BuildMath

Concrete calculator

Enter the size of your slab to see how much concrete to buy.

How much concrete do you need?

A 10 × 10 ft slab at 4 inches thick is 1.23 cubic yards of concrete. With the usual 10% extra, order about 1.36 yards, or 62 bags of 80 lb mix.

Measure the slab in feet. Thickness is almost always in inches.

The calculator turns that into cubic feet, divides by 27 for cubic yards, then counts bags from mix yield: 0.60 cubic feet per 80 lb bag, 0.45 per 60 lb bag, and 0.30 per 40 lb bag. Bags always round up.

  1. Enter length and width in feet. For a round pad, open More options and switch the shape to Circle, then enter the diameter.
  2. Enter thickness in inches. Four inches is a common sidewalk or patio slab; a driveway is often 4 to 6 inches.
  3. Leave waste at 10% unless the forms are true and the subgrade is even. You can drag Extra for waste down to 0.
  4. Order ready-mix by the cubic yard, or tap the bag count to switch 80, 60, or 40 lb bags.
  5. Open Estimate cost if you already have a plant quote per yard or a bag price.

The raw 1.23 yards is the geometry. The order line is that volume plus waste. Do not count bags from cured density (about 4,050 lb per cubic yard). Mix bags are sold by how much wet concrete they make, not by how heavy the finished slab is.

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How to calculate concrete

Multiply length by width by thickness, all in feet, then divide by 27. A 10 × 10 ft slab at 4 inches is 100 × (4 ÷ 12) ÷ 27 = 1.23 cubic yards. Add 10% and the order is 1.36 cubic yards, or 62 bags of 80 lb mix at 0.60 cubic feet per bag.

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Worked example: 10 × 10 slab at 4 inches

Worked example

Take a 10 ft by 10 ft rectangle, 4 inches thick.

10 × 10 = 100 square feet. Four inches is 4 ÷ 12 = 0.333 feet. 100 × 0.333 = 33.33 cubic feet. 33.33 ÷ 27 = 1.23 cubic yards.

80 lb bags: 33.33 ÷ 0.60 = 55.56, which rounds up to 56 bags with no extra. Add 10% waste and you buy 33.33 × 1.10 = 36.67 cubic feet, which is 1.36 yards, or 62 bags of 80 lb mix. Switch to 60 lb bags on the same 0% job and 33.33 ÷ 0.45 = 74.07, which rounds up to 75 bags.

That 10 × 10 × 4 inch job is the default on this page, with 10% extra already on the order line. See Bags vs a truck for minimum loads and short-load fees.

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How far one cubic yard of concrete goes

ThicknessArea one cubic yard covers80 lb bags for 100 sq ft
2 in162 sq ft28 bags
3 in108 sq ft42 bags
4 in81 sq ft56 bags
6 in54 sq ft84 bags

Bag counts in that table are before waste, using 0.60 cubic feet per 80 lb bag, rounded up.

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Questions

How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, 1.23 cubic yards. With 10% extra, order 1.36 yards, or 62 bags of 80 lb mix. Change the thickness if your spec is different.
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags of 80 lb (27 ÷ 0.60), or 60 bags of 60 lb (27 ÷ 0.45). A 1.23 yard slab is not a full yard, so it is 56 bags of 80 lb at 0% waste, not 45.
How do I calculate cubic yards of concrete?
Multiply length in feet by width in feet by thickness in feet, then divide by 27. Convert inches by dividing by 12 first. That yardage is what a ready-mix plant wants to hear.
Should I add extra concrete for waste?
Yes for most DIY slabs. Ten percent is the default here and is shown on its own order line so the raw 1.23 yards stays visible. Set waste to 0 if the forms are exact and you are comfortable running short.
How many 60 lb bags for a 10 × 10 slab at 4 inches?
75 bags with no extra, using 0.45 cubic feet of mix per bag. With 10% waste it is 82 bags. Switch the bag size on the result to see 80, 60, and 40 lb side by side.
Can I pour a round pad with this calculator?
Yes. Open More options, choose Circle, and enter the diameter in feet and the thickness in inches. A 4 ft pad 4 inches thick with 10% extra is 8 bags of 80 lb mix.
Is bag count based on concrete density?
No. Density is for a finished slab’s weight. Bags are sold by mix yield: 0.60, 0.45, or 0.30 cubic feet. An 80 lb bag at 150 lb per cubic foot of cured concrete is only about 0.53 cubic feet, so counting bags that way over-orders by about 12 to 13% versus 0.60.
When should I order ready-mix instead of bags?
When the order is much above a few dozen bags, or when you cannot mix and place fast enough. A 10 × 10 × 4 inch slab is already 56 bags before waste. See Bags vs a truck for minimum loads and short-load fees. Keep the cost unit on Per cubic yard.
How much is a yard of concrete?
Typical US price: $160 to $195 per cubic yard (Concrete Network, July 2026), adjusted with U.S. Bureau of Labor Statistics producer price trends (September 2026). Your local price may differ; delivery is extra.
How do I calculate yards of concrete for a slab?
Multiply length in feet by width in feet by thickness in feet, then divide by 27. A 10 × 10 ft slab at 4 inches is 1.23 cubic yards; with 10% extra, order 1.36 yards.
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Written by The BuildMath team. Formulas tested against worked examples. Updated October 21, 2026.