Short answer: multiply the slab's length by its width (in feet) to get square feet, multiply by the thickness (inches divided by 12) to get cubic feet, add a margin, then divide by the volume one bag yields. A 10 × 10 ft slab poured 4 in thick needs 33.3 cubic feet of concrete, about 36.7 cubic feet (1.36 cubic yards) with a 10 percent margin. That is 62 bags of 80 lb mix, 82 bags of 60 lb mix or 123 bags of 40 lb mix, using the yields on Quikrete's Concrete Mix 1101 data sheet: 0.6, 0.45 and 0.3 cubic feet per bag.

The thickness is the number no formula can give you. It depends on what the concrete will carry, the soil under it and local requirements, so ask your supplier or a professional. This guide uses 4 in only as an example.

If you just want the answer for your own measurements, use the concrete calculator. Read on to understand the result, compare bag sizes or decide whether bags are the right way to buy.

The formula, step by step

  1. Area in square feet. Length × width for a rectangle. For other shapes, split the slab into rectangles and circles and add them up.
  2. Thickness in feet. Divide inches by 12. Four inches is 0.333 ft.
  3. Volume in cubic feet. Area × thickness in feet.
  4. Margin. Multiply by 1.10 to add 10 percent.
  5. Bags. Divide by the yield of one bag and round up.

Round up only at the end. Rounding each step separately is how a count ends up one or two bags off.

Worked example: a 10 × 10 ft slab, 4 in thick

Step Calculation Result
Area 10 ft × 10 ft 100 ft²
Thickness 4 in ÷ 12 0.333 ft
Volume 100 ft² × 0.333 ft 33.33 ft³
With 10% margin 33.33 × 1.10 36.67 ft³
Cubic yards 36.67 ÷ 27 1.36 yd³
80 lb bags at 0.6 ft³ 36.67 ÷ 0.6 = 61.1 62 bags

The 62 bags weigh 4,960 lb before water (62 × 80 lb), which is worth knowing before you plan how to move them.

40, 60 or 80 lb bags?

The bag size changes the number of bags, not the amount of concrete. On the data sheet, the three sizes yield 0.3, 0.45 and 0.6 cubic feet, which is the same 0.0075 cubic feet per pound, so one cubic yard is 90, 60 or 45 bags.

Slab, 4 in thick Cubic yards (+10%) 40 lb bags 60 lb bags 80 lb bags
4 × 4 ft 0.22 20 14 10
10 × 10 ft 1.36 123 82 62
12 × 12 ft 1.96 176 118 88
20 × 20 ft 5.43 489 326 245

Smaller bags are easier to carry and mix in small batches but cost more handling for the same volume. Mixing a few hundred bags by hand is a different job from mixing five.

Quick reference: bags per 100 square feet

These values come from the same formula (area × thickness ÷ 27) with the 10 percent margin. Scale them by your area divided by 100.

Thickness Cubic yards (+10%) 40 lb bags 60 lb bags 80 lb bags
3 in 1.02 92 62 46
4 in 1.36 123 82 62
5 in 1.70 153 102 77
6 in 2.04 184 123 92

A post or a round column

Round shapes work the same way: the area of the circle is π × radius², and the "thickness" is the depth. A 1 ft diameter hole 4 ft deep holds 0.785 ft² × 4 ft = 3.14 cubic feet, or 0.116 cubic yards. With the 10 percent margin it is 3.46 cubic feet, which is 6 bags of 80 lb (5.8 rounded up). For a hole this small the margin is a few handfuls, so the bag count is the same with or without it.

Use the yield on your bag

The yields above belong to one manufacturer's mix. Other mixes, other brands and mixes with coarser stone can yield differently, and the label on the bag you actually buy is the number to use. The calculator has a field for it. A 12 × 8 ft slab, 6 in thick, with a 5 percent margin, needs 68 bags if the label says 0.75 cubic feet, but 84 bags at 0.6 cubic feet. The yield matters as much as the margin.

Bags or ready-mix?

For a post, a small pad or a repair, bags are practical. As the count climbs, so do the weight you have to carry and the batches you have to mix: the 20 × 20 ft slab above is 245 bags of 80 lb, close to 20,000 lb of dry mix. Ready-mix delivered by truck is usually the practical choice for jobs that size, and suppliers set minimum loads, so ask what yours is before you decide. We do not publish prices, which vary by supplier and location.

Common mistakes

  • Mixing units. Entering 4 as feet instead of 4 inches makes the answer twelve times too big.
  • Forgetting the margin. Uneven ground, spills and formwork use more concrete than the geometry says, and a pour that comes up short is hard to fix.
  • Rounding down. Round the bag count up, never down.
  • Treating one yield as fact. The label on your bag wins over any table, including ours.
  • Ignoring the base. The concrete volume does not include gravel, forms or rebar. For the stone under a slab, see how much gravel you need.

What this guide cannot tell you

  • The right thickness or depth, which depends on use, soil and local requirements.
  • Reinforcement, footings and structural design. Ask a professional for anything structural.
  • The price. It varies by supplier and location.

The calculator gives planning estimates, not engineering advice. Our assumptions are listed on the methodology page.

Frequently asked questions

How many 80 lb bags of concrete make a cubic yard?

45, if each bag yields 0.6 cubic feet: 27 cubic feet in a cubic yard divided by 0.6. With a 10 percent margin you would plan for about 50.

Do 40, 60 and 80 lb bags give the same amount of concrete per pound?

On the Quikrete 1101 data sheet they do: 0.3, 0.45 and 0.6 cubic feet are all 0.0075 cubic feet per pound. Other mixes and brands differ, so use the yield printed on your bag.

How many bags for a 10 × 10 ft slab 4 inches thick?

62 bags of 80 lb, 82 bags of 60 lb or 123 bags of 40 lb, with a 10 percent margin.

Should I add extra concrete to my order?

A margin for uneven ground, spills and formwork is normal. Our calculator defaults to 10 percent, which is a planning assumption you can change, not a standard.

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