How Much Concrete Do I Need for a Slab?

Key takeaways

• Concrete is sold by the cubic yard; 27 cubic feet make one yard, and 4 inches of thickness equals 0.333 feet.
• Add 5 to 10% for spillage, uneven subgrade and thickened edges. Running short mid-pour creates a cold joint you cannot undo.
• Bagged mix only makes sense under about 1 cubic yard. Above that, ready-mix is cheaper and far faster.
• Most ready-mix suppliers charge a short-load fee below 3 to 4 cubic yards, so small pours cost more per yard than the quoted price.

The slab volume formula

Every slab estimate starts with volume in cubic feet and ends in cubic yards, because that is the unit on the ready-mix ticket. The conversion is the only place most people slip: 27, not 3, cubic feet per yard.

1. Measure length and width in feet. Break L-shapes into rectangles and add the areas.
2. Convert thickness from inches to feet: divide by 12 (4 in = 0.333 ft, 5 in = 0.417 ft, 6 in = 0.5 ft).
3. Multiply area x thickness for cubic feet.
4. Divide by 27 for cubic yards.
5. Multiply by 1.05 to 1.10 for the waste allowance, then round up to the nearest quarter yard.

Worked example: a two-car parking pad

A 20 ft x 20 ft pad at 4 inches: 20 x 20 = 400 sq ft. 400 x 0.333 = 133.3 cubic feet. Divided by 27 that is 4.94 cubic yards. With 10% waste you order 5.5 yards.

Now add a thickened edge — a 12 in wide by 8 in deep perimeter footing is common where vehicles drive on. That perimeter is 80 linear feet; the extra depth is 4 inches over a 12 inch width, or 0.333 x 1 x 80 = 26.6 cubic feet, another cubic yard. Total order: 6.5 yards. Skipping this step is the single most common reason a pour runs short.

Thickness by use

Bags versus ready-mix

One 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet, so a cubic yard takes roughly 45 bags. An 60 lb bag yields about 0.45 cubic feet — about 60 bags per yard.

For a 4.94 yard slab that is 222 bags, several tons of material to haul and mix within the working time of the first batch. Any slab over about a cubic yard should be ready-mix, both for cost and because a slab poured in batches will not cure as one monolithic piece.

The base under the slab matters as much as the slab

Nearly every residential slab sits on 4 to 6 inches of compacted crushed stone, which drains water away from the underside and gives a uniform bearing surface. Budget that gravel in the same planning session — a 400 sq ft pad needs about 5 cubic yards of base at 4 inches.

In heated or conditioned spaces, a vapor retarder goes between the stone and the concrete. Where ground freezes, footings must reach below the local frost depth, which your building department publishes.

Estimate versus actual

• Subgrade dips: a half inch of average over-excavation across 400 sq ft is another 0.6 cubic yards.
• Short-load fees: below 3 to 4 yards, suppliers add a flat charge that can exceed the material cost on a tiny pour.
• Pump or wheelbarrow access: no-truck-access sites add pumping cost or hours of labor.
• Weather: hot, dry, windy days accelerate set time and may require retarders or an earlier start.

Frequently asked questions

Sources and references

• Portland Cement Association — Concrete slab basics — Reference for slab thickness, curing and mix proportioning.
• NRMCA — Concrete in practice series — Ready-mix ordering, short loads and placing guidance.