3 cubic feet to gallons

Three cubic feet is 22.4416 US gallons

5,184 cubic inches, 231 per gallon

Three cubic feet is 22.44155844155844 US gallons. The chain is short. A cubic foot is 1,728 cubic inches, so 3 cubic feet is 5,184 cubic inches, and a US gallon is 231 cubic inches. 5,184 / 231 = 22.44155844155844.

Multiply instead and the same factor appears: 3 x 7.48051948051948 = 22.44155844155844. NIST Handbook 44, Appendix C, 2024 edition prints the US gallon at exactly 231 cubic inches, with 3.785 liters and 0.833 British gallons on the same line. NIST Special Publication 811, Appendix B.8 gives the US gallon as 3.785 412 E-03 cubic meters and the imperial gallon as 4.546 09 E-03 cubic meters.

The inch is the anchor. The BIPM SI Brochure, 9th edition, 2019 fixes it at exactly 25.4 millimeters, so a cubic foot is 28.316846592 liters and 3 cubic feet is 84.950539776 liters. Divide that by 4.54609 and the imperial answer is 18.6865 gallons. A US dry gallon is 268.8025 cubic inches, so 5,184 / 268.8025 = 19.2858 dry gallons.

Three gallons, three numbers. Only the first one is the gallon printed on a milk jug or poured at a pump in the United States.

Read the table backwards and it answers the usual question. A 5 gallon bucket holds 0.6684 cubic feet, and a 55 gallon drum holds 7.3524 cubic feet, which is a bit less than two and a half times three cubic feet.

A box 36 by 12 by 12 inches is exactly 3 cubic feet

Measure the inside faces

Most volume jobs start with a tape measure. A box 36 inches long, 12 inches wide and 12 inches deep holds 36 x 12 x 12 = 5,184 cubic inches, and 5,184 / 1,728 = 3 cubic feet, which is 22.44 gallons. A box 24 by 18 by 12 inches lands on the same number, and so does one 18 by 18 by 16 inches.

That is the size of a planter, a cooler, a filter box, a battery box, a storage tote or a section of duct. The number that matters is length times width times height, with all three in the same unit before dividing by 1,728.

Measure the inside, not the outside. A tank built from half inch material loses an inch on each dimension, so 35 by 11 by 11 inches is 4,235 cubic inches, or 2.45 cubic feet. That is 18 percent less than the outside numbers suggest, and on a tank that is meant to hold 22 gallons it is four gallons of missing capacity.

Depth is the dimension people guess. A planter 36 by 12 inches needs 12 inches of soil to reach 3 cubic feet, and if the soil settles to 10 inches the volume drops to 2.5 cubic feet, or 18.7 gallons. Fill to the line you measured against, then top up after the first watering.

Buckets and drums: what 3 cubic feet looks like

Four and a half buckets, or 41 percent of a drum

The five gallon bucket is the unit that makes 3 cubic feet legible. One bucket holds 5 / 7.48051948051948 = 0.6684 cubic feet, so 3 cubic feet fills 3 / 0.6684 = 4.49 buckets. Four full buckets plus half of a fifth is the honest description of the volume.

A 55 gallon drum holds 55 / 7.48051948051948 = 7.3524 cubic feet, so 3 cubic feet fills it to 41 percent, a little under half. Two drums hold 110 gallons, which is 14.7 cubic feet, and that is close to the capacity of a small utility trailer load of loose material.

These comparisons matter when a job mixes units. A sump pump rated in gallons per minute, a tank sized in cubic feet and a bucket used to bail it out are three different scales, and the conversion is the only thing that lets them be compared.

The trash can row is worth a second look. A 32 gallon can holds more than 3 cubic feet, so a 3 cubic foot bag of soil does not fill it, and the bag label that says 3 cubic feet is describing about 22 gallons of loose material.

Worked example: 3 cubic feet of concrete

0.111 cubic yard, five bags

Three cubic feet is a small pour, well under the minimum most ready mix plants will deliver. Divide by 27 to reach cubic yards: 3 / 27 = 0.1111 cubic yard. Bags are the practical route. An 80 pound bag yields about 0.6 cubic feet, so 3 / 0.6 = 5 bags exactly, with nothing left over.

