1 cubic feet to gallons

One cubic foot is 7.48051948051948 US gallons

Where the factor comes from

A cubic foot is a box 12 inches on each side, so it holds 12 x 12 x 12 = 1,728 cubic inches. The US liquid gallon is defined as exactly 231 cubic inches. Divide one by the other and the factor appears: 1,728 / 231 = 7.48051948051948 US gallons. The long decimal is not rounding noise, it is the honest ratio between two units that were fixed centuries apart.

Going back is a multiplication by 0.1336805556, which is 231 / 1,728. Five gallons of deck stain therefore take 5 x 0.1336805556 = 0.6684 cubic feet of shelf space, and a 55 gallon drum takes 55 / 7.48051948051948 = 7.3524 cubic feet.

NIST Special Publication 811, Appendix B carries the inch, the foot and the gallon on the same table, which is how the 231 cubic inch definition travels into metric work. The BIPM SI Brochure, 9th edition, fixes the inch at exactly 25.4 mm, so one cubic foot is 0.3048 x 0.3048 x 0.3048 = 0.028316846592 cubic meters, or 28.316846592 liters, with no rounding anywhere.

Read the 27 row before ordering gravel or topsoil: a cubic yard is 27 cubic feet, so it is 201.97 gallons of volume. Nobody buys concrete by the gallon, but the number explains why a yard of anything is heavier and bulkier than it sounds.

The same space in liters, quarts and fluid ounces

The metric side is exact. One cubic foot is 0.028316846592 cubic meters, and a cubic meter is 1,000 liters, so one cubic foot is 28.316846592 liters. That is the number printed on imported tank labels and on the side of European heating equipment.

Inside the US system the factor splits cleanly. One cubic foot holds 7.48051948051948 gallons, which is 7.48051948051948 x 4 = 29.9221 quarts, 59.8442 pints, 119.6883 cups and 957.5065 fluid ounces. A 32 ounce bottle is 0.25 gallon, so one cubic foot is 29.92 of those bottles, close enough to call it 30 bottles of water.

Quarts and ounces are where a kitchen scale meets a tape measure. A 12 quart stock pot is 12 / 29.9221 = 0.4011 cubic feet, and a 1 cubic foot cooler holds 957.5 fluid ounces, which is 7.48 gallons of ice, drinks and meltwater together.

A 300 liter tank is 0.3 cubic meters, which is 10.59 cubic feet and 79.25 US gallons, a size that shows up on imported rain tanks. The tablespoon row is there to show the scale: the factor chain is exact at every step, and 957.5065 / 16 = 59.844 fl oz per pint, then 59.844 x 32 = 1,915.0 tablespoons. Rounding early at any step throws the last digit off.

Container sizes written in cubic feet

Most people meet this conversion through a container that is sold in one unit and used in the other. The table converts the nominal size, so treat the numbers as capacity before walls, ribs, gravel or foam take their share.

Check the 5 gallon bucket against its own label and the arithmetic closes. Five gallons is 5 x 231 = 1,155 cubic inches, which is 0.6684 cubic feet. The bucket is molded a little larger than its fill line so a full load does not spill at the rim, which is why a bucket measures closer to 6 gallons to the brim.

The 55 gallon barrel is the useful one for planning. It holds 7.3524 cubic feet, so a barrel that looks huge beside a downspout is one third of a cubic yard, and a serious storm fills it in minutes. A 275 gallon tote is 36.76 cubic feet, which is 1.36 cubic yards.

Measure the tank in inches, then divide by 1,728

Tanks are measured with a tape in inches and sold in gallons, so the conversion runs through cubic inches. The recipe has three steps and no shortcuts.

A cube 12 by 12 by 12 inches is 1,728 cubic inches, which is exactly 1 cubic foot and 7.4805 gallons. A 20 gallon long aquarium at 30 by 12 by 12 inches is 4,320 cubic inches, which is 2.5 cubic feet and 18.70 gallons, so the label runs about 7 percent optimistic once glass, gravel and trim are counted.

A 48 by 24 by 18 inch stock tank is 20,736 cubic inches. Divide by 1,728 and it is 12 cubic feet, then multiply by 7.48051948051948 for 89.77 gallons. A 24 by 12 by 18 inch tank is 5,184 cubic inches, 3 cubic feet and 22.44 gallons, which is a generous 20 gallon tank in real life.

Measure inside the tank, not outside. A quarter inch of glass on each side of a 12 inch cube cuts the volume by 12 percent, which is 0.9 gallons, and on a 100 gallon tank the same error costs more than 6 gallons of capacity. Depth should be the water line, not the rim.

One more check before buying a heater or a filter: nominal aquarium gallons describe the empty glass box. Gravel at 15 pounds and a piece of driftwood displace real water, so a 20 gallon tank usually holds 18 to 19 gallons, and dosing instructions follow actual water volume.

