Concrete Slab Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Volume (ft³) = Length(ft) × Width(ft) × (Thickness(in) ÷ 12) × SlabsWith waste = Volume × (1 + Waste% ÷ 100)Cubic yards = Volume(ft³) ÷ 27Bags = Volume(ft³) ÷ yield (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45, 40 lb ≈ 0.30)Weight (lb) = Volume(ft³) × 150 (normal-weight concrete)

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short mid-pour, and ready-mix is rounded up to the nearest quarter yard, which is how it is sold.

Worked example

A 12 ft × 12 ft patio at 4 inches thick, with a 10% waste allowance. Inputs: Length 12 ft, Width 12 ft, Thickness 4 in, Number of slabs 1, Waste allowance 10 %. Result: 1.96 yd³ (about 88 × 80 lb bags).

Concrete for common slab sizes

Calculated for a 4-inch slab with a 10% waste allowance. Tap a size to load it in the calculator above.

Slab size	Concrete needed (yd³)	80 lb bags
8 × 10 ft	1.09	49	Use →
10 × 10 ft	1.36	62	Use →
10 × 12 ft	1.63	74	Use →
12 × 12 ft	1.96	88	Use →
12 × 16 ft	2.61	118	Use →
16 × 16 ft	3.48	157	Use →
20 × 20 ft	5.43	245	Use →
20 × 30 ft	8.15	367	Use →
24 × 24 ft	7.82	352	Use →
24 × 24 ft driveway (6 in)	11.73	528	Use →
40 × 60 ft	32.59	1,467	Use →

Worked answers

Method & assumptions

Concrete is sold by the cubic yard (ready-mix) or by the bag (pre-mixed). We compute the slab volume in cubic feet, add your waste allowance, then convert: cubic yards = cubic feet ÷ 27.

Bag counts use the yield printed on U.S. pre-mixed concrete: an 80 lb bag yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³ (QUIKRETE and Sakrete spec sheets). We round bags up, because you cannot buy a partial bag, and round ready-mix up to the nearest quarter yard, which is how it is ordered.

At one cubic yard or more, the result flags a ready-mix comparison because the equivalent bag count is large. It does not prove which option is cheaper or feasible. Confirm thickness from the project requirements, then check product yield, plant minimums, delivery, access, placement capacity and order increments.

Pro tips and common mistakes

Use the specified support. Follow the project requirements for subgrade, base, drainage and compaction. The calculator does not choose a layer depth or guarantee crack prevention.
Order ready-mix slightly high. You cannot top up a slab once the truck leaves, so the built-in waste allowance matters. Rounding up to the next quarter yard is normal, and far cheaper than a cold joint from running short mid-pour.
Match thickness to the load. 4 inches is standard for patios and walkways; go to 5 or 6 inches for driveways and anything carrying vehicles. Too thin will crack under load, so do not shave the thickness to save a few dollars.
Reinforce anything structural. Rebar or welded wire mesh controls cracking on driveways and larger slabs. Size and space it before you pour, our rebar calculator handles the spacing and total length for you.
Plan for weather and curing. Avoid pouring in freezing temperatures or extreme heat, and keep the slab damp for the first few days. Proper curing, not just drying, is what gives concrete its rated strength.

By Calcnaut

Formula and declared-case checks run during the build; sources are listed for readers to review. Read our method and editorial policy.

Frequently asked questions

How much concrete do I need for a 12x12 slab?

A 12 ft × 12 ft slab at 4 inches thick is 48 ft³, which is about 1.78 cubic yards of concrete, or 1.96 cubic yards once you add a 10% waste allowance. At that volume, order ready-mix rather than mixing bags by hand.

How many 80 lb bags of concrete are in a cubic yard?

A cubic yard is 27 ft³, and an 80 lb bag yields about 0.60 ft³, so it takes roughly 45 × 80 lb bags to make one cubic yard. That is why bags only make sense for small jobs.

How thick should a concrete slab be?

Use the slab section specified for the loads, soil support, concrete properties, joints, exposure and local requirements. The calculator converts the thickness entered into volume but cannot determine whether it is adequate.

How much extra concrete should I order for waste?

Add 5-10% for spillage, an uneven subgrade and slight over-excavation. Ready-mix is hard to top up once the truck leaves, so erring slightly high is normal.

Sources and references

These references and structured source records support the method and material data used on this page.

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