Two gallons is 0.267361 cubic feet

Two US gallons take up 0.267361 cubic feet. A gallon is 231 cubic inches, so two gallons is 462 cubic inches, and a cubic foot is 1,728 cubic inches. Divide 462 by 1,728 and the digits come out 0.267361111.

The trip back is one multiplication. A cubic foot holds 7.480519 US gallons, so 0.267361 x 7.480519 returns 2.000000. A spec sheet that says 0.27 cubic feet and a jug label that says 2 gallons describe the same space, and the 0.002639 cubic feet between the rounded and exact answers is 4.56 cubic inches, a little under a third of a cup.

The pairing turns up on product pages more often than the arithmetic suggests. A two gallon wet/dry vacuum or parts washer is built in inches and specified in cubic feet, because the mold was cut to inch dimensions.

Every row is the gallon count times 0.133681, so ten gallons is five times two gallons: 5 x 0.267361 = 1.336806 cubic feet.

231 cubic inches is the whole definition

There is no measurement in this conversion. NIST Handbook 44, Appendix C prints the US gallon as 231 cubic inches (exactly), which is 3.785 liters. The 231 comes from the English wine gallon, and the United States kept it when the imperial gallon was rebuilt in 1824.

The cubic foot is easier. A foot is 12 inches, so a cubic foot is 1,728 cubic inches. Put the two definitions side by side and the factor falls out: 1,728 / 231 = 7.480519480519. That is the fraction 576/77, so the six digits 480519 repeat forever.

Rounding the factor to 7.5 looks harmless and is not. One hundred cubic feet at 7.5 gallons each gives 750 gallons, while the exact figure is 748.05. The shortcut runs 1.95 gallons high, and on a delivered load of heating oil that is a visible line on the invoice.

NIST SP 811, Appendix B.8 gives the gallon at 3.785 412 E-03 cubic meters and the cubic foot at 2.831 685 E-02, and dividing one by the other confirms 7.480519.

A two gallon aquarium, measured from the inside

Aquarium volume is quoted in gallons and the tank is built in inches. A small tank with inside dimensions of 12 by 6 by 6.5 inches holds 12 x 6 x 6.5 = 468 cubic inches. Divide by 231 and the tank is 2.026 gallons, which is the number on the box. Divide by 1,728 instead and the same space is 0.270833 cubic feet.

Substrate eats into that. Gravel runs near 100 pounds per cubic foot, so a 5 pound bag takes up about 0.05 cubic feet. Subtract it: 0.270833 - 0.05 = 0.220833 cubic feet of water. Multiply by 7.480519 and the tank holds 1.652 gallons once the gravel is in, which is the figure to use when dosing a water treatment.

Filter flow uses the same numbers in the other direction. A canister rated at 100 gallons per hour moves 100 / 7.480519 = 13.37 cubic feet an hour, a turnover of about 50 times an hour on 0.267 cubic feet. Weight is the last check: water at 8.33 pounds per gallon by the USGS water facts page makes 2 gallons 16.66 pounds, before gravel and glass.

Jugs, sprayers and RV tanks: gallons on the label, cubic feet in the spec

A two gallon garden sprayer holds 7.570823568 liters, from 2 x 3.785411784. If the label calls for 2 tablespoons of concentrate per gallon, a full tank takes 4 tablespoons, which is 59.15 mL. That arithmetic belongs in liters and milliliters, where the concentrate label already lives.

Cubic feet enters when the tank has to fit somewhere. One cubic foot is 7.480519 gallons, so a 2 gallon sprayer is 0.267361 cubic feet and a 1 cubic foot compartment holds it 3.74 times over. A 2 gallon RV fresh tank is 462 cubic inches, a box 12 by 7 by 5.5 inches.

A five gallon bucket is the yardstick most garages own. It holds 1,155 cubic inches, or 0.668403 cubic feet, so 2 gallons is exactly two fifths of it: 0.267361 / 0.668403 = 0.400000.

