Concrete slab calculator

How the slab calculator works

A slab is a rectangular block, so its volume is length times width times thickness. The only trap is units: length and width are in feet, but thickness is in inches, so divide thickness by 12 first. Concrete is ordered in cubic yards, and one cubic yard is 27 cubic feet.

Volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12

Cubic yards = volume (cu ft) ÷ 27

Cubic yards to order = cubic yards × (1 + waste %), rounded up to the next quarter yard

The calculator also divides the volume with waste by the yield of one bag to get a bag count, and always rounds up to a whole bag.

Worked example

A patio 24 ft long and 12 ft wide, poured 4 in thick:

1. Thickness in feet: 4 ÷ 12 = 0.3333 ft
2. Volume: 24 × 12 × 0.3333 = 96 cu ft
3. Cubic yards: 96 ÷ 27 = 3.556 cu yd
4. With 10% waste: 96 × 1.10 = 105.6 cu ft = 3.911 cu yd, so order 4 cu yd
5. Bags of 80 lb mix: 105.6 ÷ 0.60 = 176 bags

Four cubic yards is a ready-mix job. Mixing 176 bags by hand would take most of a day for two people, so the calculator recommends a truck unless the prices you enter say otherwise.

What to enter

• Length and width. Measure inside the forms in feet. For an L-shaped slab, split it into two rectangles, calculate each, and add the results.
• Thickness. 4 in is standard for patios, sidewalks and shed bases. Garage floors are often 4 to 6 in. Check depth at several points and use the deepest, because ground is rarely flat.
• Number of slabs. For several identical pads, such as a row of equipment pads, enter the count.
• Extra for waste. 10% is the usual allowance. Use more on rough ground or a long, narrow pour. See the waste factor guide.
• Prices. Optional. Enter what your store charges per bag and what your plant quotes per cubic yard delivered, and the calculator compares them.

Limitations

This is a material estimate, not an engineering design. The result can be off when:

• The ground is sloped or uneven, so the real thickness varies. Measure at the deepest point or add more waste.
• The slab is not a rectangle. Split curves and odd shapes into rectangles and circles, or use the all-in-one calculator for round pads.
• Rebar or mesh displaces a small amount of concrete. The effect is usually under 1% and is covered by the waste allowance.
• Forms bow outward under the weight of wet concrete, adding volume. Brace forms well and keep the 10%.
• Thickened edges or footings under the slab are not included. Add them with the footing option.

Slab thickness, reinforcement and base preparation should follow your local building code and, for anything structural, an engineer’s design.

Frequently asked questions