Concrete
How Much Concrete Do I Need? A Step-by-Step Guide
Calculate concrete for any slab, driveway, patio, or footing: the exact formula, worked examples, a common-projects table, waste factors, and ordering rules.
Under-ordering concrete is the most expensive small mistake in DIY construction. Ready-mix sets in about 90 minutes; if the truck's contents run out before the forms are full, the pour stops, a cold joint forms where new concrete later meets old, and you pay a second delivery fee for the balance. Over-ordering wastes money at $150–$200 per cubic yard. This guide gives you the exact method to hit the number, with worked examples you can follow line by line.
Step 1: Measure in consistent units
Measure length and width in feet. Measure thickness in inches - but convert it to feet (divide by 12) before multiplying. Mixing feet and inches without converting is the most common estimating error, and it produces results that are off by a factor of twelve.
Step 2: Apply the volume formula
Volume (yd³) = Volume (ft³) ÷ 27
Concrete is sold by the cubic yard, and one cubic yard is 27 cubic feet. A useful sanity check: one cubic yard covers 81 square feet at 4 inches thick. If your 400 ft² driveway estimate comes out at 2 yards, something's wrong (it should be about 5).
Step 3: Add a waste factor
Real slabs consume more concrete than geometry predicts. Forms deflect outward under the weight of wet concrete, the subgrade is never perfectly flat, and some material always stays in the chute, pump, and wheelbarrow. Use:
| Condition | Waste factor |
|---|---|
| Simple rectangle, firm compacted base, experienced crew | 5% |
| Typical residential slab or driveway | 10% |
| Irregular shape, sloped or rough subgrade, pump placement | 10–15% |
Step 4: Round up to the ordering increment
Ready-mix plants batch in half-yard steps. After adding waste, round up. If the result is under 3–4 yards, expect a short-load fee ($50–$150) - at small volumes, compare against bagged concrete using our bags vs. ready-mix comparison.
Skip the arithmetic. The concrete calculator applies this exact method and shows bags, weight, and cost too.
Open the Concrete CalculatorWorked example 1: 15 × 20 ft patio, 4 in thick
- Thickness: 4 ÷ 12 = 0.333 ft
- Volume: 15 × 20 × 0.333 = 100 ft³
- Cubic yards: 100 ÷ 27 = 3.70 yd³
- Waste (10%): 3.70 × 1.10 = 4.07 yd³
- Order: 4.5 cubic yards
Worked example 2: footing trench with post holes
A 24 ft fence line with six 12-in-diameter post holes, 3 ft deep:
- One hole: π × r² × depth = 3.1416 × 0.5² × 3 = 2.36 ft³
- Six holes: 14.1 ft³ = 0.52 yd³
- With 10% waste: 0.58 yd³ - far below ready-mix minimums, so use bags: 14.1 × 1.10 ÷ 0.6 = 26 bags of 80 lb mix
Common projects table
| Project | Dimensions | Thickness | Volume | Order (10% waste) |
|---|---|---|---|---|
| Shed slab | 8 × 12 ft | 4 in | 1.19 yd³ | 1.5 yd³ |
| Small patio | 10 × 10 ft | 4 in | 1.23 yd³ | 1.5 yd³ |
| Large patio | 15 × 20 ft | 4 in | 3.70 yd³ | 4.5 yd³ |
| Sidewalk | 4 × 50 ft | 4 in | 2.47 yd³ | 3 yd³ |
| 1-car driveway | 12 × 20 ft | 4 in | 2.96 yd³ | 3.5 yd³ |
| 2-car driveway | 20 × 24 ft | 5 in | 7.41 yd³ | 8.5 yd³ |
| Garage slab | 24 × 24 ft | 5 in | 8.89 yd³ | 10 yd³ |
Irregular shapes
L-shapes: split into two rectangles, calculate each, add. Circles: volume = π × radius² × thickness. Slopes and thickened edges: many slabs go from 4 in in the field to 8–12 in at the perimeter; treat the thickened band as its own rectangle at the deeper dimension. When a slab varies continuously in thickness, use the average depth - measure at several points, not just the ends.
When to call a professional
This method covers non-structural flatwork. Slabs carrying structures or heavy vehicles, foundations, and anything on soft, expansive, or filled ground need design by a licensed engineer - thickness, reinforcement, and base requirements come from loading and soil conditions, not from a calculator. Most building departments require permits for structural concrete.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
At 4 inches thick, a 10x10 slab needs 1.23 cubic yards. Add 10% waste and round up: order 1.5 cubic yards of ready-mix or about 62 bags of 80 lb mix.
How do I calculate concrete in cubic yards?
Multiply length x width in feet, multiply by thickness in inches divided by 12, then divide by 27. Example: 12 x 20 ft at 4 in = 12 x 20 x 0.333 / 27 = 2.96 cubic yards.
How much does a yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches thick, 65 square feet at 5 inches, and 54 square feet at 6 inches.
Is it cheaper to order slightly more concrete?
Yes. Leftover concrete costs a few dollars per cubic foot; running short mid-pour creates a cold joint, a second delivery fee ($75-200), and often a visible seam that weakens the slab.
How thick should my slab be?
4 inches is standard for patios, sidewalks, and shed slabs; 5-6 inches for driveways and garage floors that carry vehicles. Local codes and soil conditions can require more - verify before pouring.