Knowing the volume of your septic tank helps you determine if it complies with regulations or recommended specifications. Some jurisdictions require a minimum sized septic tank to be installed in new constructions, regardless of the prospective usage. Even if regulations do not require it, there may be size recommendations with which you wish to comply. A popular way to determine the size a septic tank should be is to consider the number of bedrooms in the house. In this system, a zero to two bedroom home should have a 750 gallon septic tank. Three, four and five to six bedroom homes should have 1,000, 1,200 and 1,500 gallon septic tanks, respectively.

Measure the width, length and depth of the septic tank with a tape measure. Use units of feet.

Multiply the width times the length times the depth to calculate the volume, measured in cubic feet. As an example, if you measured 4 feet by 8 feet by 6 feet, you would have 192 cubic feet.

Multiply the cubic feet measurement by 7.5 to calculate the volume in gallons. In the example, 192 cubic feet converts to 1,440 gallons.

Measure the diameter of the tank and divide by two to calculate the radius. The diameter is the centre distance across the circular end of the septic tank. This measurement should be in units of feet

Measure the depth or length of the septic tank in units of feet.

Square the radius by multiplying it by itself and them multiply this figure by the height times 3.14 to calculate the volume in cubic feet. As an example, a septic tank with a radius of 3 feet and a depth of 6 feet would have a volume of 170 cubic feet.

Multiply the volume in cubic feet by 7.5 to convert to gallons. In the example, 170 cubic feet converts to 1,275 gallons.