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Concrete Slab Calculator

Length × width × thickness, with the bags or yards to order. Works for patios, shed pads, sidewalks, driveways and garage floors. Results update as you type.

What are you pouring?
Units

Slab, patio or driveway

4 in is typical for patios and walks; 5–6 in for driveways and garage floors.

Bag size
Add your prices (optional)

Use your supplier's prices. We don't show average prices because they vary too much by area.

Concrete needed

1.36 cubic yards

36.7 ft³ · 1.04 m³ · includes 10% extra

You need about 1.36 cubic yards of concrete, or 62 bags of 80 lb mix.

Bags of premix

  • 12340 lb bags
  • 9850 lb bags
  • 8260 lb bags
  • 6280 lb bags
  • Before extra1.23 yd³ (33.3 ft³)
  • Weight of bags4,960 lb
  • Ready-mix costAdd a price per yd³
  • Bagged costAdd a price per bag

One 80 lb bag makes about 0.60 ft³ (manufacturer's published yield). Bags are rounded up.

At 1.36 yd³ (62 × 80 lb bags, 4,960 lb), bags and a small ready-mix load are both reasonable. Ask local suppliers for their minimum load and short-load fee before deciding.

Estimate only, not a quote or engineering advice. Disclaimer

How to calculate concrete for a slab

  1. Multiply length by width in feet to get the area in square feet.
  2. Divide the thickness in inches by 12 and multiply: that is the volume in cubic feet.
  3. Divide cubic feet by 27 for cubic yards, the unit ready-mix is sold in.
  4. Add extra for waste and an uneven base. This calculator adds 10% unless you change it.
  5. For bags, divide the cubic feet by the bag's yield (0.60 ft³ for an 80-lb bag) and round up.
Cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27

Example: a 12 × 16 ft patio 4 inches thick is 192 sq ft × 4 ÷ 12 = 64 ft³, or 2.37 yd³. With 10% extra that is 2.61 yd³, or 118 bags of 80-lb mix. That many bags is a lot of mixing; see ready-mix vs bags.

Concrete per 100 square feet

Volume for every 100 sq ft of slab, before waste
Thicknessft³yd³sq ft per yd³80-lb bags
3.5 in29.21.089349
4 in33.31.238156
5 in41.71.546570
6 in50.01.855484
8 in66.72.4741112

How thick should a slab be?

Four inches is the usual thickness for patios, walkways and shed pads. Driveways, garage floors and pads for heavy items are often poured 5 to 6 inches, sometimes with thickened edges. Soil, climate and what will sit on the slab all matter, so follow your local building department or the plans for anything structural.

Odd shapes

Break an L-shaped or irregular slab into rectangles, run each one, and add the results. For a round slab, use the column calculator with the slab thickness as the height (convert inches to feet: 4 in = 0.33 ft).

Common sizes have their own answer pages: 10×10, 12×12, 20×20 and 24×24.

Frequently asked questions

How many yards of concrete do I need for a slab?

Multiply length × width in feet by the thickness in inches, divide by 12, then by 27. A 10 × 10 ft slab 4 inches thick is 1.23 yd³, or 1.36 yd³ with 10% extra.

How many square feet does a yard of concrete cover?

One cubic yard covers 81 sq ft at 4 inches thick, 65 sq ft at 5 inches and 54 sq ft at 6 inches.

How many bags of concrete for a 4-inch slab?

Each 80-lb bag (0.60 ft³) covers about 1.8 sq ft at 4 inches thick, so plan on roughly 56 bags per 100 sq ft before waste, or 62 with 10% extra. With 60-lb bags (0.45 ft³) it is about 75 per 100 sq ft before waste.

Should I add extra concrete for waste?

Yes. Bases are never perfectly flat, forms bow, and some concrete is left in the mixer or truck. Five to ten percent extra is common; use more for small or irregular pours. Running short in the middle of a pour is worse than a little left over.

Related

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