5 cubic feet to gallons

Five cubic feet is 37.4026 US gallons

8,640 cubic inches, 231 to the gallon

Five cubic feet is 37.4025974025974 US gallons. The chain is short. A cubic foot is 1,728 cubic inches, so five of them is 8,640, and a US gallon is 231 cubic inches. 8,640 / 231 = 37.4025974025974.

Go the other way and the same factor shows up: 5 x 7.48051948051948 = 37.4025974025974. 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 lists 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 what the whole thing hangs on. The BIPM SI Brochure, 9th edition, 2019 fixes it at exactly 25.4 millimeters, so a cubic foot is 28.316846592 liters and five cubic feet is 141.58423296 liters. Divide that by 4.54609 and the imperial answer is 31.1442 gallons. A US dry gallon is 268.8025 cubic inches, so 8,640 / 268.8025 = 32.1426 dry gallons.

Three gallons carry the same name. Only the first is the one printed on a milk jug or sold at a pump in the United States, and it is the only one that belongs in a tank size.

Smaller units are worth having too. Five cubic feet is 149.61 quarts, 299.22 pints and 4,788 fluid ounces. A case of 24 twelve ounce bottles is 2.25 gallons, so five cubic feet is 16.6 cases, and a 12 ounce can is 0.0938 gallons, which makes the same volume 399 cans.

Read the table backwards and a tank question answers itself. A 50 gallon rain barrel is 6.684 cubic feet, so five cubic feet leaves it about three quarters full and short by 12.6 gallons.

A 40 gallon breeder tank is 5.25 cubic feet

13.33 inches of water over 36 by 18

A 40 gallon breeder is 36 inches long and 18 inches wide. Filled to 14 inches it holds 36 x 18 x 14 = 9,072 cubic inches, which is 5.25 cubic feet and 39.28 gallons. Put exactly five cubic feet into that footprint and the water line lands at 13.33 inches, since 8,640 / (36 x 18) = 13.333.

That difference is why tank size and tank volume are two different questions. Tanks are sold by a nominal figure that assumes a fill to the rim, and no one runs one to the rim. A 55 gallon tank loses the top two inches to the trim and the brace, a sump runs half full, and sand and rock take space the water cannot use. A 40 breeder holding five cubic feet of water is holding 37.4 gallons, not the figure on the box.

A new tank is not ready the day it is filled. Tap water carries chlorine or chloramine, and a dechlorinator handles the first; ammonia from fish waste then starts the nitrogen cycle, which runs three to six weeks and shows as a nitrite spike before it clears. Test every other day through the cycle, then change a quarter of the water weekly, which on 37.4 gallons is 9.35 gallons a week.

Weight is the part that gets forgotten. Five cubic feet of water is 312 pounds at 62.4 pounds per cubic foot. Sand is heavier than water. Two inches of pool filter sand over 36 by 18 inches is 0.75 cubic feet, and at roughly 100 pounds per cubic foot that is 75 pounds sitting on the bottom glass before any rock goes in.

Saltwater is mixed by weight, not by the scoop. Artificial sea salt at 35 grams per liter makes a specific gravity near 1.026, so 141.58 liters takes 4,955 grams, or 10.9 pounds of mix. A refractometer earns its price here; a swing arm hydrometer drifts with temperature and biofilm, and a tank run at the wrong salinity grows algae and kills snails.

The heater and the filter are sized on the same figure. A 200 watt heater holds a 40 breeder about 10 degrees Fahrenheit above room temperature, which is enough for tropical fish in a heated room and not enough in a cold one. The filter wants a turn of the whole water volume four to six times an hour, so 37.4 gallons needs 150 to 225 gallons per hour of rated flow, and the rating on the box is measured with an empty canister.

Add it up before the stand is built. Water at 312 pounds, sand at 75, glass and trim at 70, a stand at 40 and a filter with water at 30 puts the whole thing near 530 pounds on a footprint of 4.5 square feet. That is 118 pounds per square foot, which a concrete slab does not notice and an upper floor with 2 by 8 joists at 16 inches may. Run the load past a joist span table before filling, and put the tank across the joists rather than along them.

Every row is the same volume at a different shape, and every row is 37.4 gallons. The shallow wide tank is easier to light and easier to reach into, and it needs 7.5 inches of water to reach five cubic feet instead of 15.

A rain barrel, a drum and seven and a half buckets

Containers sized in gallons

The five gallon bucket is the unit that makes five cubic feet legible. One bucket holds 5 / 7.48051948051948 = 0.6684 cubic feet, so five cubic feet fills 37.4026 / 5 = 7.48 buckets. Seven buckets plus a bit less than half of an eighth is the honest description.

