Tank Volume Calculator
Capacity in gallons for cylinder and rectangular tanks — and how much is actually in there at a given depth.
Leave blank for full capacity. Measure from the inside bottom of the tank.
Measure inside dimensions for true capacity; outside measurements overstate it by the wall thickness. Weight uses the density of the liquid you pick.
Measuring a tank you cannot see inside
Most tank questions start the same way: someone has a cylinder in a crawlspace, a poly tank on a stand or a stock tank in a field, and no plate on the side saying how many gallons it holds. A tape measure answers it. Capacity is geometry, and for the three shapes that cover almost every domestic tank — a cylinder on its side, a cylinder standing up, and a rectangular box — the arithmetic is short enough to check by hand.
Two rules before the numbers. Measure the inside if you can; on a thick-walled poly tank the outside diameter can overstate capacity by several percent. And measure the straight part of the barrel only. Dished or bulged ends on a steel tank add volume this calculator does not include, which is the direction you want to be wrong in.
Worked example: a 48 x 72 inch horizontal tank, full
A common heating-oil or water tank is 48 inches in diameter and 72 inches long, lying on its side. Radius is 24 inches, so the end area is 3.1416 x 24 x 24 = 1,809.6 square inches. Multiply by the 72-inch length and you get 130,288 cubic inches. A US gallon is defined as exactly 231 cubic inches, so that is 564 gallons — about 2,135 liters, or 75.4 cubic feet. Full of fresh water it weighs roughly 4,700 pounds.
The same tank, half and quarter full
At 24 inches of depth the tank is exactly half full: 282 gallons. A circle is symmetrical about its centre line, so the mid-point reading is the one depth where the ruler tells the truth. Drop to 12 inches — a quarter of the diameter — and intuition breaks. The answer is 110 gallons, or 19.6% of capacity, not 25%. Go the other way to 36 inches and you have 454 gallons, 80.4%. The tank is fat in the middle and thin at top and bottom, so gallons pile up fastest around the centre line.
The clearest way to feel it is gallons per inch. On this tank the first inch off the bottom holds about 2.9 gallons. An inch taken across the middle holds about 15 gallons — more than five times as much for the same movement of the stick. That is why a fuel gauge that reads a quarter can empty far faster than you planned, and why calibrated dipsticks have marks that crowd together at the ends.
Where the partial number comes from
The wetted cross-section of a horizontal cylinder is a circular segment. Its area is r² x arccos((r − h) / r) minus (r − h) x √(2rh − h²), where r is the radius and h the depth of liquid. Multiply that area by the tank length for the volume. You do not need to derive it — the point is that an arccosine sits in the middle of it, and no arccosine ever produced a straight line. That single term is the whole reason depth and volume refuse to be proportional.
Upright and rectangular tanks are the easy case
A vertical cylinder or a rectangular box has the same cross-section at every height, so partial volume really is proportional to depth. A 60 x 30 x 40 inch aquarium is 72,000 cubic inches, or 312 gallons, and filled to 20 inches it holds exactly half that. For upright tanks a plain ruler marked in even gallons works perfectly, which is why nobody sells calibrated sticks for them.
Weight, and the floor under it
Fresh water is about 8.34 pounds per US gallon; salt water is nearer 8.55, and diesel around 7.1. Those numbers matter more than the volume itself when the tank sits indoors. A 100-gallon tank is 834 pounds concentrated on a small footprint, and a 300-gallon aquarium on a suspended timber floor is a structural question, not a hobby one. Spread the load across joists, or put the tank against a load-bearing wall where the spans are shortest.
What this calculator does not model
It assumes flat ends, straight walls and no internal fittings. Real tanks come with dished or elliptical heads that add a few percent, sloped or coned bottoms that remove some, baffles, sumps, heaters and pumps. Aquarium substrate and rock typically claim 10 to 15% of the glass volume. Treat the result as the geometric capacity and subtract for whatever you know is inside — and never fill a closed tank to 100% of it, because liquids expand and vents need headspace.
Sources & further reading
- NIST Office of Weights and Measures — legal definitions of the US gallon, cubic inch and related volume units
- US EPA Underground Storage Tanks — requirements for tank capacity, filling limits and leak detection
- USGS Water Science School — water density figures used to convert tank volume into weight
Frequently asked questions
Why isn't a horizontal tank half full at half the depth?
At exactly the mid-line it is — a round tank is symmetrical, so half the diameter really is half the volume. Everywhere else the relationship bends, because the tank is widest in the middle and narrow at top and bottom. On a 48-inch tank a 12-inch reading is about 19.6% of capacity, not the 25% the ruler suggests.
Can I use this for an aquarium?
Yes, for the glass volume. Measure the inside length, width and height of a rectangular tank and you get the nominal gallons stocking rules are quoted in. Remember that substrate, rock and decor typically take up 10 to 15%, so a 55-gallon tank often holds closer to 48 gallons of water.
How much does a full tank weigh?
Fresh water weighs about 8.34 lb per US gallon, so a 564-gallon tank holds roughly 4,700 lb of water plus the tank itself. That is well past what a typical timber floor is rated for, so anything over about 100 gallons upstairs or on a deck deserves a look at the joist spans. Diesel is lighter at about 7.1 lb per gallon, saltwater slightly heavier.
Why doesn't my dipstick match this calculator?
A straight stick marked in even gallons only works on a rectangular or upright tank. On a horizontal cylinder the gallons per inch change with depth, which is why calibrated fuel sticks have uneven markings. Use this tool to build your own chart: enter the tank once, then step the fill depth an inch at a time and note the gallons.