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Concrete Slab Calculator

Use this calculator to work out how much concrete you need for a rectangular slab such as a patio, shed base, garage floor or equipment pad. Enter the slab size and thickness, choose a waste allowance and bag size, and you will get the volume to order from a ready-mix supplier or the number of pre-mixed bags to buy.

Enter your measurements

Measurements
ft
ft
in

Typical: 4 in (10 cm) for patios and walkways, 5–6 in (13–15 cm) for driveways.

Material & assumptions
%

Covers spillage, uneven subgrade and form movement. 5–10% is a common allowance.

Yields are typical values. Check the yield printed on your product.

Enter your measurements and press Calculate to see quantities, a material breakdown and an optional cost estimate.

How the calculation works

  • Volume = Length × Width × Thickness
  • Total volume = Volume × (1 + Waste % ÷ 100)
  • Bags = Total volume ÷ Bag yield (rounded up)

All three dimensions must be in the same unit before multiplying. In imperial units, divide the thickness in inches by 12 to convert it to feet, multiply to get cubic feet, then divide by 27 to get cubic yards.

In metric, convert the thickness from centimetres to metres (divide by 100) and the result is in cubic metres.

Assumptions used

  • The slab is rectangular with a uniform thickness.
  • Bag yields are typical label values: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³. Use the custom option for your product's exact yield.
  • Bag counts are rounded up to the next whole bag.

Example calculation

A 20 ft × 12 ft patio poured 4 inches thick with a 10% waste allowance.

  1. 1Convert thickness: 4 in ÷ 12 = 0.333 ft
  2. 2Volume: 20 × 12 × 0.333 = 80 ft³
  3. 3Cubic yards: 80 ÷ 27 = 2.96 yd³
  4. 4Add 10% waste: 2.96 × 1.10 = 3.26 yd³ (88 ft³)
  5. 580 lb bags at 0.60 ft³ each: 88 ÷ 0.60 = 146.7 → 147 bags

Result: Order about 3.25–3.5 yd³ of ready-mix. Mixing 147 bags by hand is a very large job, so ready-mix is the practical choice here.

How to measure your project

  • Measure the inside dimensions of the formwork, not the outside of the boards.
  • Check the thickness at several points. If the subgrade is uneven, use the average depth or the deepest typical value.
  • For L-shaped slabs, split the area into two rectangles, calculate each one and add the results.
  • Thickened edges or footings under the slab should be calculated separately with the concrete footing calculator.

Frequently asked questions

How thick should a concrete slab be?

Patios and walkways are commonly 4 inches (10 cm) thick, while driveways and slabs carrying vehicles are often 5–6 inches (13–15 cm). Local codes, soil conditions and loads can require more. See our slab thickness guide.

How much waste should I add for a concrete slab?

Most people add 5–10%. Use the higher end if the subgrade is uneven, the forms are long or you are placing the concrete by wheelbarrow, because running short in the middle of a pour causes cold joints.

Is it cheaper to use bags or ready-mix concrete?

For small jobs under about one cubic yard, bags are usually convenient. Beyond that, ready-mix is often more economical and far less labour, though suppliers may charge short-load fees for small orders. Compare local prices with the cost estimator.

How many 80 lb bags of concrete are in a cubic yard?

A cubic yard is 27 ft³. With a typical yield of 0.60 ft³ per 80 lb bag, that is 27 ÷ 0.60 = 45 bags, before any waste allowance.

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Results are estimates based on the measurements and assumptions you enter. Always confirm product specifications, local building codes and final quantities with your supplier or a qualified professional. See our disclaimer.