Tank Volume Calculator

Three shapes cover most tanks

Three solids cover nearly every tank a homeowner meets. A rectangular cistern or stock tank is a box, so its volume is length x width x height. A rain barrel or pressure tank standing upright is a cylinder, so its volume is pi x radius squared x height. A fuel tank or trough lying on its side has the same body volume as the upright version of itself, but a depth reading no longer divides it evenly, which gets its own section below.

The tank volume calculator above runs all three in one place. Give it dimensions in inches, feet, meters or centimeters and it reports US gallons, liters and cubic feet, plus 42 gallon barrels for anyone comparing against petroleum figures. It also handles a partial fill: enter a liquid depth and it returns the water in the tank at that level, not the nameplate number.

Everything else, cones, domes, rounded corners, is a correction on top of these three. The safe habit is to compute the plain solid first, then adjust for the shape details you can see from outside.

Worked example: a rectangular tank

Take a rectangular tank 48 inches long, 24 inches wide and 24 inches tall. The raw volume is 48 x 24 x 24 = 27,648 cubic inches. Divide by 231, the exact number of cubic inches in a US gallon, and the tank holds 119.7 US gallons. The same volume reads 453.1 liters and 16.0 cubic feet, which is the figure to carry when the question is floor loading or shelf spacing.

Now set the water line at 18 inches instead of the rim. Only the height term changes: 48 x 24 x 18 = 20,736 cubic inches, which is 89.8 gallons. That is 75 percent of capacity from a tank filled three quarters of the way up, because a box fills evenly with height. Every inch of depth in this tank is worth 48 x 24 / 231, just under 5 gallons, which is a handy mental shortcut once you know one level.

Measure inside the walls, not outside. A quarter inch wall takes half an inch off each dimension, and 47.5 x 23.5 x 23.5 = 26,232 cubic inches, or 113.6 gallons. The same tank measured over the outside shows 119.7, so the tape measure alone created a 6 gallon gap, about 5 percent of the real number.

Vertical cylinders and the diameter trap

The mistake that costs people here is typing a diameter where the formula wants a radius. Volume grows with the square of the radius, so using the diameter instead makes the answer four times too big.

A rain barrel 24 inches across and 48 inches tall has a radius of 12 inches. Pi x 12^2 x 48 = 21,715 cubic inches, which is 94.0 US gallons, 355.8 liters and 12.6 cubic feet. Feed the same barrel its 24 inch diameter as if it were the radius and the arithmetic reports 376 gallons, four times what the barrel can hold.

Capacity also climbs faster than diameter. Widen the barrel from 24 to 30 inches at the same 48 inch height and the volume multiplies by (15 / 12)^2 = 1.5625: 94.0 gallons becomes 146.9. Height is more modest, since it enters the formula as a plain factor: adding 12 inches of height adds exactly 25 percent.

Horizontal tanks and partial fills

A tank on its side holds the same total as it would upright, but a dipstick reading maps through a circular segment instead of a rectangle. The area of liquid in the circular end is r^2 x acos((r - h) / r) - (r - h) x sqrt(2rh - h^2), where h is the liquid depth. Multiply that area by the tank length and divide by 231 to reach gallons.

Worked example: a fuel tank 48 inches in diameter and 72 inches long, with liquid 12 inches deep. The radius is 24 inches, and the segment area works out to 353.8 square inches. Times the 72 inch length that is 25,476 cubic inches, or 110.3 gallons, which is 19.6 percent of the tank. Full, the same tank holds pi x 24^2 x 72 = 130,288 cubic inches, or 564.0 gallons.

The curve is the part intuition misses. Half the diameter, 24 inches of depth, holds exactly half the volume, 282.0 gallons. But 18 inches of depth, three quarters of the way up the diameter, holds 193.2 gallons, only 34.3 percent of the tank. The bottom quarter and the top quarter of a horizontal cylinder each carry a small share and the middle carries most of it, which is why a quarter mark on a cylindrical fuel gauge means something different from a quarter of the stick.

