Concrete Calculator

Multiply length × width × thickness in feet and divide by 27. A 10 × 10 ft slab at 4 inches thick needs about 1.23 cubic yards of concrete.

It takes about 45 bags of 80 lb concrete mix to make one cubic yard, since each 80 lb bag yields roughly 0.6 cubic feet.

A 10×10 ft slab poured 4 inches thick needs about 1.23 cubic yards (33.3 cubic feet) of concrete — roughly 56 bags of 80 lb mix, though ready-mix delivery is usually cheaper at this size.

Yardage is just cubic yards. Multiply length by width by thickness — all in feet — then divide by 27, because a cubic yard is 27 cubic feet. A 4-inch slab is 0.333 ft thick, so a 10 by 10 pad is 10 × 10 × 0.333 ÷ 27, or 1.23 cubic yards.

Measure the slab in feet, convert the thickness from inches to feet by dividing by 12, multiply all three, then divide by 27 for cubic yards. Order 5–10% over: forms bow outward, the subgrade is never perfectly level, and a short load leaves a visible cold joint you cannot undo.

Square yards measure area, not volume, so they cannot tell you how much concrete to order. Divide the square footage by 9 to get square yards. To order, you need cubic yards — that same area multiplied by the slab thickness in feet, then divided by 27.

Divide the pour volume in cubic feet by the bag yield: an 80 lb bag makes about 0.6 ft³, a 60 lb bag 0.45 ft³, and a 40 lb bag 0.3 ft³. That is 45, 60 and 90 bags per cubic yard. Past roughly 50 bags, order ready-mix instead.

Multiply length by width in feet. A 10 by 12 slab is 120 square feet. Square footage sets the forms, the rebar grid and the finishing labour, but it does not size the pour — for that you also need the thickness, which turns the area into cubic yards.