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How Much Concrete Do I Need? Slab Formula, Table and Examples

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RenoCalcs Team

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To find how much concrete you need, multiply the length, width and thickness of the slab in feet, then divide by 27 to convert cubic feet to cubic yards. Add 5 to 10% for waste and uneven ground. A 10 × 10 ft slab that is 4 inches thick needs about 1.23 cubic yards, or about 1.36 cubic yards with 10% extra.

Step-by-step: how to calculate concrete for a slab

  1. Measure length and width in feet. For L-shaped areas, split them into rectangles.
  2. Convert the thickness to feet. Divide inches by 12: 4 in = 0.333 ft, 6 in = 0.5 ft.
  3. Multiply length × width × thickness to get cubic feet.
  4. Divide by 27 to get cubic yards, the unit ready-mix suppliers use.
  5. Add a waste allowance of 5 to 10%, more if the ground is uneven.

What each step looks like on site

Here is how those measurements look on a real slab, from the empty form to the finished pour.

Wooden form staked on a gravel base with a tape measure along the length and width
Step 1. Measure the length and width inside the form. For an L-shaped slab, measure each rectangle separately.
Tape measure inside the form measuring slab thickness from the gravel base to the top of the form
Step 2. Measure from the compacted base to the top of the form board. This depth is your slab thickness.
Concrete mix bags next to a wheelbarrow of mixed concrete and a hoe
Step 3. Buy the number of bags the calculator shows, including the waste allowance, and mix one bag at a time.
Gloved hands pulling a straight board across fresh concrete to level the slab
Step 4. Fill the form and pull a straight board across the top in a sawing motion to level the concrete.

Concrete needed for common slab sizes

Volumes below are for a 4-inch-thick slab, without waste. Bag counts use about 0.60 cu ft per 80 lb bag and are rounded up.

Slab sizeCubic feetCubic yards80 lb bags
4 × 4 ft5.30.209
8 × 8 ft21.30.7936
10 × 10 ft33.31.2356
12 × 12 ft48.01.7880
10 × 20 ft66.72.47112

How many bags of concrete are in a yard?

One cubic yard is 27 cubic feet. With typical yields of 0.60, 0.45 and 0.30 cubic feet per bag, that works out to about 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb concrete per cubic yard. Yields vary by brand, so check the bag.

When to order ready-mix instead of bags

Once a pour goes past about one cubic yard, bags become heavy work and it is hard to mix fast enough to keep the slab consistent. Ready-mix trucks deliver the whole volume at once; ask suppliers about minimum orders and short-load fees for small jobs.

Common mistakes

  • Forgetting to convert inches to feet before multiplying, which makes the result 12 times too large.
  • Measuring depth in only one spot on uneven ground.
  • Ordering exactly the calculated amount with no allowance for spillage and form bulges.

This guide covers material quantities only. Slab thickness, base preparation and reinforcement depend on the load and on local building codes; for structural work, consult a qualified professional.

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