Round tanks and pipes need the circle first

A cylinder adds one step: area of the circle, then height. Area in square feet is 3.14159 x radius squared, with the radius in feet. Volume is area x height in feet, and gallons are volume x 7.48051948051948.

A 24 inch diameter tank 48 inches tall has a radius of 1 foot, so the area is 3.14159 square feet and the volume is 3.14159 x 4 = 12.5664 cubic feet, or 94.00 gallons. A 12 inch diameter pipe needs 15.28 inches of length to hold one cubic foot: the area is 0.7854 square feet, and 1 / 0.7854 = 1.2732 feet, which is 15.28 inches.

Pipes are quoted by the foot, so the figure to memorize is gallons per foot. A 4 inch pipe carries 3.14159 x (1/6) x (1/6) = 0.0873 square feet of cross section, which is 0.6528 gallons for every foot of run. A 2 inch pipe carries a quarter of that, 0.1632 gallons per foot.

The 24 inch row is the one that catches people: doubling the diameter quadruples the volume per foot, because area grows with the square of the radius. Two 12 inch tanks do not hold what one 24 inch tank holds.

Rain on a roof and water on a garden

Watering and harvesting turn this conversion into a planning tool. One inch of water over one square foot is 1 / 12 = 0.08333 cubic feet, which is 0.6234 gallons. Over 100 square feet the same inch is 62.34 gallons, and over a 1,000 square foot roof it is 623.4 gallons.

That last number is the ceiling for a rain barrel. A 55 gallon barrel holds 7.3524 cubic feet, so it captures the first 0.088 inches of rain off 1,000 square feet and overflows on everything after. A 1,000 square foot roof in a 1 inch storm sheds 623 gallons, which is more than eleven barrels.

Garden watering runs the same arithmetic in reverse. A vegetable plot 20 feet by 30 feet is 600 square feet, and half an inch of water over it is 600 x 0.5 / 12 = 25 cubic feet, or 187.0 gallons. That is why hose end timers are sold in gallons and why a drip line beats a sprinkler on a hot afternoon.

The EPA WaterSense outdoor page puts residential outdoor use at nearly 8 billion gallons a day nationwide, mostly landscape irrigation, which is the reason these per square foot figures show up on utility bill inserts. Catchment never reaches the theoretical number: gutters spill, first flush rinses the roof, and a barrel is full long before the storm ends.

What a cubic foot of water weighs

Volume becomes weight at 8.33 pounds per gallon, so one cubic foot of water is 7.48051948051948 x 8.33 = 62.31 pounds. The USGS Water Science School lists 62.416 pounds per cubic foot at 32 F and 8.33 pounds per gallon, so 62.4 is the working figure for typical tap water and 62.3 is what the per gallon rate gives.

That weight decides decks, stands and vehicle loads. A 55 gallon barrel of water is 55 x 8.33 = 458 pounds plus the barrel, which is not a two person lift and not a shelf bracket task. A 50 gallon water heater holds 416.5 pounds of water before the steel tank and the anode rod.

Depth matters more than area for a floor. One foot of water exerts 62.4 pounds on every square foot under it, so a 4 foot deep hot tub puts 249.6 pounds per square foot on the deck, and a 1 inch spill puts 5.2 pounds per square foot, which most floors ignore.

One cubic yard of water weighs 1,684.8 pounds, which is why a yard of wet gravel, at roughly 2,700 pounds, is a different problem for the same wheelbarrow. The conversion changes the unit, never the substance.

Dry gallons, imperial gallons and labels that round

Not every gallon is the liquid US gallon. The US dry gallon is 268.8025 cubic inches, so one cubic foot is 1,728 / 268.8025 = 6.4285 dry gallons. That unit survives on bags of potting soil, grain and produce, and the liquid gallon used for water is 14 percent smaller than it.

The imperial gallon is larger, not smaller: 4.54609 liters against 3.785411784, a ratio of 1.20095. One cubic foot therefore holds 6.2288 imperial gallons. A UK label that says 10 gallons is 12.01 US gallons, and a Canadian recipe written before metrication carries the same gap.

Rounding is the last snag. Using 7.5 instead of 7.48051948051948 overstates the volume by 0.26 percent, which is nothing in a bucket and 2.6 gallons in a 1,000 gallon tank. Using 7.48 understates it by 0.007 percent, which is why the short form is safe for a fish tank and the long form belongs on a shipping ticket.

A 1 cubic foot bag of potting soil is sold as dry volume, so it fills 6.43 dry gallons of space, and a 7.5 gallon nursery pot is 1.0026 cubic feet by the liquid gallon and 1.1667 by the dry one. Buy soil by the cubic foot and the ambiguity disappears.