Concrete Slab Calculator
Size a rectangular or L-shaped slab as two rectangles and get the concrete in cubic yards and bags. Leave the second rectangle at zero for a simple slab.
Calculator
A slab of 150 sq ft at 4" is about 50.0 cu ft — 1.85 yd³ or 84 bags of 80 lb. For an L-shape, add the two rectangles.
Most real slabs are not perfect rectangles. A patio wraps around a corner, a shed pad has a bump-out, a driveway apron flares at the street. This calculator handles that the easy way: describe the shape as two rectangles, add their areas, and convert the combined volume to cubic yards and bags. For a plain rectangle, leave the second one at zero.
Formula
Sum the rectangle areas, multiply by thickness, then convert:
area = (L₁ × W₁) + (L₂ × W₂)
cu ft = area × (thickness_in ÷ 12)
cubic yards = cu ft ÷ 27 · bags = ceil(cu ft ÷ bag yield)
Worked example
An L-shaped slab, one 10 × 10 ft bay plus a 5 × 10 ft leg, at 4 in:
- Area = (10 × 10) + (5 × 10) = 100 + 50 = 150 sq ft
- Volume = 150 × (4 ÷ 12) = 50 cu ft
- Cubic yards = 50 ÷ 27 = 1.85 yd³
- Bags (80 lb) = ceil(50 ÷ 0.60) = 84 bags
Breaking a shape into rectangles
For anything more complex than an L, slice the footprint into non-overlapping rectangles on paper first, then add them two at a time. Round curves and diagonals up to the enclosing rectangle — you will slightly over-order, which is exactly what you want on a monolithic pour. If the slab steps down in thickness (a thickened edge, for instance), treat each thickness zone as its own calculation and total the volumes.