A 60 pound bag yields about 0.45 cubic feet, so 3 / 0.45 = 6.667, which means 7 bags. A 50 pound bag yields 0.375 cubic feet, so 3 / 0.375 = 8 bags. The bag count moves faster than the price per bag, so compare the cost per cubic foot rather than the ticket on one sack.

Post holes are the classic three cubic foot job. An 8 inch hole 30 inches deep is a cylinder with a radius of 4 inches and a height of 30: 3.1416 x 16 x 30 = 1,508 cubic inches, or 0.8727 cubic feet. Three cubic feet fills 3.44 holes, so three holes with enough left for a fourth if the digging runs clean.

Buy the whole bag even when the sum lands on a whole number, because a bag that has been open for a month is a bag to throw away. Mix to the water figure on the bag and not by eye, since extra water is the usual reason a small pour crumbles at the edge.

Worked example: 3 cubic feet of soil and of water

36 square feet at one inch, and 187 pounds

Spread over ground, 3 cubic feet covers 3 x 12 = 36 square feet at one inch, 18 at two inches, 12 at three, 9 at four and 6 at six inches. A raised bed 4 by 8 feet holds 32 square feet, so a 3 cubic foot bag fills it 1.125 inches deep, and six inches of fill in that bed needs 16 cubic feet, which is 5.33 bags, so 6.

Water is the easy weight. At 62.4 pounds per cubic foot, 3 cubic feet weighs 187.2 pounds, the figure the USGS Water Science School prints for cold water. Through the gallon route the answer is 22.4416 x 8.33 = 186.9 pounds, and the two pound gap is rounding in the density rather than a different result.

Soil is heavier. A moist potting mix runs about 75 to 100 pounds per cubic foot, so 3 cubic feet of it is 225 to 300 pounds, which is the number that decides whether a shelf, a balcony or a truck bed is up to the load. Dry sand is heavier again at roughly 100 pounds per cubic foot.

The bag column is the 32 square foot bed multiplied by the depth in feet and divided by 3, then rounded up at the store. At 6 inches that is 16 cubic feet and 5.33 bags, so 6. At 8 inches it is 21.33 cubic feet and 7.11 bags, so 8.

Reverse check: gallons back to cubic feet on a delivery ticket

Reading a bill in gallons

Suppliers often bill liquid in gallons and bulk material in cubic feet or cubic yards. To read a delivery ticket, run the factor backwards. One US gallon is 0.133680556 cubic feet, so a ticket showing 5 gallons is 0.668403 cubic feet, and a ticket showing 22.4416 gallons is exactly 3 cubic feet.

The large containers on a job site are worth memorising. A 55 gallon drum holds 7.352431 cubic feet. A 275 gallon tote holds 36.762153 cubic feet. A 1,000 gallon tank holds 133.680556 cubic feet, which is 4.951 cubic yards. Multiply the gallon figure by 0.133680556 and the answer is in cubic feet, with no rounding needed until the last step.

Imperial gallons do not belong in that table. One imperial gallon is 0.160544 cubic feet, about 20 percent larger, so a UK ticket of 22.44 gallons describes 3.6 cubic feet, not 3.0. Check which gallon the invoice uses before you compare it to a tank volume.

Price per gallon against price per cubic foot

The same jug, two rates

A price per gallon and a price per cubic foot describe the same liquid. Multiply by 7.48051948051948 to move from one to the other. At $0.80 per gallon, a cubic foot costs 0.80 x 7.48051948051948 = $5.98, and 3 cubic feet costs 22.4416 x 0.80 = $17.95.

Bulk changes the comparison in the other direction. Soil or gravel at $42 per cubic yard is 42 / 27 = $1.56 per cubic foot, so 3 cubic feet is $4.67, about a quarter of the jug price. The catch is the minimum order: most yards sell by the half yard, 13.5 cubic feet, which is four and a half times this volume and a lot of leftover material.

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