How Much Concrete Do I Need? Slab & Footing Math

Ordering concrete is unforgiving in both directions: come up short mid-pour and you get a visible cold joint; over-order and you pay to dispose of the excess. The estimate itself is simple volume math — the craft is in the unit conversions and the waste allowance. Here's the full method for slabs, footings, and columns.

The core formula

Volume = Length × Width × Thickness

The catch: thickness is almost always measured in inches while length and width are in feet. Convert thickness to feet first (divide inches by 12), or the result will be off by an order of magnitude.

*Example — patio slab:* 12 ft × 10 ft, 4 inches thick. Thickness = 4 ÷ 12 = 0.333 ft Volume = 12 × 10 × 0.333 = 40 cubic feet

Converting to cubic yards

Ready-mix concrete is sold by the cubic yard. One cubic yard = 27 cubic feet:

40 ÷ 27 = 1.48 cubic yards

Working in metric? Volume in m³ is just length × width × thickness with everything in meters — the Volume Converter and Length Converter handle any mixed-unit cleanup.

Add a waste factor

Real sites aren't perfect rectangles. Subgrade dips, form flex, spillage, and pump line loss all consume concrete. Standard practice:

• 5% for simple slabs on well-compacted, even base
• 10% for typical residential work (the safe default)
• 15% for irregular excavations or hand-mixed pours

1.48 × 1.10 ≈ 1.6 cubic yards — that's the order quantity for our patio.

Bags or ready-mix?

An 80 lb bag yields about 0.60 cubic feet of concrete; a 60 lb bag about 0.45. Our 40 ft³ slab would need:

Mixing 74 bags by hand is a brutal day. The usual rule of thumb: below about 1 cubic yard, bags are reasonable; above it, ready-mix wins on cost, consistency, and your back. Many suppliers have a 1-yard minimum, which our patio clears.

Footings and columns

• Rectangular footing: same L × W × D formula. A 20 ft run, 16 in wide, 8 in deep = 20 × 1.33 × 0.667 = 17.7 ft³ ≈ 0.66 yd³.
• Cylindrical column/pier: V = π × r² × height. A 12-inch diameter pier (r = 0.5 ft) at 3 ft deep = 3.1416 × 0.25 × 3 = 2.36 ft³. Twelve of them ≈ 1.05 yd³.

The Concrete Volume Calculator handles all three shapes, applies the waste factor, and converts to yards or bags in one step.

Checklist before ordering

1. Measure the *formed* dimensions, not the plan dimensions — forms move.
2. Probe slab thickness at several points; subgrade is rarely flat.
3. Round up to the supplier's increment (usually quarter yards).
4. Confirm the truck can reach the pour, or budget for a pump.

FAQ

How many cubic feet are in a yard of concrete? 27. A quick sanity check: a cubic yard covers 81 square feet at 4 inches thick.

How thick should a concrete slab be? Common practice: 4 inches for patios and walkways, 4–6 inches for driveways depending on vehicle loads, with local codes taking precedence. Verify requirements with your building department before pouring.

How many 80 lb bags make a cubic yard? About 45 bags (27 ÷ 0.60). It's doable for small jobs, but at that quantity ready-mix is usually cheaper per yard and far more consistent.

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*Get an exact order quantity — slab, footing, or pier — with the free Concrete Volume Calculator.*

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