What is the formula for calculating how much product I need to purchase to complete my project?

I don't have a formula, but we filled a hole with one bag and the hole was about 1ft by 2ft and averaged 4 inches deep. Hope that helps.

Measure the area of each pothole - length x width in inches - and add them together. Multiply the total by the depth in inches, yielding total volume in cubic inches. Divide this result by 650, the approximate volume of tamped asphalt per bag (3/8 cu. ft). Measure generously - for example, if a pothole is approximately round, measure as a square. For example: 13" across, measure 13 x 13 = 269 x 2" thick = 338 cu-in. Then 338 / 650 = 0.52, meaning it will take about 1/2 bag to fill that one pothole. I recommend 1-1/2" - 2" thickness of asphalt on a hard-packed base. If deeper than 2", fill with clay or other suitable material that can be tamped firmly.