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Concrete Calculator

Calculate cubic yards of concrete for a slab or footing, plus 60 and 80 lb bag counts with a waste factor.

Results update instantly.Change any value and the estimate recalculates on the spot.

Everyday

Concrete

Find cubic yards and bag counts for a slab or footing.

ft
ft
in
%

Concrete needed

-

  • Cubic meters-
  • 60 lb bags-
  • 80 lb bags-

Estimate only. Bag yields vary by mix; check the bag label. Order a little extra - running short mid-pour is worse than leftovers.

How to use this calculator

1. Pick slab or footing. 2. Enter length, width, and thickness. 3. Keep the 10% waste allowance unless your forms are exact, then read cubic yards and bag counts.

Worked example

A 10 by 10 ft slab, 4 inches thick, with 10% waste: about 1.36 cubic yards (1.04 cubic meters), or 82 sixty-pound bags, or 62 eighty-pound bags.

How the math works

Volume = length x width x thickness (converted to feet), divided by 27 for cubic yards. The waste percentage pads the result for spillage and uneven subgrade. A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.6 cubic feet, so bags = volume / yield, rounded up.

How thick should a slab be?
4 inches suits walkways and patios; driveways usually need 5 to 6 inches. Footings follow local frost depth and load rules - check your building code.
Why add a waste allowance?
Forms are never perfect, some concrete sticks to the truck and tools, and subgrade varies. Ten percent is the standard cushion for small jobs.
How many bags for a small repair?
Use the bag counts. A single 80 lb bag covers about 0.6 cubic feet - fine for a fence post hole, not for a slab.
Do I need rebar or mesh?
Most slabs benefit from wire mesh or rebar for crack control, but that is a structural choice, not a volume one. Ask your supplier or local code.

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How it works

How the Concrete Calculator works

Quick answer: A 10 × 10 ft slab, 4 inches thick, with 10% waste needs 1.36 cubic yards of concrete, or 62 80-lb bags.

Formula

Cubic feet = length × width × (thickness in inches ÷ 12) × (1 + waste)
Cubic yards = cubic feet ÷ 27

An 80-lb bag yields about 0.6 cubic feet; a 60-lb bag about 0.45 cubic feet.

Worked example

10 ft × 10 ft × 4 in with 10% waste: 1.36 cubic yards (1.04 m³), 82 bags at 60 lb or 62 bags at 80 lb.

10 × 10 ft slab, 10% waste

ThicknessCubic yards80-lb bags
3 in1.0246Open
4 in1.3662Open
5 in1.7077Open
6 in2.0492Open

Assumptions and limits

  • Ready-mix trucks often have a minimum order; bags are practical for small pours.

Frequently asked questions

How thick should a slab be?

4 inches suits walkways and patios; driveways usually need 5 to 6 inches. Footings follow local frost depth and load rules - check your building code.

Why add a waste allowance?

Forms are never perfect, some concrete sticks to the truck and tools, and subgrade varies. Ten percent is the standard cushion for small jobs.

How many bags for a small repair?

Use the bag counts. A single 80 lb bag covers about 0.6 cubic feet - fine for a fence post hole, not for a slab.

Do I need rebar or mesh?

Most slabs benefit from wire mesh or rebar for crack control, but that is a structural choice, not a volume one. Ask your supplier or local code.

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