Gallons, liters, cubic feet and barrels

Unit conversion is where tank math goes wrong most often, so anchor everything to two definitions. The US gallon is 231 cubic inches by definition, and NIST Special Publication 811, Appendix B lists it as 3.785412 liters. Every other figure follows: a cubic foot holds 1,728 cubic inches and therefore 7.4805 gallons, and the petroleum barrel is 42 US gallons, the unit the US Energy Information Administration uses for crude oil statistics.

Run the 119.7 gallon example up the ladder: 119.7 x 3.785412 = 453.1 liters, 119.7 / 7.4805 = 16.0 cubic feet, and 119.7 / 42 = 2.85 barrels. Scale the same steps to a 1,000 gallon cistern and it is 3,785.4 liters, 133.7 cubic feet, or 23.81 barrels. The widget on this page reports all three beside the gallon figure so nobody has to run that ladder by hand twice.

Real capacity versus the nameplate

The printed capacity is almost always more than the water you can use. Freeboard is the gap between the liquid line and the rim, left so splash and thermal expansion do not push water over the top. Fill the 119.7 gallon example to 90 percent of its height and you have 107.7 gallons of usable water; the missing 12 gallons are freeboard, not slop in the arithmetic.

The same tank read at four fill levels, which is what a gauge or a dipstick is telling you:

Shape details shave further volume. Dished or sloped bottoms hold less than the box math, internal baffles and fittings displace water, and an outlet that sits a couple of inches above the floor strands the last few gallons below it. A rain barrel drained through a spigot partway up the side gives up its bottom band permanently, which is one reason two barrels labeled the same size can deliver different amounts.

Wall thickness is the error that hides inside the tape measure, and on the 48 by 24 by 24 example it is worth about 6 gallons, roughly 5 percent. Plan around usable volume rather than printed volume whenever the water feeds something, a pump, an irrigation line, an aquarium, that cares where the level sits.

Rain barrels and roof catchment

Sizing a rainwater tank starts with the roof, not the tank. The US Geological Survey rain and precipitation page calculates that 1 inch of rain over 1 acre delivers 27,154 gallons. An acre is 43,560 square feet, so 1,000 square feet of roof collects about 623 gallons from a 1 inch storm. A pair of 55 gallon barrels is full after a tenth of an inch, which is why people who store rainwater seriously end up shopping for cisterns measured in hundreds of gallons.

Worked example: a 1,200 square foot roof under a 2 inch storm collects 1,496 gallons. A pair of 60 inch diameter, 60 inch tall cisterns holds 734.4 gallons each, 1,468.8 gallons together, and they come up about 27 gallons short of catching the whole storm. That is the useful kind of close: storage sized near the storm you want to catch, not near the roof area that produces it.

Vertical tanks also make level checks easy, because they fill linearly with depth. Twelve inches of water in that 60 inch cistern is 146.9 gallons, exactly 20 percent of capacity. Compare that with the horizontal tank in the earlier section, where 12 inches of depth held under 20 percent, and the reason the shape selector matters becomes obvious.

Mistakes that skew the number

• Diameter in the radius slot: makes a cylinder four times too big, 376 gallons instead of 94.0 on the rain barrel above.
• Mixed units in one formula: a tank entered as 48 by 2 by 24 inches instead of 48 by 24 by 24 reports 10.0 gallons instead of 119.7, because the 2 was feet.
• Outside dimensions on a thick walled tank: about 6 gallons of ghost water on the example tank, 5 percent of the real figure.
• Treating a horizontal tank dipstick as linear: 18 inches of depth is 34.3 percent of the tank, not 75 percent.
• Counting freeboard as capacity: a tank filled to 90 percent of height delivers 90 percent of the printed gallons, not all of them.
• Forgetting the flat end assumption: the segment math assumes flat ends, and a dished or sloped bottom holds less.

One last check that catches most of these at once: run the same tank twice, once in inches and once in feet, and the two answers should land within rounding of each other. If they do not, a unit went in wrong, and it is faster to find before the concrete pad is poured.