How Much Concrete Do I Need for a Slab? A 10×10 Example

The 10 x 10 slab, step by step

Let’s start with a real case instead of a formula. You want a small square slab, 10 feet long and 10 feet wide, and 4 inches thick. How much concrete is that?

Step 1: convert the thickness to feet. Four inches is 4 / 12 of a foot, or 0.3333 ft.

Step 2: multiply. 10 x 10 x 0.3333 = 33.33 cubic feet.

Step 3: convert to cubic yards. There are 27 cubic feet in a cubic yard, so 33.33 / 27 = 1.23 cubic yards.

That is the volume of the slab itself, with nothing spilled and nothing left in the wheelbarrow. Real jobs need a bit more, which brings us to waste.

The formula in one line

For any rectangular slab in imperial units:

cubic feet = length (ft) x width (ft) x (thickness in inches / 12)

cubic yards = cubic feet / 27

Everything else in this guide builds on those two lines. Keep the units straight: length and width in feet, thickness in inches, and the division by 12 handles the mismatch.

Why a waste allowance is built in

Ground is rarely perfectly level, forms are not perfectly tight and a little always gets spilled or left behind. A waste allowance adds a margin so you do not come up short. Our calculator starts at 10%, and you can change it.

To apply it, multiply the volume by 1 plus the allowance as a fraction. For our 10 x 10 example: 33.33 x 1.10 = 36.67 cubic feet, which is 36.67 / 27 = 1.36 cubic yards.

You can see how sensitive the result is by trying other allowances on the same slab. At 5% the volume is 33.33 x 1.05 = 35.00 cubic feet, or 1.30 cubic yards. At 15% it is 33.33 x 1.15 = 38.33 cubic feet, or 1.42 cubic yards. Each five points of allowance adds about 1.67 cubic feet to this slab, which is a modest amount but a real one.

Ten percent is a starting point, not a rule. If you want to see the effect of no allowance at all, set it to 0 and compare. The difference for this slab is about 3.33 cubic feet, or 0.12 cubic yards.

Turning volume into bags

If you are mixing from bags, you need a count. Each bag makes a certain volume of concrete, called its yield, and it is printed on the bag. Common yields are 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. Check the label on the bag you actually plan to buy, because yields differ.

Divide the volume by the yield and round up, since you cannot buy part of a bag.

With 10% waste you need 123 bags of 40 lb, 82 bags of 60 lb or 62 bags of 80 lb. That is a lot of bags, which is worth knowing before you start.

Other sizes at a glance

Changing the length or thickness changes everything in proportion. Here are two more examples using the same formulas and the same 10% allowance.

Notice what thickness does. Going from 4 inches to 6 inches on the same 10 x 10 footprint raises the volume by half, from 33.33 to 50.00 cubic feet. A small change in depth is a big change in concrete.

If you measure in metres

The same idea works in metric. Enter length and width in metres and thickness in centimetres. The volume in cubic metres is length x width x (thickness / 100).

For a slab of 3 m by 3 m and 10 cm thick, that is 3 x 3 x 0.10 = 0.90 cubic metres. With 10% waste it is 0.99 cubic metres. To compare with the imperial figures, multiply cubic metres by 1.30795 to get cubic yards: 0.99 x 1.30795 is about 1.29 cubic yards. In bags, using the same yields as above, that is 117 bags of 40 lb, 78 bags of 60 lb or 59 bags of 80 lb.

Questions to settle before you order

The arithmetic is the easy part. These are the decisions that change the real quantity. For each one, ask a local supplier or a qualified professional. We do not set rules for thickness, reinforcement or base here, because they depend on your site and local requirements.

1. Is the ground level, or will the slab need to be deeper in places?
2. What thickness is suitable for how the slab will be used?
3. Will there be anything under the concrete, such as a prepared base, that changes the depth?
4. Is the slab one simple rectangle, or do you need to split it into sections?
5. Will you mix from bags, or is the amount large enough to ask about delivered concrete?
6. Have you checked the yield on the bag you plan to buy?

When the total climbs towards a couple of cubic yards, hauling and mixing that many bags becomes a serious job. Our guide to how many bags of concrete are in a cubic yard compares bag sizes, and a local ready-mix supplier can tell you what suits a bigger pour.

If your project also includes a roof or other structure, how to calculate roof pitch covers that side of the maths, and the Home and Energy Calculators page lists the other tools.

Frequently asked questions

Tools used in this guide

• Concrete Slab Calculator

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Photo: Mélyna Côté on Unsplash