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 →

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.

Above roughly one cubic yard, hand-mixing bags stops making sense on cost and labor, so the result recommends ready-mix. Confirm the thickness your project needs: 4 inches is standard for patios and walkways, 5-6 inches for driveways and anything carrying vehicle loads.

Pro tips and common mistakes

Prep a compacted base first. Pour over 4 to 6 inches of compacted gravel, not bare soil. A solid, level subgrade stops the slab from cracking and settling, and keeps your concrete volume accurate to what you ordered.
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.

How we check this

Every formula is shown in full and verified with automated tests against manufacturer spec sheets and ASTM and ACI standards. We show our work so you can check it.

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?

Four inches is standard for patios, walkways and shed bases. Use 5-6 inches for driveways or anything that carries vehicles. Structural slabs should follow an engineer or local code.

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.

References

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