A 50 gallon rain barrel holds 6.684 cubic feet, so five cubic feet takes it to about three quarters, 37.4 gallons, and a second barrel is needed to catch the rest of the same storm. A 55 gallon drum holds 7.352 cubic feet and swallows five cubic feet with 2.35 cubic feet to spare.

The 32 gallon trash can is the odd one. It holds 4.278 cubic feet, which is less than five, so a five cubic foot bag of soil does not fit it and the lid will not close on the last few gallons. A 44 gallon Brute can is 5.88 cubic feet and takes the whole bag with room to spare.

None of these containers is a cube, so the conversion is the only way to compare them. A barrel is tall and narrow, a bucket is tapered and a trash can is wider at the top than the bottom, and all of them are quoted in gallons. Cubic feet is the common ground.

The last two rows are the ones that catch people out. Five cubic feet does not fill a rain barrel or a drum, so a job that needs five cubic feet and a barrel to put it in needs 0.75 of a barrel, not one.

Box shapes that come out to 5 cubic feet exactly

Measure the inside faces

Thirty by eighteen by sixteen inches is the easy one: 30 x 18 x 16 = 8,640 cubic inches, and 8,640 / 1,728 = 5 cubic feet. Three more boxes land on the same number, and all of them are sizes you can buy or build: a 48 by 15 by 12 inch tote, a 40 by 18 by 12 inch crate, and a 24 by 24 by 15 inch cube.

Measure the inside. A cooler built from one inch walls loses two inches on every dimension, so a 30 by 18 by 16 inch shell measures 28 by 16 by 14 inside, which is 6,272 cubic inches, or 3.63 cubic feet. That is 27 percent less than the outside suggests, and it is the gap between a label that says five cubic feet and a box that holds 27 gallons.

Depth is the dimension people guess wrong. A planter 30 by 18 inches needs 16 inches of soil to reach five cubic feet. If the mix settles two inches, the volume falls to 3.75 cubic feet, or 28 gallons, and the bag count moves with it. Fill to the line you measured against, water it in, and top up after the first soak rather than buying an extra bag up front.

Ice is the one material where the box matters more than the weight. A cubic foot of ice weighs 57.4 pounds, so five cubic feet is 287 pounds, and it melts down to about 4.6 cubic feet of water, which is 34.3 gallons. A rotomolded cooler sold as 45 quarts holds 11.25 gallons, or 1.504 cubic feet, so five cubic feet of ice fills three of them and a third of a fourth.

For a trailer or a truck bed the same arithmetic decides whether a load is legal. A 4 by 8 foot bed with 12 inch sides holds 32 cubic feet, so five cubic feet is a shallow layer 1.9 inches deep. A pickup with a 6 foot bed, 5 feet between the wheel wells and 20 inch sides holds 50 cubic feet, so it swallows five cubic feet without anyone noticing the springs.

The last row is the same footprint with two inches less height, and it is the size most planters and totes turn out to be. A five cubic foot bag does not fill it, and the leftover is about 9 gallons of soil with nowhere to go.

Worked example: 5 cubic feet of concrete

0.1852 cubic yard, nine bags

Five cubic feet is a small pour, well under the minimum most ready mix plants will send. Divide by 27 to reach cubic yards: 5 / 27 = 0.1852 cubic yard. Bags are the practical route at this size. An 80 pound bag yields about 0.6 cubic feet, so 5 / 0.6 = 8.333, which means 9 bags and 720 pounds of mix to carry.

A 60 pound bag yields 0.45 cubic feet, so 5 / 0.45 = 11.11, which means 12 bags. A 50 pound bag yields 0.375 cubic feet and needs 14 bags, and a 40 pound bag needs 17. The bag count climbs faster than the yield per bag falls, so compare the cost per cubic foot rather than the ticket on one sack.

Post holes are where five cubic feet usually goes. A 10 inch hole 36 inches deep is a cylinder with a 5 inch radius: 3.1416 x 25 x 36 = 2,827 cubic inches, or 1.636 cubic feet. Five cubic feet fills 3.06 holes, so three holes take 4.9 cubic feet and a fourth needs another bag. A 12 inch hole at the same depth is 2.356 cubic feet, and five cubic feet covers 2.12 of them.

Mix to the water figure on the bag, not by eye. Extra water is the usual reason a small pour crumbles at the edge and dusts on top, and a slump that looks right in a wheelbarrow can be two quarts over the spec.

The weight column is there because a bag count that looks light on paper is 720 pounds of material in a cart. Five cubic feet of cured concrete weighs about 725 pounds at 145 pounds per cubic foot, so the finished pour and the material that goes into it land in the same place.