The drum row catches people out. A 55 gallon drum is 7.35 cubic feet, not 7.5, and four of them on a pallet carry 29.4 cubic feet of liquid before packaging.

Propane: 2 gallons fills a bit over 40 percent of a 20 lb cylinder

Grill cylinders are sold by the pound of propane they hold, not by volume. A standard 20 pound cylinder is built to hold 47.6 pounds of water, which is 47.6 / 8.33 = 5.71 gallons of shell volume, or 0.764 cubic feet. The 80 percent fill rule leaves 4.57 gallons of propane, and cylinder makers list about 4.7 gallons at 60 F.

Against either figure, 2 gallons is a bit over 40 percent of a full tank: 2 / 4.7 = 0.4255, and 2 / 4.57 = 0.4377. In cubic feet the liquid is 0.267361 inside a 0.764 cubic foot shell, so it fills roughly a third of the steel.

The energy is where the numbers get large. The EIA energy units page lists propane at 91,452 Btu per gallon, so 2 gallons is 182,904 Btu. A grill rated 30,000 Btu per hour runs 182,904 / 30,000 = 6.1 hours on that, and a 10,000 Btu camp stove runs 18.3 hours.

The cylinder's water capacity is the only volume figure stamped on the collar, and 47.6 pounds of water only becomes gallons after the 8.33 pounds per gallon step.

Concrete and mortar: 0.267 cubic feet is 59 percent of a 60 lb bag

A 60 pound bag of concrete mix lists a yield of about 0.45 cubic feet on the bag, which is 3.37 gallons. Two gallons is 0.267361 cubic feet, so one bucket of that size uses 0.267361 / 0.45 = 0.594 of a bag, or 35.6 pounds of dry mix. Two bags for a two gallon pour leave most of the second bag in the garage.

Mixed concrete weighs about 150 pounds per cubic foot, so 0.267361 x 150 = 40.1 pounds in the bucket. That is the number to quote when someone asks whether a two gallon pour is a one person lift.

Spread it thin and the area is small. A 2 gallon batch is 462 cubic inches, and poured 4 inches thick it covers 462 / 4 = 115.5 square inches, which is 0.80 square feet. That is a patch around a fence post, not a step.

Moving cartons: 2 gallons takes a fifth of a 16 by 12 by 12 box

Carton sizes are printed in inches and movers price storage in cubic feet. A 16 by 12 by 12 inch carton is 2,304 cubic inches, which is 1.333333 cubic feet or 9.974 gallons. A 2 gallon item fills 0.267361 / 1.333333 = 0.2005, just over a fifth of the box.

The empty space is what gets billed. Subtract the item and 1.333333 - 0.267361 = 1.066 cubic feet of void remains, which is 7.97 gallons of packing peanuts.

The 12 inch cube lands exactly on 1 cubic foot and 7.4805 gallons, and every carton with whole inch sides is a whole number of cubic inches. The last row also answers a common question: 30 two gallon jugs need 30 x 0.267361 = 8.02 cubic feet, so a 6 cubic foot carton will not take them.

Imperial and dry gallons in the same cubic foot

The imperial gallon is 4.54609 liters, defined as the volume of 10 pounds of water at 62 F, and that is 277.419433 cubic inches. Two imperial gallons are 554.838866 cubic inches, or 0.321087 cubic feet. Against the US answer of 0.267361 the imperial figure is 20.1 percent larger: 0.321087 / 0.267361 = 1.200950.

The US dry gallon is a third unit, used for grain and produce, and it is 268.8025 cubic inches, one eighth of a 2,150.42 cubic inch bushel. Two dry gallons are 537.605 cubic inches, or 0.311114 cubic feet, which sits between the liquid and imperial answers.

This is why a tank sold in the UK needs a second look. A two gallon tank from a British supplier holds 0.321 cubic feet, so a filter rated for 0.27 cubic feet is undersized for it by 20 percent.

The liter column settles the question. Both the dry and the imperial figure sit above 8 liters for two gallons while the US liquid figure is 7.57, so the metric equivalent shows which gallon a vendor meant.