Gravel/Stone Estimator Use these calculators to estimate the amount of gravelor rock you''ll need for various projects. Be sure to verify your results with your gravel supplier. For Slabs, Walkways, and Similar Projects: Square Footage: For items with straight sides and 90-degree corners, multiply height x width. For more complex shapes, graph on paper, one spare per foot. Count partial squares as filled. Square Feet Thickness: For items with varying thickness use the average thickness or calculate each section separately. Inches Volume of gravel/stone needed: Cubic Yards Cubic Feet