Tank Size Calculator
Tank size calculator for volume and partial fill: cylinders, horizontal tanks, rectangles, spheres and capsules, in gallons, litres and cubic feet.
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How the Tank Size Calculator Formula Works
The volume of a tank is the area of its cross-section multiplied by its length, so each shape has its own version. An upright cylinder is the area of the circle at the base, pi times the radius squared, times the height. A rectangular tank is length times width times height. A sphere holds four thirds of pi times the radius cubed, and a capsule is a cylinder with a sphere's worth of volume added at the ends.
A partly full tank is where it gets interesting. For an upright cylinder or a rectangle the liquid volume is simply the base area times the liquid depth. In a horizontal cylinder, the liquid cross-section is a circular segment, the slice of the circle below the liquid line, and the volume is that area times the tank length. A partly filled sphere holds a spherical cap.
The calculator takes any of these shapes in inches, feet, centimetres, metres or millimetres, and gives the capacity in US gallons, imperial gallons, litres, cubic metres and cubic feet. One US gallon is exactly 231 cubic inches, or 3.785411784 litres.
cylinder: V = pi × r2 × h; rectangle: V = l × w × h; sphere: V = 4/3 × pi × r3; horizontal tank at depth h: V = [r2 × arccos((r − h)/r) − (r − h) × √(2rh − h2)] x length- diameterin, ft, cm, m or mm24Inside diameter of a cylindrical, spherical or capsule tank.
- lengthsame unit32Height of an upright cylinder, or straight length of a horizontal cylinder or capsule.
- length, width, heightsame unitInside dimensions of a rectangular tank.
- liquid depthsame unitDepth of the liquid; leave at 0 for a full tank.
- diameter
- in, ft, cm, m or mm
- Inside diameter of a cylindrical, spherical or capsule tank.
- length
- same unit
- Height of an upright cylinder, or straight length of a horizontal cylinder or capsule.
- length, width, height
- same unit
- Inside dimensions of a rectangular tank.
- liquid depth
- same unit
- Depth of the liquid; leave at 0 for a full tank.
Using the Tank Size Calculator
Enter tank type
Choose from Round (cylinder, sphere, capsule), Rectangular.
Enter round tank shape
Choose from Upright cylinder, Horizontal cylinder, Sphere, Horizontal capsule (rounded ends).
Enter dimension unit
Choose from Inches, Feet, Centimetres, Metres, Millimetres.
Enter inside diameter
Type the figure you already have, then press Calculate.
Enter height or straight length (ignored for a sphere)
Type the figure you already have, then press Calculate.
Enter inside length
Type the figure you already have, then press Calculate.
Enter inside width
Type the figure you already have, then press Calculate.
Enter inside height
Type the figure you already have, then press Calculate.
Enter liquid depth (0 = full tank)
Type the figure you already have, then press Calculate. This one is optional.
Read the result
Press Calculate to update the result. Until you do, the last result is dimmed and marked as out of date, so you always know which numbers belong to the values in the boxes.
A Real Worked Example
An upright cylindrical tank is 24 inches across and 32 inches high. The radius is 12 inches, so the volume is pi x 12 squared x 32 = 14,476.7 cubic inches.
Dividing by 231 gives 62.67 US gallons, which is 237.2 litres, 52.2 imperial gallons or 8.38 cubic feet. With no liquid depth set, the tank is treated as full.
Horizontal tank, 48 x 96 in, filled 12 in deep
A horizontal cylinder is 48 inches across and 96 inches long, so it holds 752.0 US gallons when full. The liquid is 12 inches deep, one quarter of the diameter.
The liquid cross-section is a circular segment, and the volume is 556.5 litres, 147.0 US gallons. That is only 19.6 percent of the tank, so a quarter of the depth is much less than a quarter of the volume.
A rectangular tank, 100 x 50 x 40 cm, water 35 cm deep
A rectangular tank 100 cm long, 50 cm wide and 40 cm high holds 100 × 50 × 40 = 200,000 cubic centimetres, which is 200 litres or 52.8 US gallons.
With water 35 cm deep it holds 175 litres, 87.5 percent of the tank, leaving 6.6 US gallons of room.
