Convert thickness before calculating volume
The rectangular formula is length × width × thickness. Convert inches to feet first: 4 in ÷ 12 = 0.333 ft. For 10 × 10 ft, 100 ft² × 0.333 ft = 33.33 ft³.
Divide by 27 ft³ per yd³ to get 1.23 yd³. The concrete slab calculator keeps cubic feet and cubic yards visible so a bag estimate and a ready-mix quote use the same base quantity.
Add waste after the base quantity
A 10% allowance changes 33.33 ft³ to 36.67 ft³, or 1.36 yd³. An 80 lb bag with a 0.60 ft³ cited yield produces 62 bags after rounding up; the product label and supplier increment still govern the order.
Thickness is a structural choice, not only a material input. Soil, reinforcement, loads, edges, joints, frost conditions, and local code can require a design that this quantity page does not provide.