How to Estimate Concrete Volume for Slabs, Footings, and Posts

Ordering concrete is one of the few home projects where guessing is genuinely expensive. You cannot buy a little more later at the same price, and a short pour leaves a cold joint that weakens the whole slab. The fix is unglamorous: measure in feet, convert to cubic yards, and add a waste allowance. Here is the arithmetic for the three shapes that cover almost every job.

The Unit Everyone Forgets: Cubic Yards

Ready-mix concrete is sold by the cubic yard, not by the bag or the square foot. A cubic yard is 3 × 3 × 3 = 27 cubic feet. Every volume you calculate must end in cubic feet before you divide by 27.

Cubic yards = Total cubic feet ÷ 27

Bags are a fallback for small jobs only: an 80-pound bag of concrete mix yields about 0.6 cubic feet, so a full cubic yard is roughly 45 bags. Past about 1 yard, ready-mix delivery is cheaper and far less work.

Shape 1: A Rectangular Slab

Volume = Length × Width × Thickness, all in feet. A 12 ft × 10 ft patio poured 4 inches thick converts thickness to feet first (4 ÷ 12 = 0.333 ft):

• 12 × 10 × 0.333 = 40.0 cubic feet
• 40.0 ÷ 27 = 1.48 cubic yards

Round up to 1.6 yards to cover uneven subgrade and spillage. That half-yard of cushion costs far less than a second truck trip.

Shape 2: Footings and Trenches

Continuous footings are just long, narrow slabs. A 40-foot footing that is 16 inches wide and 8 inches deep:

• Width in feet: 16 ÷ 12 = 1.333
• Depth in feet: 8 ÷ 12 = 0.667
• 40 × 1.333 × 0.667 = 35.6 cubic feet = 1.32 cubic yards

Shape 3: Round Posts and Piers

Use the cylinder formula π × radius² × height. A 10-inch diameter post hole 3 feet deep has a radius of 5 inches, or 0.417 feet:

• π × 0.417² × 3 = 1.64 cubic feet per hole
• Six holes: 1.64 × 6 = 9.83 cubic feet = 0.36 cubic yards

Hand-mixing six bags of fast-setting concrete is normal here; a truck cannot pour 0.36 yards economically.

Putting the Shapes in One Table

The driveway example shows why the units matter. A single inch of thickness across 800 square feet adds 66.7 cubic feet — about 2.5 extra yards, or several hundred dollars. Measure thickness twice.

The Waste Allowance Rule of Thumb

Professional estimators order 5-10% above the calculated volume. Use 5% for flat, well-compacted forms and 10% for irregular shapes, sloped ground, or lots of rebar that displaces less volume than expected. On the driveway above, 10% turns 12.35 yards into 13.6 — order 14.

Why the Math Beats the Eye

Eyeballing works until the shapes stop being simple rectangles. The moment you mix a slab with footings and posts on the same job, you need a single common unit to add them together. That unit is the cubic foot, converted to yards at the end. For irregular or mixed jobs you can add the volumes in a general volume and unit calculator, converting inches to feet as you go and tallying the total before you call the plant.

FAQ

How much does a cubic yard weigh? Standard concrete is about 4,050 pounds per cubic yard; lightweight mixes run lower.

Can I pour in stages? Yes, but each stoppage creates a cold joint. If you must, stop at a planned expansion joint, not mid-slab.

How long until I can walk on it? Typically 24-48 hours for foot traffic, but full cure strength takes about 28 days.

The Bottom Line

Concrete estimating reduces to one habit: get every measurement into feet, multiply to get cubic feet, divide by 27 for yards, then add 5-10% for waste. Do that and you will order the right amount the first time — which is the only version of this project that stays on budget.