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Concrete slab calculator

Enter the slab's length, width and thickness, or pick a preset such as a 4-inch patio or a 6-inch driveway, to see how much concrete to order. Compare bagged mix with a ready-mix truck for bigger pours.

Units
Typical thickness
Extra for waste and spills
Bag size
Concrete to order
1.36 cubic yards
1.04 m³ · 36.7 cu ft, with 10% extra

Bags of concrete mix

80 lb bags (0.6 cu ft each)
62
60 lb bags (0.45 cu ft each)
82
50 lb bags (0.375 cu ft each)
98
40 lb bags (0.3 cu ft each)
123

Standard concrete mix yields. Total weight about 5,500 lb (2,495 kg).

Amounts

Exact volume (no extra)
1.23 cu yd
Cubic feet to order
36.7
Ready-mix order (to the next ¼ yard)
1.5 cu yd
That's 62 bags to mix by hand. Above about 1 cubic yard, a ready-mix truck is usually faster and costs about the same.
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How to work out concrete for a slab

  1. 1

    Pick a typical thickness, such as Patio 4″ or Driveway 6″, or type your own.

  2. 2

    Enter the length and width of the slab. Split an L-shaped slab into two rectangles and add them.

  3. 3

    Keep 10% extra for an uneven base, or more if the ground is soft.

  4. 4

    Order the cubic yards shown for a ready-mix truck, or the bag count for a small pad.

Features

  • Thickness presets

    One click sets the usual thickness for patios, walks, shed bases, driveways and garage floors.

  • Ready-mix rounding

    See the order rounded up to the next quarter yard, the way most plants sell it.

  • Bags for small pads

    For pads under about a cubic yard, the bag count for every bag size is right there.

  • Cost both ways

    Enter a bag price and a price per yard to see which way of buying is cheaper for your slab.

Typical slab thickness

SlabCommon thicknessNotes
Patio, sidewalk, path4 inFoot traffic and furniture
Shed or playhouse base4 inThicken under heavy equipment
Garage floor4 to 6 inThicker for workshops and heavy vehicles
Driveway4 to 6 in6 in for RVs, trucks and boats
Hot tub or AC pad4 to 6 inFollow the manufacturer's spec

These are typical figures, not a design. Soil, climate and local codes can call for more, and anything structural needs an engineer.

Worked examples

  • Driveway, 12 × 24 ft at 5 in: 120 cubic feet, or 4.44 cubic yards. With 10% extra, order about 4.89 yards: that's a ready-mix job.
  • Two-car garage, 20 × 20 ft at 4 in: 133.33 cubic feet, or 4.94 cubic yards before waste.
  • L-shaped patio: measure it as two rectangles, work each one out, and add the cubic yards together. Overlapping corners only count once.

Get the base and edges right

A slab is only as good as what's under it. Strip turf and topsoil, compact the subgrade, and lay a compacted gravel base, usually around 4 inches. Indoors or under a garage, a vapor barrier under the slab keeps ground moisture out. Forms need to be level along their length but can slope slightly away from buildings for drainage: about 1/8 to 1/4 inch per foot is common for patios.

Control joints let the slab crack where you choose rather than at random. Wire mesh or rebar helps hold cracks tight but does not stop them. Plan where joints go before you pour, and cut or tool them the same day.

Work out the gravel base with the gravel calculator, or measure an odd-shaped area with the square footage calculator. For posts and footings, use the full concrete calculator.

Frequently asked questions

How much concrete do I need for a 10 × 10 slab?

At 4 inches thick, a 10 × 10 ft slab is 33.3 cubic feet, or 1.23 cubic yards. With 10% extra, order about 1.36 cubic yards, or 62 bags of 80 lb mix.

How thick should a driveway slab be?

Driveways for passenger cars are commonly 4 to 5 inches thick, and 6 inches where heavier vehicles such as RVs or trucks park. Many builders thicken the edges as well. Your local code may set a minimum.

Do I need gravel under a concrete slab?

Most slabs are poured on a compacted base of gravel or crushed stone, often about 4 inches deep. It gives an even, well-drained surface so the slab does not crack where soft spots settle.

How far apart should control joints be?

A common rule is a spacing in feet of two to three times the slab thickness in inches: every 8 to 12 feet for a 4-inch slab. Cut them about a quarter of the slab depth, and keep panels roughly square.

Is a ready-mix truck worth it for a small slab?

For about a cubic yard or less, bags are usually simpler. Above that, a truck saves hours of mixing and gives one even pour, but ask about short-load fees for orders under the plant minimum.

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