Rain on a roof and what a full barrel weighs

60 square feet at one inch

Five cubic feet is 60 square feet at one inch, 30 at two, 20 at three, 15 at four, 10 at six and 7.5 at eight. The arithmetic is 5 x 12 / depth, with depth in inches, and it works for soil, gravel, mulch and water alike.

Rain is the same sum with a roof in place of a bed. One inch of rain on a 60 square foot section of roof is exactly five cubic feet, or 37.4 gallons, before splash and first flush losses. A 1,000 square foot roof sheds 83.33 cubic feet from the same inch, which is 623 gallons. That is why one downspout fills a 50 gallon barrel in a few minutes of steady rain and why the overflow matters more than the inlet.

Weight decides where the barrel sits. Five cubic feet of water is 312 pounds, and a barrel holding that plus 25 pounds of plastic is close to 340. On a deck rated at 40 pounds per square foot, a barrel standing on two square feet of base is a 170 pound per square foot point load. A sheet of plywood spreads the same load over four square feet and halves it.

A rain garden is sized on the same runoff figure. A planted swale 18 inches deep and 3 feet wide carries 4.5 cubic feet for every foot of its length, so the 50 cubic feet from that inch of rain needs 11.1 feet of swale before it overtops. Clay soil percolates slowly and needs twice the length of a sandy loam, which is the difference between a bed that drains overnight and one that breeds mosquitoes.

Gutter and downspout sizing runs on the same numbers. One inch of rain on 600 square feet of roof is 50 cubic feet, or 374 gallons, and a 3 inch downspout moves that in a few minutes at full flow. A 2 inch outlet on the barrel is the bottleneck in most home setups, and the overflow should point somewhere that can take 37 gallons in a hurry.

Soil is heavier than water. Moist potting mix runs 75 to 100 pounds per cubic foot, so five cubic feet is 375 to 500 pounds. Dry sand at 100 pounds per cubic foot is 500 pounds flat, and gravel at 105 is 525. A raised bed that takes five cubic feet of any of them is a load worth checking against the deck or the balcony it stands on.

The gallons column is depth in feet times 7.4805. At one inch that is 0.0833 x 7.4805 = 0.623 gallons on every square foot, which is the figure to use when a storm is coming and the barrel is already half full.

Imperial gallons, dry gallons and the jug on the shelf

31.14 and 32.14 are not 37.40

Three different gallons share the name. The US gallon is 231 cubic inches, or 3.785411784 liters. The imperial gallon is 4.54609 liters, which is 1.20095 US gallons. The US dry gallon is 268.8025 cubic inches, which is 1.16365 US gallons.

For five cubic feet the three answers are 37.4026 US gallons, 31.1442 imperial gallons and 32.1426 dry gallons. The imperial figure is 16.7 percent smaller than the US one, and that gap is why a can sold as 5 imperial gallons in Britain holds 6.005 US gallons.

Labels blur the line. A Canadian gallon is the imperial one, so a fuel figure quoted in Canada needs the 1.20095 factor before it goes into a US tank. Fertilizer and seed are sometimes sold by the dry gallon, which is a volume of 268.8025 cubic inches and not the liquid gallon.

One number never moves. Five cubic feet is always 8,640 cubic inches, whatever gallon is printed on the container, and that is the figure to hold on to when a supplier quotes something unfamiliar.

Check a conversion by going back through liters. 37.4026 x 3.785411784 = 141.5842, and 31.1442 x 4.54609 is the same 141.5842. If the round trip lands on the number you started with, the gallon you picked was the right one.

Price per gallon against price per cubic foot

One liquid, two rates

A price per gallon and a price per cubic foot describe the same liquid, and the factor between them is 7.48051948051948. At $0.80 per gallon a cubic foot costs $5.98, and five cubic feet costs 37.4026 x 0.80 = $29.92.

Bulk moves the other way. Soil or gravel at $42 per cubic yard is 42 / 27 = $1.56 per cubic foot, so five cubic feet is $7.78, about a quarter of the jug price. The catch is the minimum load. Most yards sell by the half yard, which is nearly three times this volume and a pile to get rid of.

Concrete is the third case. Nine bags of 80 pound mix at $6.50 each is $58.50 for five cubic feet, or $11.70 per cubic foot. Delivered ready mix at $150 per yard is $5.56 per cubic foot, but a short load fee on less than a yard can double that, and five cubic feet is not enough to justify the truck.

Keep the units straight when comparing three quotes. A quote in gallons, a quote in cubic feet and a quote in cubic yards can be put on one line by converting all three to cubic feet first, then multiplying by the volume the job needs.

The arithmetic on this page, including the calculator above, runs in your browser. Nothing you type is uploaded, stored or shared.