Measure, in inches, the length, width and depth of the space to be paved with asphalt. For example, a sidewalk project might have a length of 150 inches, a width of 50 inches and a depth of 2 inches.
Multiply the length by the width by the depth to obtain the volume of asphalt required in cubic inches. Performing this step, for example, leads to 150 inches times 50 inches times 2 inches, or a volume of 15,000 cubic inches.
Divide the volume by 231 to convert to gallons, because a single gallon equals 231 cubic inches. Completing the example, you arrive at 15,000 cubic inches divided by 231 cubic inches per gallon, or an asphalt amount of 64.9 gallons.