How to Calculate Concrete for Footings, Slabs, and Columns

Ordering the wrong amount of concrete is one of the most expensive mistakes in construction. Too little means a second delivery charge and a cold joint. Too much means you’re paying for waste. Here’s how to get the numbers right.

Concrete is Sold by the Cubic Yard

Everything starts with volume in cubic yards (CY). The formula:

Volume (CY) = (Length × Width × Thickness) ÷ 27

The ÷ 27 converts cubic feet to cubic yards (1 CY = 27 CF).

Slab Calculations

Example: 20’ × 30’ slab, 4” thick:

Volume = (20 × 30 × 0.333) ÷ 27 = 200 CF ÷ 27 = 7.41 CY

Add 5–10% waste factor for spillage, low spots, and subgrade variance:

Order: 7.41 × 1.08 = 8.0 CY → round up to 8.5 CY

Typical slab thicknesses:

4” — residential driveway, patio, sidewalk
5” — light commercial, parking
6” — heavy vehicle traffic, industrial

Footing Calculations

Continuous footings (for foundation walls):

Volume (CY) = (Width × Depth × Length) ÷ 27

Example: 16” wide × 10” deep continuous footing, 80 linear feet:

Volume = (1.333 × 0.833 × 80) ÷ 27 = 88.8 CF ÷ 27 = 3.29 CY

Spread footings (for columns/posts):

Volume (CY) = (L × W × D) ÷ 27

Example: 4 footings, each 24” × 24” × 12” deep:

Each footing: (2 × 2 × 1) ÷ 27 = 0.148 CY
4 footings: 0.148 × 4 = 0.59 CY → order 0.75 CY minimum

Note: small pours under about 1 CY are usually better served by bags rather than a ready-mix truck.

Column and Pier Calculations

Round piers (Sonotubes):

Volume (CY) = π × r² × Height ÷ 27

Example: 12” diameter, 4 feet deep:

r = 0.5 ft
Volume = 3.14159 × 0.25 × 4 ÷ 27 = 3.14 CF ÷ 27 = 0.116 CY per pier

For 6 piers: 0.116 × 6 = 0.70 CY. Add 10% waste: order 0.77 CY.

Square columns:

Volume (CY) = Side² × Height ÷ 27

Wall Calculations

Poured concrete walls:

Volume (CY) = (Length × Height × Thickness) ÷ 27

Example: 40’ long × 8’ tall × 10” thick foundation wall:

Volume = (40 × 8 × 0.833) ÷ 27 = 266.7 CF ÷ 27 = 9.88 CY

Don’t subtract for door openings unless they’re large — the rebar cage and pour complexity often means you’ll use the extra anyway.

Waste Factors by Pour Type

Pour Type	Waste Factor
Slabs (flat, good subgrade)	5–7%
Slabs (rough subgrade, grade changes)	8–12%
Walls	5–8%
Footings	5–10%
Round piers	10–15%

When in doubt, add 10%. The cost of a half-yard overage is far less than a second truck.

Ordering Tips

Minimum order charges: Most ready-mix suppliers have a minimum (often 1 CY or a short-load fee). For small pours, pre-mix bags may be cheaper.
Specify PSI: Standard residential is 3,000 PSI. Driveways often spec 4,000 PSI for durability. Ask your engineer if in doubt.
Air entrainment: Required in freeze-thaw climates — specify 5–7% air content.
Slump: Request 4–5” slump for most pours. More than 6” means excess water and reduced strength.
Timing: Have your crew and tools ready. Concrete waits for no one.