Tank Volume Calculator turns the inside dimensions of a rectangular, cylindrical, oval, capsule, cone bottom, or spherical tank into capacity and gallons at the depths you measure.
| Depth | Gallons | % Full |
|---|---|---|
| 1 in | 1.04 gal | 10.0% |
| 2 in | 2.08 gal | 20.0% |
| 3 in | 3.12 gal | 30.0% |
| 4 in | 4.16 gal | 40.0% |
| 5 in | 5.19 gal | 50.0% |
| 6 in | 6.23 gal | 60.0% |
| 7 in | 7.27 gal | 70.0% |
| 8 in | 8.31 gal | 80.0% |
| 9 in | 9.35 gal | 90.0% |
| 10 in | 10.39 gal | 100.0% |
How the Tank Volume Calculator Measures Capacity
The Tank Volume Calculator finds the full capacity of a tank and the amount of liquid in it at any depth. Homeowners use it to read heating oil tanks, and farmers, truckers and fleet shops use it for fuel, water and DEF tanks.
Pick one of 11 tank shapes, enter the inside dimensions, and add the liquid depth from a dipstick or sight gauge. Dimensions can be in inches, feet, centimeters, meters or millimeters.
US units return gallons, and metric units return liters. The results also show the fill percentage, the weight of the liquid, and a dip chart for the whole tank.
Tank Volume Calculator Formulas by Shape
Straight-walled tanks are simple. A rectangular tank is length times width times height, and a vertical cylinder is the circle area times the height.
$$V = L \times W \times H \qquad V = \pi r^2 h$$
For a partial fill, the liquid depth replaces the height. That is why these two shapes fill evenly from bottom to top.
A tapered tank, or frustum, uses both radii. The cone section of a cone bottom tank uses the same formula, with the outlet as the small end.
$$V = \frac{\pi h}{3}\left(r_1^2 + r_1 r_2 + r_2^2\right)$$
A sphere at partial fill is a spherical cap, where f is the liquid depth.
$$V = \frac{\pi f^2 (3r – f)}{3}$$
The Tank Volume Calculator works in cubic millimeters and then converts. A US gallon is exactly 231 cubic inches, so one cubic foot holds about 7.48 gallons.
A common mistake is measuring the outside of the tank. The wall and any insulation are not storage space, so always enter inside dimensions.
Why Depth Isn’t Volume in Round Tanks
In a horizontal cylinder, the liquid surface is a slice of a circle. The narrow bottom and top hold less than the wide middle, so volume does not follow depth.
The Tank Volume Calculator finds that slice with the circular segment formula, the same geometry the Hydraulic Institute uses for horizontal tank equations. It then multiplies by the tank length.
$$V = L\left[r^2 \cos^{-1}\!\left(\frac{r – f}{r}\right) – (r – f)\sqrt{2rf – f^2}\right]$$
Take the default horizontal tank, 24 inches across and 48 inches long. It holds 94.0 gallons full. At 18 inches deep, which is 75% of the height, it holds 75.6 gallons, or 80.4% of capacity.
Scaling capacity by depth would give 70.5 gallons, about 5.1 gallons low. The alert box shows that gap for every horizontal, oval, capsule and spherical tank.
The effect flips near the bottom. At 6 inches, or 25% of the depth, the same tank holds only 19.6% of its capacity.
Reading a 275-Gallon Oil Tank
The most common home heating oil tank is the 275-gallon obround. Granby lists its standard model at 27 inches wide, 44 inches tall and 60 inches long.
Enter it in the Tank Volume Calculator as a Vertical Oval with those numbers. The flat-end model gives 267.9 gallons, about 2.6% below the 275-gallon nominal rating. For exact deliveries, check the result against the manufacturer’s own tank chart.
The rounded bottom matters for low readings. The first inch holds only about 1.8 gallons, while each inch in the straight middle holds about 7.0 gallons.
That is why a tank reading 11 inches is not a quarter full. It holds about 57 gallons, only 21% of capacity.
The Dip Chart and Gauge Marks
Below the result cards, the Tank Volume Calculator builds a dip chart for your exact tank. Each row pairs a depth with its volume and percent full, and the row nearest your reading is highlighted.
The fourth card shows how much one inch, or one centimeter, of depth holds at your current level. It also gives the depths where the tank is one quarter and one half full by volume.
Those marks are useful for painting a gauge or setting a float alarm. On a round tank, the quarter mark sits well above a quarter of the height.
The first card shows the liquid at your depth, the percent full, and the room left for a delivery. The second card gives full capacity in the other volume units, including UK gallons and cubic feet or cubic meters.
Weight of What’s in the Tank
In the third card, the Tank Volume Calculator multiplies the volume by the density of the liquid you pick. Gasoline is preset at 0.745 kg/L, or about 6.22 lbs per gallon.
At the defaults, 7.79 gallons of gasoline weighs 48.45 lbs. The result covers the liquid only, not the empty tank.
For more on how fuel weight changes with type and temperature, the fuel weight calculator covers 18 fuels with a temperature correction.
Limits the Tool Enforces
Every dimension and the density must be greater than zero, and liquid depth cannot be negative. The Tank Volume Calculator also rejects a depth taller than the tank and names the tank height in the alert.
Some shapes have extra checks. A capsule’s overall length or height must be at least its diameter, and an oval’s long side must be at least its short side.
On a cone bottom tank, the outlet cannot be wider than the top, and the cylinder or cone height can be zero but not both. A custom density below 0.4 or above 2 kg/L triggers a warning.
Measuring Mistakes to Avoid
Measuring the air gap from the top instead of the liquid depth from the bottom gives the empty space, not the fill. Subtract the reading from the tank height if your stick measures from the top.
Mixing up the sides of an oval tank flips the shape. For a vertical oval, the width is the narrow side and the height is the tall side.
Leaving the liquid on gasoline when the tank holds water or DEF throws off the weight card. Pick the right liquid, since water weighs about a third more per gallon than gasoline.
Tank Volume Questions People Ask
How do I find the gallons in a partly full horizontal tank?
Measure the inside diameter, the length and the liquid depth. The circular segment formula then gives the liquid area, which you multiply by the length. A 24 by 48 inch tank at 18 inches deep holds 75.6 gallons. Don’t scale the full capacity by the depth percentage, since that would come out about 5 gallons low here.
How many gallons are in a cubic foot?
About 7.48 US gallons. A cubic foot is 1,728 cubic inches, and a US gallon is exactly 231 cubic inches. A cubic foot is also 28.32 liters, or about 6.23 UK gallons. The second result card shows total capacity in cubic feet for inch or foot entries, and in cubic meters for metric ones.
Is a tank volume calculator accurate for oil tanks?
It is as close as the shape model allows. The tool assumes perfect geometric shapes, while real tanks have rounded seams, dished ends and fittings. For the 275-gallon obround, the math lands about 2.6% under the nominal rating. Use the result for planning and the maker’s chart for billing-grade readings.
How do I work out a cone bottom tank?
Split it into two parts. The cylinder on top is the circle area times its height. The cone below is a frustum from the top diameter down to the outlet. Add the two for full capacity. At any depth inside the cone, volume grows slowly at first, then faster as the cone widens, and the alert box shows what share of the tank the cone holds.