Calculateur de Dalle en Béton — Formule

How to Calculate Concrete for a Slab

Concrete is ordered in cubic yards. To figure out how many you need, calculate the volume of the slab in cubic feet (length x width x thickness), then divide by 27 (since there are 27 cubic feet in a cubic yard). A standard 10x10 slab at 4 inches thick needs about 1.23 cubic yards before waste.

The Formula

Cubic Yards = (Length x Width x Thickness in feet) / 27

If your thickness is in inches, divide by 12 first to convert to feet. A 4-inch slab is 0.333 feet thick.

Always Add a Waste Factor

Order 5-10% extra. Concrete doesn't pour perfectly into forms. Some spills, some sticks to the truck, and ground surfaces aren't perfectly level. For a 3 cubic yard pour, order 3.3 yards. Running short mid-pour is much worse than having a little left over. Most suppliers won't deliver less than 1 cubic yard, and a short load fee applies if you order less than their minimum (usually 3-4 yards).

Typical Slab Thickness

Sidewalks and patios: 4 inches

Driveways (cars): 4-5 inches

Driveways (heavy vehicles): 6 inches

Garage floors: 4-6 inches

Shed foundations: 4 inches

Footings: 8-12 inches minimum (check local code)

Thicker isn't always better. A well-compacted 4-inch slab on proper base material can handle most residential loads. But if the subgrade is poor (clay, organic soil, or uncompacted fill), going thicker helps.

Common Mistakes

Measuring in the wrong units. Mixing inches and feet in the calculation is the number one error. Convert everything to feet before multiplying.

Not accounting for the slab shape. L-shaped patios, curved sidewalks, and stepped foundations need to be broken into rectangles and calculated separately.

Ordering too close to exact. Concrete suppliers charge the same whether you use the last 0.25 yards or not. Having a wheelbarrow of extra is fine. Being half a yard short means a second delivery, cold joint, and a weaker slab.