How to Calculate Concrete for a Slab, Footing, or Column

Ordering concrete is expensive and unforgiving — short one yard and you're stuck with a cold joint; order too much and you pay for what goes in the dumpster. Here's how pros calculate concrete for slabs, footings, and columns without guessing.

The core formula

Every concrete calculation — slab, footing, column, or stair — is just volume math. The formula is the same:

Volume (cubic yards) = (Length × Width × Thickness in feet) ÷ 27

The divisor is 27 because one cubic yard equals 27 cubic feet (3 ft × 3 ft × 3 ft). From there, the only difference between shapes is how you measure them.

Step-by-step for a concrete slab

Measure length, width, and thickness in feet

Convert inches to feet by dividing by 12. A 4-inch slab is 0.333 ft thick. A 6-inch slab is 0.5 ft. This is the step most DIYers get wrong — plugging inches into a cubic-yard formula gives results that are 1,728× too large.

Multiply to get cubic feet

For a 20 ft × 30 ft × 4 in slab: 20 × 30 × 0.333 = 200 cubic feet.

Divide by 27 to get cubic yards

200 ÷ 27 = 7.41 cubic yards. This is the raw concrete needed, before waste.

Add waste factor (5–10%)

Spillage, over-excavation, uneven subgrade, and form bulging eat concrete. Industry standard is 5% for slabs with rigid forms and 10% for footings or rough subgrade. For the slab above: 7.41 × 1.05 = 7.78 cubic yards — order 8 yards (most plants sell in 0.25 yard increments, but you rarely want to cut it that close).

Convert to bags (only for pours under 1 yard)

If you're mixing from pre-mix bags instead of calling a ready-mix truck:

1 cubic yard ≈ 45 × 80 lb bags (Quikrete, Sakrete)
1 cubic yard ≈ 60 × 60 lb bags
1 cubic yard ≈ 90 × 40 lb bags

Above 1 cubic yard, bags become both more expensive and more labor-intensive than ready-mix delivery.

Footings

A footing is just a long, narrow slab. Same formula: Length × Width × Depth ÷ 27. A 30 ft continuous footing, 18 inches wide and 12 inches deep, works out to 30 × 1.5 × 1 = 45 cubic feet = 1.67 cubic yards. Add 10% waste for dug trenches: 1.84 cubic yards.

For stepped footings, calculate each step separately and sum.

Columns and piers

Round columns use the cylinder formula:

Volume = π × radius² × height ÷ 27

A 10-inch diameter column, 8 feet tall: radius = 0.417 ft, so π × 0.417² × 8 = 4.37 cubic feet = 0.162 cubic yards. Add 10% waste: 0.178 cubic yards. Below 0.25 cubic yards, use bags.

Stairs

Stairs are the ugliest calc in the book because every step is a wedge. The shortcut:

Calculate the triangular side area: ½ × total rise × total run
Multiply by stair width
Divide by 27

This gives the minimum volume (the wedge under the steps). For cast-in-place stairs, add 20% to account for the actual tread/riser geometry that sits above the wedge.

Pro tip: ready-mix plants usually charge a “short-load fee” for deliveries under 3–5 cubic yards (typically $100–$250 depending on market). If you're borderline, it's often cheaper to order the minimum truck load and waste the extra.

Common mistakes

Using inches in the formula. Cubic yards requires all dimensions in feet.
Forgetting waste factor. Your slab will always need 5–10% more than the geometric volume. Concrete doesn't "stretch."
Rounding down. Never round down when ordering. If the math says 7.41 yards, don't order 7 — you'll be paying an emergency short-load fee when the truck runs out before the pour finishes.
Ignoring slab reinforcement volume. Rebar and welded wire mesh take up negligible volume, so you can ignore them. Post-tension cables similarly. But if you have large drain pockets, subtract their volume.

Skip the math — use the calculator

BuildCalc Pro's concrete calculator handles slabs, footings, columns, and stairs automatically, including bag counts and waste factor. Free to use, no signup required.

Open Concrete Calculator →