A spherical tank, 2 m across, filled 0.5 m deep
A sphere 2 metres across has a radius of 1 metre and holds 4/3 x pi = 4.189 cubic metres, which is 4,188.8 litres or 1,106.6 US gallons.
Filled to 0.5 metres, the liquid is a spherical cap of pi x 0.25 × 2.5 / 3 = 0.6545 cubic metres. That is 654.5 litres, 15.6 percent of the tank.
A capsule tank, 1.2 m across, 3 m straight, 0.72 m deep
A capsule tank has a cylinder 1.2 m across and 3 m long, plus rounded ends equal to one sphere. Its volume is 3.393 + 0.905 = 4.298 cubic metres, or 4,297.7 litres.
With liquid 0.72 m deep, 60 percent of the diameter, it holds 2,711.9 litres, which is 63.1 percent of the capacity.
A half-full horizontal tank, 6 ft x 20 ft
A horizontal tank 6 feet across and 20 feet long holds 4,230 US gallons when full. Fill it to a depth of 3 feet, half the diameter.
The liquid fills exactly half, 8,006.4 litres, or 2,115 US gallons. At the half-depth point, depth and volume agree.
What Else to Know
How the Tank Size Calculator Works
The calculator multiplies the cross-section of a tank by its length or height, using the formula for the shape you choose. It covers upright cylinders, horizontal cylinders, rectangular tanks, spheres and horizontal capsules, in inches, feet, centimetres, metres or millimetres.
The result is shown at once in US gallons, imperial gallons, litres, cubic metres and cubic feet. Add a liquid depth and it also shows how much liquid is in a partly filled tank, what percentage of the tank that is, and how much room is left.
It works the same way for a water tank, a fuel tank, a fish tank, a sump or a process vessel.
Tank Volume Formulas by Shape
An upright cylinder holds pi x r squared x height. A horizontal cylinder holds the same, with the length in place of the height. A rectangular tank is length x width x height. A sphere holds 4/3 x pi x r cubed. A capsule is a horizontal cylinder plus a sphere.
The shape table lists the full and the partly full formula for each. Use the inside dimensions in every case, because the liquid occupies only the inside.
To convert, one US gallon is exactly 231 cubic inches or 3.785411784 litres, and an imperial gallon is 4.54609 litres.
Partly Full Tanks
For an upright cylinder or a rectangle, the liquid volume is the base area times the liquid depth, so volume rises in proportion to depth.
A horizontal cylinder is different. The liquid cross-section is a circular segment, with area r squared x arccos((r - h)/r) minus (r - h) x the square root of (2rh - h squared), as given for a circular segment on Wikipedia. Multiply by the length for the volume.
Depth and volume are not proportional. A quarter of the diameter holds only 19.6 percent of the volume, and half the diameter holds exactly 50 percent. The fill table lists the curve.
Spheres, Capsules and Other Shapes
A partly filled sphere holds a spherical cap, with volume pi x h squared x (3r − h) / 3 for a liquid depth h and sphere radius r, as stated on Wikipedia. A capsule tank adds the cylinder part, using the circular segment, to the cap.
The calculator does not model dished or elliptical heads, cones or irregular tanks. For those, use the tank maker's rated capacity or split the tank into simpler shapes and add the volumes.
For propane and some fuel tanks the capsule shape is a good approximation, but the rated capacity on the label is the figure to use.
Choosing a Tank Size
To size a tank for a required volume, choose a height and solve for the diameter, d = 2 x the square root of V / (pi x h). The size table lists the diameters for 50 to 2,500 gallons at four heights.
A taller tank needs a smaller diameter, so the choice depends on the space you have and the load on the floor. The calculator gives the volume for any dimensions you try, so you can adjust until the capacity covers your need.
It does not know your use, your local rules or the tank's rated working capacity. Check those separately.
Aquariums and Water Tanks
A rectangular fish tank is length x width x height in inches divided by 231 for US gallons. A tank 36 × 18 × 16 inches is 44.9 gallons. The water is less than that when the tank is not filled to the brim, so enter the water depth.
For a round water tank or a cistern, use the upright or horizontal cylinder. The table of upright cylinders lists capacities for common diameters and heights, from 12 to 72 inches.
Glass thickness, gravel, decorations and equipment take up room, so the water volume is a little lower than the geometric figure.
