Being able to calculate the gallon capacity of a sink or other container is important when you are tackling household chores. Many cleaning products must be diluted in a gallon of water. If you don't know how many gallons your sink or bucket will hold, you may not be able to determine the correct amount of cleaner to use. The shape of your sink or container will determine which calculations you should use to determine its capacity.
Note on Measurements
Most of the calculations for volume or capacity require measuring the height or depth of the sink. When taking this measurement, do not measure to the top edge of the sink. Measure only to the level to which the sink is usually filled --- generally 2 to 3 inches below the top edge. The calculations also require dividing the cubic inches of the sink by 231, because one gallon has a volume of 231 cubic inches.
Square or Rectangular Sink
Square or rectangular sinks are the same width at the top and bottom and have straight sides. They may have slightly rounded corners. To calculate the capacity of a square or rectangular sink, measure the length, width and depth. Multiply the three measurements together to get the number of cubic inches that the sink will hold. Divide this number by 231 to determine the number of gallons the sink will hold. The mathematical formula is Gallon Capacity = (L x W x D) / 231.
To calculate the capacity of a round sink, you need two measurements: depth and radius. To determine the radius, measure across the top of the sink and divide by 2. Square the radius -- that is, multiply it by itself. Multiply the result by the depth. Multiply that result by 3.14. This result represents the number of cubic inches that the sink will hold. Divide this number by 231 to determine the number of gallons the sink will hold. The mathematical formula is Gallon Capacity = ([(R x R) x D] x 3.14) / 231.
If your sink is round and is smaller at the bottom than at the top, it is a special shape called a frustum -- this is the part of a cone that is left when the tip is cut off. To determine the capacity of this sink, you will need three measurements: the top radius, the bottom radius and the depth. To determine the top radius, measure across the top of the sink and divide by 2. To determine the bottom radius, measure across the bottom of the sink and divide by 2. Square both radii -- that is, multiply each one by itself. Multiply the two radii together. Now add the two squares to the product of the two radii. Call this the "combined radius." Multiply the depth by 3.14. Divide the result by 3. Now multiply that result by the combined radius to get the number of cubic inches that the sink will hold. Divide this number by 231 to determine the number of gallons the sink will hold. The mathematical formula is Gallon Capacity = [(3.14 x D) / 3] x [(T x T) + (T x B) + (B x B)] / 231, where T is the top radius and B is the bottom radius.