,728 in³, or 7.48 US gallons, so a 48-gallon bath is about 6.4 ft³ of water.

Hot water and heating energy

The heating energy line is the energy needed to take the whole bath from a 15 °C (59 °F) cold supply to 40 °C (104 °F), using 4.186 kJ per kilogram per degree and treating a litre of water as a kilogram. For the US defaults that is 5.25 kWh, or about 17,900 BTU (1 kWh is 3,412 BTU). It makes no difference whether the heater supplies all of that or you mix hot with cold; the water still has to gain the same heat. The figure has no allowance for heater efficiency, standing losses or water cooling in the pipes, so the energy you actually pay for is higher. Multiply the kWh by your own tariff and divide by your heater's efficiency for a running cost.

Hot water demand is a smaller number than the bath volume, because the bath is a mix. With a 120 °F tank and 59 °F cold water, the hot share of a 104 °F bath is (104 − 59) ÷ (120 − 59), which is 74%, so the 48-gallon default draws about 35 gallons from the heater. Kohler's sheet asks for a hot supply of at least 70% of the bath's capacity, which matches that arithmetic. A UK cylinder stored at 60 °C with 15 °C cold water needs a smaller share, 25 ÷ 45 or 56%, so the 181-litre metric default takes about 100 litres of hot water. Compare that hot share with your heater's first-hour rating, or with a cylinder's usable volume, rather than with the tank size printed on the label.

Displacement and weight

The result is the water in an empty tub. A person getting in pushes the level up by about their own body weight in litres, since the body is close to the density of water, so a 75 kg adult lying back with only their head out displaces roughly 70 litres (about 18 US gallons). That is why a bath is run well below the overflow: fill it to the overflow, climb in, and that water goes straight down the overflow pipe.

Weight matters for the floor. Water is 1 kg per litre, or 8.34 lb per US gallon, so the default bath holds about 181 kg (roughly 400 lb) of water before anyone climbs in. The Kohler tub is cast iron and weighs 327 lb (148 kg) empty; filled to its rated 44 gallons it adds 367 lb (166.6 kg) of water, and with a 165 lb (75 kg) bather a small patch of floor is carrying about 860 lb, or 390 kg. Large soaking tubs and cast-iron baths on timber joists are worth checking before they go in.

What this tool does not do

It estimates water volume from a box and a factor. It does not replace the manufacturer's stated capacity, which is the better number whenever a spec sheet exists. It does not size a water heater or cylinder, design pipework or drainage, or check that a floor can carry a full tub. For those, a licensed plumber, a Gas Safe registered engineer for gas appliances in the UK, or a chartered structural engineer for floor loading is the right person to confirm the figures.

Getting the most from the result

Every input can be changed, including the fill factor, and the result updates as you type. Switch between US units and metric at the top of the calculator and the inputs change to inches or millimetres. The two sets of defaults describe the same tub, so both land on 181 L, but numbers you have typed need entering again in the new units. Everything runs in your browser; nothing you enter is sent anywhere. With a volume you trust, the next useful number is usually the hot-water share above, since that is the one a heater has to meet.