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Concrete Calculator

How much concrete to order for a slab, footing, post holes or steps, in cubic yards or bags.

What are you pouring?
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Forms bow, ground dips and some concrete always stays in the mixer. Running short mid-pour is far worse than a little left over.
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Price is
Concrete needed 1.36 cubic yards That's 36.7 cu ft (1.04 m³), including 10% extra.

Volume

Cubic feet
36.7 cu ft
Cubic meters
1.04 m³
Cubic yards before extra
1.23 cu yd

Or bags of concrete mix

80 lb bags (0.60 cu ft)
62 bags
60 lb bags (0.45 cu ft)
82 bags
40 lb bags (0.30 cu ft)
123 bags

Cost

How much concrete do I need?

Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27 for cubic yards. Add about 10% extra. For bags, divide the cubic feet by the bag yield: 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.30 for 40 lb. A 10 × 10 ft slab 4 inches thick needs 1.36 cubic yards with 10% extra, or 62 bags of 80 lb mix.

Worked example: a 10 × 10 ft patio slab, 4 inches thick

A concrete slab 10 feet long, 10 feet wide and 4 inches thick: length times width times thickness gives the volume. Length 10 ft Width 10 ft Thickness 4 in 10 × 10 × ⅓ ft = 33.3 cu ft ÷ 27 = 1.23 cu yd · ÷ 0.60 = 56 bags (80 lb)
  1. Volume: 4 in is 1/3 ft, so 10 × 10 × 1/3 = 33.3 cu ft (1.23 cu yd).
  2. Extra: add 10%: 36.7 cu ft.
  3. Cubic yards: 36.7 ÷ 27 = 1.36 cu yd (1.04 m³).
  4. Bags: 36.7 ÷ 0.60 = 62 bags of 80 lb, or 82 of 60 lb, or 123 of 40 lb.

That is a lot of mixing by hand. For anything over about a cubic yard, compare the price of a ready-mix delivery.

Concrete for common slab sizes (4 inches thick)

Including 10% extra. Bags are 80 lb (0.60 cu ft each).

SlabCubic yards80 lb bags
10 × 10 ft1.3662
12 × 12 ft1.9688
10 × 20 ft2.72123
20 × 20 ft5.43245
24 × 24 ft (2-car garage)7.82352

Each extra inch of thickness adds about 25%. At 4 inches, 100 sq ft is 1.23 cubic yards before extra; at 6 inches it is 1.85.

Bags per cubic yard

A cubic yard is 27 cubic feet. Using Quikrete's published yields, it takes:

  • 45 bags of 80 lb concrete mix (0.60 cu ft each)
  • 60 bags of 60 lb (0.45 cu ft each)
  • 90 bags of 40 lb (0.30 cu ft each)

Other brands are close but not identical, so check the yield printed on the bag you buy.

Choosing the right shape

  • Slab: patios, walkways, shed bases and driveways. Enter the finished thickness.
  • Footing or wall: a long strip, such as a footing under a block wall. Length × width × depth, times how many.
  • Post hole or column: round holes for fence or deck posts, or round columns (sonotubes). This fills the whole hole; the post itself takes up a little room, so the result is slightly generous.
  • Steps: solid steps poured on the ground, each one rise taller than the last.

For an odd shape, split it into rectangles, work each one out and add them.

Before you pour

  • Check the thickness your job needs. Walkways and patios are commonly 4 inches; driveways and slabs that carry vehicles are thicker. Your local building department sets the minimum.
  • Order a little extra. Running out halfway through leaves a weak joint. 5 to 10% extra is normal.
  • Ready-mix minimums: many plants charge a short-load fee for small orders. Ask before deciding between bags and a truck.
  • Gravel base: most slabs sit on a compacted gravel base. The gravel calculator works that out.
  • Block walls: for the blocks, mortar and core fill above the footing, use the concrete block calculator.

Frequently asked questions

How many bags of concrete are in a yard?
45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb, based on Quikrete's yields of 0.60, 0.45 and 0.30 cu ft per bag.
How many yards of concrete do I need for a 10 × 10 slab?
At 4 inches thick, 1.23 cubic yards, or about 1.36 with 10% extra. At 6 inches thick, 1.85 cubic yards before extra.
How many 80 lb bags of concrete do I need for a 10 × 10 slab?
About 56 bags at 4 inches thick with no extra (33.3 cu ft ÷ 0.60), or 62 bags with 10% extra.
How much concrete do I need for a post hole?
Work out the hole's volume: π × radius² × depth. A 12 in wide, 4 ft deep hole holds about 3.1 cu ft, roughly 5.2 bags of 80 lb mix with no extra. The post takes up some of that space, so this is slightly generous.
Should I use 40, 60 or 80 lb bags?
They are the same concrete in different sizes. 80 lb bags are the cheapest per cubic foot and mean fewer bags to open; 40 and 60 lb bags are easier to lift and carry.
How do I convert cubic feet of concrete to cubic yards?
Divide by 27. For example, 54 cubic feet ÷ 27 = 2 cubic yards.

Guides

Related calculators

Concrete Block CalculatorBlocks, mortar bags and core fill for any block wall, with cost.Gravel CalculatorTons, cubic yards and bags of gravel for a driveway, path or patio.Asphalt CalculatorTons of hot-mix asphalt for a driveway or patch, with cost.Rebar CalculatorRebar grid for a slab: bars, linear feet, 20 ft sticks and weight.

Sources and method

Last checked October 12, 2026. How we build and test every calculator: methodology.