Gallons to Cubic Feet Converter

The conversion factor: one cubic foot is 7.48 gallons

The two units come from different systems that never coordinated. A cubic foot is a box one foot on each side; a US gallon is legally defined as exactly 231 cubic inches. Put the definitions side by side and the factor falls out: a cubic foot is 12 x 12 x 12 = 1,728 cubic inches, and 1,728 ÷ 231 = 7.48052 gallons. That awkward decimal is not rounding error, it is the honest ratio between two units defined centuries apart.

The exact figure is 7.48052 US gallons per cubic foot, and the National Institute of Standards and Technology publishes these relationships in its SI conversion tables. For everyday work, 7.48 rounds off cleanly and stays accurate to about one part in ten thousand: a 100 gallon claim computed with 7.48 is off by about 0.007 gallons.

Going the other direction, one US gallon is 1 ÷ 7.48052 = 0.13368 cubic feet, so 7.5 gallons is a touch over 1 cubic foot. Keep one number in your head, 7.48 gallons per cubic foot, and both directions are a single multiply or divide away. The converter above does either direction, plus the imperial gallon comparison this page covers below.

Converting gallons to cubic feet, worked both ways

Gallons to cubic feet: divide by 7.48052. One hundred gallons is 100 ÷ 7.48052 = 13.368 cubic feet. Fifty gallons is 50 ÷ 7.48052 = 6.684 cubic feet.

Cubic feet to gallons: multiply by 7.48052. A 2 cubic foot storage tote holds 2 x 7.48052 = 14.96 gallons, a whisker under 15. A 0.5 cubic foot box holds 0.5 x 7.48052 = 3.74 gallons.

The imperial column exists because UK recipes, Canadian product labels and older engineering documents use the imperial gallon, which is 1.2 US gallons. It is the classic gotcha: convert gallons to cubic feet without checking which gallon, and the answer is wrong by a fifth.

Finding tank volume in gallons, step by step

Most gallon to cubic foot questions are mostly tank questions. The recipe is always the same three steps.

Take a rectangular tank 24 inches long, 12 inches wide and 18 inches deep. Step one, volume in cubic inches: 24 x 12 x 18 = 5,184 cubic inches. Step two, convert to cubic feet by dividing by 1,728: 5,184 ÷ 1,728 = 3 cubic feet. Step three, convert to gallons: 3 x 7.48052 = 22.44 US gallons. That is a standard 20 gallon long aquarium, more or less, once glass thickness and trim take their share.

For cylinders, like a well pressure tank, the area comes from the circle first: area = 3.1416 x radius squared, then volume = area x height. An 18 inch diameter, 48 inch tall tank has a radius of 0.75 feet, area 3.1416 x 0.75 x 0.75 = 1.767 square feet, volume 1.767 x 4 = 7.068 cubic feet, and 7.068 x 7.48052 = 52.88 gallons, about 53.

Pools scale the same recipe up. A 16 x 32 foot pool with an average depth of 4.5 feet holds 16 x 32 x 4.5 = 2,304 cubic feet, which is 2,304 x 7.48052 = 17,235 gallons. When the seller quotes pool volume, that is the arithmetic underneath, and average depth is the estimate that makes or breaks it.

US gallons versus imperial gallons

Both countries call their unit a gallon and the two disagree by 20 percent, which has stranded enough recipes and fuel calculations to deserve its own section.

The US gallon is 231 cubic inches, about 3.785 liters. The imperial gallon, defined in Britain in 1824 and still used for some purposes there and historically in Canada, is about 277.42 cubic inches, about 4.546 liters. The ratio is 4.546 ÷ 3.785 = 1.20095, so an imperial gallon is 1.2 US gallons, and one cubic foot holds 7.48052 ÷ 1.20095 = 6.2288 imperial gallons.

So 100 US gallons is 100 ÷ 1.20095 = 83.27 imperial gallons, and 100 imperial gallons is 100 x 1.20095 = 120.1 US gallons. The USGS Water Science School keeps these water measurement basics handy if you want the gallons per cubic foot figures in a teaching context.

Practical rule: anything from the US, use 7.48. Anything British from before metrication, cookbooks aside, use 6.23. When a number looks 20 percent off from your expectation, the gallon mismatch is the first suspect.

Cubic feet to gallons for common containers

Working the factor backward tells you what everyday containers hold:

Two of these rows are worth a second look. The rain barrel: 55 ÷ 7.48052 = 7.353 cubic feet, which is why a barrel that looks huge next to a downspout empties in one serious storm; a 1,000 square foot roof sheds 6 inches of rain into 500 cubic feet, about 3,740 gallons. And the aquarium: nominal gallon sizes describe the empty glass box, so a 20 gallon tank with gravel, filter and decor displaces real water and holds a couple of gallons less than the label says.

Where these conversions show up in real life

A handful of recurring situations, with the numbers already worked:

Rain harvesting. One inch of rain on 1,000 square feet of roof is 1,000 x (1 ÷ 12) = 83.33 cubic feet of water, and 83.33 x 7.48052 = 623 gallons. Gutters and barrels never capture all of it, but that is the ceiling, and it explains why serious rain catchment means tanks, not barrels.

Hot tubs and stock tanks. A common 8 foot stock tank runs about 2 feet deep: 8 x 2 x 2 = 32 cubic feet, roughly 239 gallons. Knowing the gallons is what lets you dose dechlorinator or stock tank cleaner by the label, which speaks in gallons.

Moving and shipping. Furniture boxes quote cubic feet; tanks and liquid limits quote gallons. A cargo area of 40 cubic feet could hold 40 x 7.48052 = 299 gallons of volume if water were the cargo, which is a useless fact for furniture but a useful sanity check for water: a standard bathtub holds around 42 gallons, which is 42 ÷ 7.48052 = 5.6 cubic feet, and most of a shipping bay dwarfs it.

Concrete and landscaping also speak cubic feet and gallons in the same breath; a cubic yard of concrete is 27 cubic feet, and at 7.48052 that is 27 x 7.48052 = 201.97 gallons of volume, a number that surprises people who picture a yard as small.

Precision, weight and when it matters

Volume and weight travel together in water work, so two reference numbers round out the picture. A US gallon of water weighs about 8.34 pounds, so a cubic foot of water weighs 7.48052 x 8.34 = 62.39 pounds. Tank stands, floor loads and roof catchment all care about that second number: a 55 gallon barrel of water is nearly 460 pounds, not a two person lift.

How much precision do you need? For a fish tank or a tote, 7.48 is plenty; the error is smaller than the slop in your measurements. For chemical dosing, fuel inventories or anything billed by the gallon, use the full 7.48052 and state which gallon you mean. The US gallon is 3.785 liters, the imperial gallon 4.546, and a contract or a recipe that just says gallon is ambiguous by design of history, not by your mistake.

Temperature shifts water volume slightly, about 0.2 percent between cold tap and hot bath, which matters to metrologists and nobody dosing an aquarium. And remember the conversion converts space, not substance: gasoline, milk and honey all obey the same 7.48 per cubic foot, but a cubic foot of gasoline weighs about 42 pounds against water's 62.4, so never mix volume conversions with weight conversions mid calculation.