Estimating a concrete pour
Concrete is ordered by volume — cubic yards in the U.S. — so the job is to measure the space the concrete will fill, convert every dimension to feet, multiply, and divide by 27. Thickness is almost always given in inches, which is where most estimates go wrong: 4 inches is 0.333 ft, not 0.4. Small jobs use bagged premix, whose label states the volume each bag makes.
The formulas
Slab or footing: ft³ = length (ft) × width (ft) × thickness (in) ÷ 12 Round column: ft³ = π × (diameter (in) ÷ 24)² × height (ft) Cubic yards = ft³ × quantity × (1 + waste %) ÷ 27 Bags = ⌈ total ft³ ÷ bag yield ⌉ (80 lb ≈ 0.60, 60 lb ≈ 0.45, 40 lb ≈ 0.30 ft³)
Worked example
A 10 × 10 ft patio, 4 in thick: 10 × 10 × 4/12 = 33.33 ft³ = 1.235 yd³. With 10% waste: 36.67 ft³ = 1.36 yd³, or 62 bags of 80 lb (36.67 ÷ 0.6 = 61.1, rounded up).
Two 12 in round piers, 4 ft deep: π × 0.5² × 4 = 3.14 ft³ each, 6.28 ft³ total, + 10% = 6.91 ft³ = 12 bags of 80 lb.
Notes
- Bag yields are typical label values for Quikrete and Sakrete concrete mix; high-strength and fast-setting mixes can differ slightly — check the bag.
- The estimate covers concrete only — not gravel base, rebar, wire mesh or forms.
Sources: Quikrete, Concrete Mix product data (yield per bag); Portland Cement Association, concrete basics; International Code Council, International Residential Code R506.1 (slab thickness).
Frequently asked questions
How many yards of concrete do I need for a 10×10 slab?
At 4 inches thick: 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 cubic feet, ÷ 27 = 1.23 cubic yards. With 10% extra for uneven ground and spillage, order about 1.36 yards — most ready-mix suppliers sell in quarter- or half-yard steps, so 1.5 yards.
How many bags of concrete are in a yard?
A cubic yard is 27 cubic feet. An 80 lb bag of pre-mixed concrete makes about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³, so one yard takes 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb. Beyond about one yard, ready-mix delivery is usually cheaper and gives a more uniform pour.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Driveways that carry cars are usually 4–5 inches, and 6 inches for heavier vehicles or poor soil. Check your local building code — the International Residential Code sets 3½ inches as the minimum for floor slabs on ground.
How much extra concrete should I order?
Add 5–10%. Subgrades are rarely perfectly flat, forms bow, and some concrete is always left in the wheelbarrow or truck chute. Running short in the middle of a pour leaves a cold joint, which is far worse than a little leftover.