How to Calculate Concrete

Calculating concrete is a four-step process: measure, convert to a consistent unit, divide by 27 to get cubic yards, then add a waste factor. Below is the math, two worked examples, and the rules of thumb that turn the formula into a real-world order quantity.

Step 1 — Measure your pour

Length and width go in feet; thickness goes in inches. Concrete thickness is conventionally given in inches because slabs are relatively thin (4–8″) and using feet would produce confusing decimal fractions.

For irregular shapes, break the pour into rectangles, calculate each separately, and sum the volumes. L-shaped patios, stepped foundations, and wraparound walks are all just multiple rectangles.

Step 2 — Convert to cubic feet

Multiply length × width × (thickness ÷ 12). The division by 12 converts thickness from inches to feet so all three dimensions are in the same unit.

Step 3 — Convert to cubic yards

Concrete is sold by the cubic yard in the US. Divide your cubic feet by 27 (a cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³).

Step 4 — Add a waste factor

Real-world pours never use exactly the calculated volume. Sub-grade settles, forms leak, and a small amount spills during placement. Add a waste factor — multiply by (1 + waste%):

• 10% — slabs, footings, walls. The industry default.
• 5% — sonotubes and round columns. Forms are clean and the volume is predictable.
• 15% — DIY pours over uneven sub-grade.

Worked example: a 20 × 10 × 4″ patio

1. Volume in ft³: 20 × 10 × (4 ÷ 12) = 66.67 ft³.
2. Convert to cubic yards: 66.67 ÷ 27 = 2.47 yd³.
3. Add 10% waste: 2.47 × 1.10 = 2.72 yd³ — order ~2.75 yd³.
4. Bagged equivalent (60 lb bags): 73.34 ft³ ÷ 0.45 ft³/bag ≈ 163 bags.

At ~$160/yd³ delivered plus a $60 short-load fee (under the 3 yd³ threshold), that's about $500 in concrete — before forms, gravel base, and labor.

Worked example: a strip footing

A 40-foot footing, 16″ wide and 8″ deep, for a one-story addition:

1. Convert width and depth to feet: 16 ÷ 12 = 1.33 ft, 8 ÷ 12 = 0.67 ft.
2. Volume: 40 × 1.33 × 0.67 = 35.55 ft³.
3. Cubic yards: 35.55 ÷ 27 = 1.32 yd³.
4. Add 10% waste: 1.32 × 1.10 = 1.45 yd³.

Below the 3 yd³ short-load threshold, so expect a delivery fee — or consider bagged for this size pour.

Slab thickness rules of thumb

• 4″ — sidewalks, patios, standard driveways, garage floors
• 5–6″ — heavy / RV driveways, light commercial
• 8″+ — engineered slabs, high-load commercial

Always confirm with local building code. Thickness requirements depend on soil bearing capacity, freeze-thaw cycles, and intended load.

Use the calculators

Skip the math and use one of the built-in calculators — they apply the waste factor and produce yards, bag counts, weight, and cost in one pass:

• Concrete Slab Calculator — the homepage tool, with thickness presets
• Footing calculator
• Round column calculator
• Bag calculator — for DIY pours
• Cost calculator — ready-mix vs. bagged comparison