You can calculate the size of a given tank using solid formulas. The volume of a shape is the amount of space inside of it. If you measure a tank in feet, convert to meters, and use the appropriate formula, you can find approximately how big it is inside. You must then find the volume of the substance to be stored in the tank. Gases like propane and hydrogen are sometimes compressed, so you need to know the compression ratio to find the storage capacity.
Determine which shape fits the tank most closely. For example, a corn silo is cylinder-shaped, so you know the height, you can measure the circumference with a rope marked off at 10-foot intervals, and use the cylinder volume formula. Some tanks may require multiple shapes. Suppose you want to find the volume of a propane tank. Since a propane tank is cylindrical, you can use the cylinder formula to find its volume and storage capacity. Check the formulas for the dimensions you need. The cylinder formula requires radius and length. The radius will be half the tank's diameter.
Take the measurements you need. For a propane tank, measure the length of the tank from end to end. Then measure the diameter of the tank. Call the diameter and length D and L, respectively. The radius, R, is D / 2. Convert these measurements to metric: A 190" long tank with a diameter of 41" has a radius of 20.5 inches. The radius, then, is 1.0 meter rounded down to the nearest tenth, and the length is 4.8 meters rounded to the nearest tenth.
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Solve the volume formula: in this case, the volume is 1 * 1 * 4.8 * 3.14 is 15 cubic meters rounded to the nearest tenth, where 3.14 is pi to 2 digits. We want the volume in meters because measurements of most gases and liquids are generally given in metric.
Use the compression ratio of the substance to be stored to find the storage capacity. The compression ratio for liquid to vaporous propane is 1:270. 15 * 270 = 4050, so your tank can hold 4050 liters or 1069 gallons; a propane tank is not a perfect cylinder, so you can assume that your tank will hold about 1000 gallons. Note that this is at 100% capacity; most propane tanks stay around 80% capacity.
These are the volume formulas you will need for most tanks, with 3.14 substituted for pi: Cylinder: 3.14 * length * radius^2. Sphere: 3.14 * radius^2 Cube or box: height * width * length