Accuracy and Limits
The geometry is exact for the shapes covered, so the answer depends on your measurements. Dimensions that are squared in the formula, such as the diameter, magnify errors: a 2 percent error in diameter is a 4 percent error in volume.
The calculator does not account for wall thickness unless you use inside dimensions, fittings, baffles, dished ends or tilt. A tank that is not level has a different liquid volume from the one given.
For other pump and boiler sizing, see the boiler feed pump calculator, which uses a tank size for the feedwater receiver.
Reference Tables
| Term | Meaning |
|---|---|
| Capacity | The volume the tank holds when full |
| Liquid depth | How deep the liquid is, measured from the lowest inside point |
| Circular segment | The part of a circle cut off by a straight line; the liquid cross-section in a horizontal tank |
| Spherical cap | The part of a sphere cut off by a plane |
| Capsule | A cylinder with a hemisphere at each end |
| US gallon | Exactly 231 cubic inches, 3.785411784 litres |
| Imperial gallon | 4.54609 litres, about 20 percent larger than the US gallon |
| Shape | Full volume | Partly full |
|---|---|---|
| Upright cylinder | pi x r2 x height | pi x r2 x depth |
| Horizontal cylinder | pi x r2 x length | circular segment area x length |
| Rectangular | length x width x height | length x width x depth |
| Sphere | 4/3 x pi x r3 | spherical cap: pi x h2 × (3r − h) / 3 |
| Capsule | pi x r2 x length + 4/3 x pi x r3 | segment x length + spherical cap |
| Diameter (in) | 24 in high | 36 in high | 48 in high | 60 in high | 72 in high |
|---|---|---|---|---|---|
| 12 | 11.8 | 17.6 | 23.5 | 29.4 | 35.3 |
| 18 | 26.4 | 39.7 | 52.9 | 66.1 | 79.3 |
| 24 | 47.0 | 70.5 | 94.0 | 117.5 | 141.0 |
| 30 | 73.4 | 110.2 | 146.9 | 183.6 | 220.3 |
| 36 | 105.8 | 158.6 | 211.5 | 264.4 | 317.3 |
| 48 | 188.0 | 282.0 | 376.0 | 470.0 | 564.0 |
| 60 | 293.8 | 440.6 | 587.5 | 734.4 | 881.3 |
| 72 | 423.0 | 634.5 | 846.0 | 1,057.5 | 1,269.0 |
| Diameter (in) | 48 in long | 72 in long | 96 in long | 120 in long | 240 in long |
|---|---|---|---|---|---|
| 24 | 94 | 141 | 188 | 235 | 470 |
| 30 | 147 | 220 | 294 | 367 | 734 |
| 36 | 212 | 317 | 423 | 529 | 1,058 |
| 48 | 376 | 564 | 752 | 940 | 1,880 |
| 60 | 588 | 881 | 1,175 | 1,469 | 2,938 |
| 72 | 846 | 1,269 | 1,692 | 2,115 | 4,230 |
| Depth / diameter | Volume filled | Volume empty |
|---|---|---|
| 0.05 | 1.9% | 98.1% |
| 0.10 | 5.2% | 94.8% |
| 0.20 | 14.2% | 85.8% |
| 0.25 | 19.6% | 80.4% |
| 0.30 | 25.2% | 74.8% |
| 0.40 | 37.4% | 62.6% |
| 0.50 | 50.0% | 50.0% |
| 0.60 | 62.6% | 37.4% |
| 0.70 | 74.8% | 25.2% |
| 0.75 | 80.4% | 19.6% |
| 0.80 | 85.8% | 14.2% |
| 0.90 | 94.8% | 5.2% |
| 0.95 | 98.1% | 1.9% |
| Length x width x height (in) | Cubic inches | US gallons | Litres |
|---|---|---|---|
| 20 × 10 × 12 | 2,400 | 10.4 | 39.3 |
| 24 × 12 × 16 | 4,608 | 19.9 | 75.5 |
| 30 × 12 × 18 | 6,480 | 28.1 | 106.2 |
| 36 × 18 × 16 | 10,368 | 44.9 | 169.9 |
| 48 × 18 × 21 | 18,144 | 78.5 | 297.3 |
| 60 × 24 × 24 | 34,560 | 149.6 | 566.3 |
| 72 × 36 × 24 | 62,208 | 269.3 | 1,019.4 |
| 96 × 48 × 48 | 221,184 | 957.5 | 3,624.6 |
| Target gallons | Height 36 in | Height 48 in | Height 60 in | Height 72 in |
|---|---|---|---|---|
| 50 | 20.2 in | 17.5 in | 15.7 in | 14.3 in |
| 100 | 28.6 in | 24.8 in | 22.1 in | 20.2 in |
| 250 | 45.2 in | 39.1 in | 35.0 in | 32.0 in |
| 500 | 63.9 in | 55.4 in | 49.5 in | 45.2 in |
| 1,000 | 90.4 in | 78.3 in | 70.0 in | 63.9 in |
| 2,500 | 142.9 in | 123.8 in | 110.7 in | 101.1 in |
| From | To | Multiply by |
|---|---|---|
| US gallons | Litres | 3.785411784 |
| Imperial gallons | Litres | 4.54609 |
| Litres | US gallons | 0.264172 |
| Cubic feet | US gallons | 7.48052 |
