Concrete slab calculator

Enter the size and thickness of your patio slab. We work out the cubic yards, the bags of 80 lb or 60 lb mix, the gravel under it, the reinforcing and where to cut joints.

A 120 sq ft slab, 4 in thick, takes 1.63 cubic yards with 10% extra. In bags that is 74 bags of 80 lb or 98 bags of 60 lb. Under it, order about 1.6 cu yd (2.3 tons) of gravel. Cut control joints every 8 ft: 2 x 2 panels.

Concrete before waste: 40.0 cu ft
Concrete with 10% extra: 44.0 cu ft, 1.63 cu yd
80 lb bags: 74 at 0.6 cu ft each
60 lb bags: 98 at 0.45 cu ft each
Control joints: 2 cuts, 1.0 in deep, panels about 5.0 x 6.0 ft
Welded wire mesh: 3 sheets of 5 x 10 ft

Bag counts follow the Quikrete Concrete Mix 1101 yields. For more than about 1 cubic yard, a ready-mix truck is usually easier than bags. L-shapes and circles are cut on the bounding rectangle for the joint layout.

Thinking of a deck instead? See ground-level deck plans

A slab is permanent and hard to change. A low deck over the same spot is lighter work and can sit over a slope. It costs more per square foot.

Planning figures for a patio slab on foot traffic, not an engineer's design. Check local rules for thickness, base and reinforcing, and call before you dig.

How the concrete slab calculator works

The concrete slab calculator turns a size and a thickness into the concrete to order and the materials around it. For the 10 by 12 ft example, a 4 in slab holds 40.0 cubic feet. With 10% for waste that is 1.63 cubic yards, 74 bags of 80 lb mix or 98 bags of 60 lb mix.

Cubic yards for a slab

Area in square feet times thickness in feet gives cubic feet, and cubic feet divided by 27 gives cubic yards. Add 5 to 10% for an uneven base, spillage and forms that bow. A slab 6 in thick instead of 4 in takes 2.44 cubic yards in the same example, which is 50% more.

Bags of 80 lb and 60 lb mix

Divide the cubic feet to order by the yield of one bag, and round up. Bags are practical for a small slab. They are heavy: the example is 74 bags of 80 lb, about 3.0 tons to mix by hand. For slabs over about a cubic yard, ask a ready-mix supplier for a price per yard.

What goes under and inside the slab

Gravel base

A compacted gravel base keeps water from collecting under the slab. The calculator starts at 4 in, with 10% extra for compaction. For the 10 by 12 ft example that is 1.6 cubic yards, or 2.3 tons.

Wire mesh or rebar

Mesh comes in sheets of 5 by 10 ft and is lapped 6 in. Rebar is laid on a 24 in grid each way with 10% extra for laps. Both belong in the middle of the slab on chairs, not on the gravel. If you choose neither, the cracks are controlled only by the joints.

Control joints

How far apart

Concrete shrinks as it cures, and it cracks where it is weakest. A control joint is a planned weak line. Space them 2 to 3 feet for each inch of thickness, so every 4 in slab gets a joint every 8 to 12 ft. The 10 by 12 ft example at 8 ft spacing has 2 by 2 panels and 2 cuts.

How deep, and when

Cut each joint a quarter of the slab thickness deep, which is 1.0 in for a 4 in slab. Tool them in while the concrete is wet or saw them as soon as it will take a blade without ravelling.

A slab or a deck?

A slab is permanent and hard to change, and it needs a flat site. A low deck over the same space works on a slope and can be moved or rebuilt later, but it costs more per square foot. See the ground-level deck plans, or count a deck with the deck calculator. If pavers suit you better, use the paver calculator.

Where the numbers come from

Bag yields are the figures Quikrete publishes for Concrete Mix 1101: 0.6 cubic feet for an 80 lb bag and 0.45 cubic feet for a 60 lb bag. Other brands differ by a few percent, so check the bag you buy. One cubic yard is 27 cubic feet.

The control joint rule, 2 to 3 feet of spacing per inch of thickness, is a widely quoted rule of thumb, and the joint depth of one quarter of the slab thickness is from the Portland Cement Association. A 2018 Portland Cement Association publication gives 24 to 30 times the thickness, which is 2 to 2.5 feet per inch, so the calculator starts at the tighter end. The 4 in thickness, the 10% waste, the 4 in gravel base and the reinforcing layouts are editorial and common practice.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

Multiply length by width by thickness, with the thickness in feet, to get cubic feet. Divide by 27 for cubic yards, then add 10% for waste. A 10 by 12 ft slab at 4 in is 40.0 cubic feet, and 1.63 cubic yards with 10% extra.

Take the area in square feet, multiply by the thickness in inches and divide by 12 to get cubic feet. For 120 sq ft at 4 in that is 40 cubic feet. Divide by 27 for cubic yards: 1.48 before waste.

A 10 by 10 ft slab at 4 in thick needs 62 bags of 80 lb mix or 82 bags of 60 lb mix with 10% extra. Quikrete lists 0.6 cubic feet for an 80 lb bag and 0.45 for a 60 lb bag. Past about 1 cubic yard, ready-mix is easier.

A common rule is 2 to 3 times the slab thickness in inches, as feet: a 4 in slab gets a joint every 8 to 12 ft. Keep panels close to square and cut the joints a quarter of the slab thickness deep, soon after the concrete is firm enough to cut.

Not always, and neither stops cracks. Reinforcing holds a crack tight if one forms. A 10 by 12 ft slab takes 3 sheets of 5 by 10 ft wire mesh, or about 14 sticks of No. 3 rebar on a 24 in grid. Ask your building department, since some require it.