| Cubic metres | Litres | 1,000 |
| Cubic inches | US gallons | 0.004329 (divide by 231) |
| US gallons | Imperial gallons | 0.832674 |
| Mistake | Effect | Fix |
|---|---|---|
| Using outside instead of inside dimensions | Over-estimates capacity | Measure inside the tank walls |
| Treating a half-full depth as half the volume of a non-cylinder | Wrong fill volume | Use the shape's own fill formula |
| Mixing US and imperial gallons | About 20 percent error | Check which gallon your supplier uses |
| Ignoring dished ends and fittings | Small error | Use the maker's stated capacity |
| Filling to the brim | No expansion room | Leave headspace as the tank maker advises |
Frequently Asked Questions About the Tank Size Calculator
How do I calculate tank volume?
Multiply the cross-section area by the length or height. For an upright cylinder that is pi x radius squared x height, so a tank 24 inches across and 32 inches high holds 62.7 US gallons. A rectangular tank is length x width x height. Pick the shape in the calculator. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
What is the formula for the volume of a cylindrical tank?
V = pi x r2 x h, where r is the inside radius and h the height of an upright tank or the length of a horizontal one. A 48 inch diameter tank 96 inches long holds 752 US gallons. Divide cubic inches by 231 to get gallons. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How do I find the volume of a partly full horizontal tank?
The liquid cross-section is a circular segment, with area r2 x arccos((r - h)/r) − (r - h) x the square root of (2rh - h2), where h is the liquid depth. Multiply by the tank length. The fill table shows the share of the volume for each depth. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How many gallons are in a tank of a given size?
It depends on the shape. A 48 × 18 × 21 inch rectangular tank is 78.5 gallons and an upright cylinder 36 inches across and 48 inches high is 211.5. The tables list common sizes. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
What size tank do I need for a given volume?
Pick a height and solve for the diameter: d = 2 x the square root of (V / (pi x h)). For 500 US gallons at a height of 60 inches the diameter is 49.5 inches. The size table lists other combinations. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How do I convert tank volume to gallons?
Cubic inches divided by 231 gives US gallons, cubic feet times 7.48 gives US gallons and litres divided by 3.785 gives US gallons. A tank of 12,000 cubic inches is 51.9 gallons. The calculator shows gallons, imperial gallons, litres and cubic metres together. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How many litres are in a tank?
Multiply cubic metres by 1,000, or US gallons by 3.785. A tank of 500.00 US gallons holds 1,893 litres. A rectangular tank 100 × 50 × 40 cm holds 200 litres, since 200,000 cubic centimetres is 200 litres. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
What is the difference between a US gallon and an imperial gallon?
A US gallon is exactly 231 cubic inches, 3.785411784 litres. An imperial gallon is 4.54609 litres, about 20 percent larger. The calculator shows both, so you can match the supplier or country you are working with, which matters when ordering tanks or fuel. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How do I calculate the volume of a fish tank?
Multiply length x width x height in inches and divide by 231 for US gallons. A tank 36 × 18 × 16 inches is 44.9 gallons. The water is a little less if the tank is not filled to the top, so enter the water depth for a fill level. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How much water is in a partly filled tank?
For an upright cylinder or a rectangular tank, multiply the base area by the water depth. For a horizontal tank use the circular segment. A 100 × 50 × 40 cm tank with water 35 cm deep holds 175 litres, which is 87.5 percent of its capacity. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How much does a half-full horizontal tank hold?
Exactly half, because the circle is symmetric: at a depth of half the diameter the liquid fills 50 percent of the volume. At one quarter of the diameter it fills 19.6 percent, and at three quarters 80.4 percent, so depth and volume are not proportional. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How do I calculate a spherical tank?
A sphere holds 4/3 x pi x r3. A tank 2 metres across holds 4,189 litres. Partly filled to a depth h, the liquid is a spherical cap with volume pi x h2 × (3r − h) / 3. Choose the sphere shape and enter the depth. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
What is a capsule tank?
A capsule tank is a cylinder with a hemisphere at each end, common for propane and some fuel tanks. Its volume is the cylinder plus one full sphere. Enter the diameter and the straight length of the cylinder part, not the overall length. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
Can I use this to size a water storage tank?
Yes, for the volume. Work out how much water you need, then try diameters and heights until the capacity covers it with room to spare. The calculator does not know your daily use, local rules or the tank's rated working capacity, so confirm those separately. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
Can I use this for a fuel or oil tank?
It gives the geometric volume of the shape, which is a starting point. Real fuel and oil tanks may have dished ends, fittings and a safe fill level lower than the full volume, so check the manufacturer's rated capacity and any regulations that apply to your tank. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
Should I use inside or outside dimensions?
Use the inside dimensions, since the liquid occupies only the inside. Outside dimensions overstate the volume by the wall thickness on every side. For a thin steel tank the difference is small, but for a thick plastic or concrete tank it can be large. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How much headspace should I leave?
This calculator cannot say, because it depends on the liquid, the tank and the rules that apply. Many tanks are not filled to the brim to allow for expansion. Use the fill depth box to see the volume at any level you choose, and follow the tank maker's guidance. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How do I measure the liquid depth?
Measure from the lowest inside point of the tank up to the liquid surface, in the same unit as the dimensions. For a horizontal tank that is the bottom of the circle, not the outside of the shell. A dipstick or sight gauge reads this depth directly. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
Why is a horizontal tank's depth not proportional to its volume?
Because the circle is narrower near the bottom and top than in the middle. The first 10 percent of depth holds only 5.2 percent of the volume, while the middle 10 percent of depth, from 45 to 55 percent, holds about 12.7 percent. The fill table shows the curve. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
What units can I use?
Choose inches, feet, centimetres, metres or millimetres for the dimensions. The results are shown in US gallons, imperial gallons, litres, cubic metres and cubic feet at once, so you do not have to convert afterwards. All the dimensions and the liquid depth use the same unit. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How do I calculate a tank with dished or elliptical ends?
This calculator does not model dished or elliptical heads. For a rough figure, use the capsule shape for rounded ends or add the head volumes from the maker's data. Tank makers publish the capacity for their designs, which is the best figure to use. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How accurate is the tank calculator?
The geometry is exact for the shapes it covers, so the answer is only as accurate as your measurements. Small errors in diameter are squared, so a 2 percent error in the diameter gives about a 4 percent error in volume. Measure carefully and use inside dimensions. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
How do I use this tank size calculator?
Choose a round or rectangular tank, pick the shape, choose the unit and enter the dimensions. Add a liquid depth for a partly full tank, or leave it at zero for a full tank. The results show the capacity and the filled volume in gallons, litres and cubic feet. Enter your own dimensions in the calculator for an exact answer in gallons, litres and cubic feet, and check the figure against the tank maker's label.
References
- [1] Wikipedia contributors. “Circular segment.” Wikipedia. Accessed 2026-10-06.
- [2] Wikipedia contributors. “Spherical cap.” Wikipedia. Accessed 2026-10-06.
- [3] Wikipedia contributors. “Gallon.” Wikipedia. Accessed 2026-10-06.
- [4] Omni Calculator. “Tank volume calculator.” Omni Calculator. Accessed 2026-10-06.
- [5] CalculatorSoup. “Tank volume calculator.” CalculatorSoup. Accessed 2026-10-06.
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