The amount of concrete you need equals the volume of the space you’re filling. Calculate that volume, add 10% for waste, then convert to cubic yards or cubic metres depending on your supplier. Most residential projects — slabs, footings, driveways — fall between 1 and 10 cubic yards (0.76–7.65 m³). Get that number wrong and you’re either stopping work to reorder or paying for concrete you can’t use.
How to calculate concrete volume — the core formula
The formula is the same for every project: Volume = Length × Width × Thickness. Use consistent units throughout. For slabs and flatwork, thickness in imperial is usually expressed in inches, which must be converted to feet before multiplying (divide inches by 12). In metric, thickness in millimetres must be converted to metres (divide by 1000).
Worked example — imperial: A slab 12 ft × 20 ft × 4 in thick = 12 × 20 × (4/12) = 80 cu ft. Divide by 27 to convert to cubic yards: 80 ÷ 27 = 2.96 cubic yards. Add 10% waste: 3.26 cubic yards — order 3.5 yds to be safe.
Worked example — metric: A slab 3.6 m × 6 m × 100 mm thick = 3.6 × 6 × 0.1 = 2.16 cubic metres. Add 10% waste: 2.38 m³. Round up to 2.5 m³ when ordering.
Use the Concrete Slab Calculator to handle the arithmetic automatically — input length, width, and thickness in any unit and it returns cubic yards, cubic feet, cubic metres, and bag counts simultaneously.
Concrete volumes for common project types
Different projects have standard thickness requirements that directly drive volume. The table below shows representative scenarios across the most common project types. These are real-world dimensions, not minimums.
| Project | Typical Dimensions | Thickness | Approx. Volume |
| Residential driveway | 16 ft × 20 ft (4.9 × 6.1 m) | 5 in / 125 mm | 5.9 yd³ / 4.5 m³ |
| Standard patio | 12 ft × 16 ft (3.6 × 4.9 m) | 4 in / 100 mm | 2.9 yd³ / 2.2 m³ |
| Sidewalk (50 linear ft) | 50 ft × 3 ft (15.2 × 0.9 m) | 4 in / 100 mm | 1.9 yd³ / 1.4 m³ |
| Garage floor | 22 ft × 24 ft (6.7 × 7.3 m) | 4 in / 100 mm | 6.5 yd³ / 5.0 m³ |
| Column footing | 24 in × 24 in × 12 in deep (0.6 × 0.6 × 0.3 m) | 12 in / 300 mm | 0.44 yd³ / 0.34 m³ |
| Shed pad | 10 ft × 12 ft (3 × 3.6 m) | 3.5 in / 90 mm | 1.3 yd³ / 1.0 m³ |
For complex shapes — L-shaped patios, round piers, stepped footings — break the area into rectangles or cylinders, calculate each section separately, then add the volumes. The Concrete Volume Multi-Shape Calculator handles composite shapes in one pass.
Imperial vs metric: unit conversion that matters
Ordering errors happen most often at the unit conversion step. In the US, concrete is ordered in cubic yards. In the UK, Canada, and Australia, suppliers quote in cubic metres. The conversion factor is: 1 cubic yard = 0.765 cubic metres; 1 cubic metre = 1.308 cubic yards.
For bagged concrete: a standard 60 lb / 27 kg bag yields approximately 0.45 cu ft (0.0127 m³) of mixed concrete. An 80 lb / 36 kg bag yields roughly 0.60 cu ft (0.017 m³). At those yields, a 10 yd³ pour requires approximately 450 × 60 lb bags — which is why ready-mix becomes cost-effective above 1 cubic yard (0.76 m³) for most regions. The Concrete Bags Calculator converts your required volume directly into bag counts by bag size.
When working across unit systems, the Imperial to Metric Concrete Converter avoids manual conversion errors — particularly useful when a US project spec is being quoted to a metric supplier.
How the 10% waste factor works — and when to adjust it
The 10% waste allowance accounts for spillage, over-excavation, subbase irregularities, and the difficulty of mixing the last fraction of a batch consistently. It is not optional — undercounting leads to short pours where the top lift hardens before you can finish it.
Adjust the waste factor upward in these situations:
Irregular or sloped subgrade: Use 12–15%. Soft spots that require additional fill can consume 20% more material than planned on poorly surveyed sites.
Forms with complex curves or angles: Budget 12% — curves are harder to fill completely without voids.
Hand-mixing from bags: Add 15% because batch-to-batch variation in water content affects yield by up to 5%.
Pumped concrete: Pump lines hold 0.2–0.5 yd³ (0.15–0.38 m³) of concrete that cannot be recovered — factor this as a fixed addition to your total.
The Concrete Waste Factor Calculator lets you set a custom waste percentage and see the adjusted order quantity and its cost impact in a single calculation.
Common mistakes when estimating concrete volume
The four errors that account for the majority of mis-orders:
1. Not converting thickness to the right unit before multiplying. Calculating 12 × 20 × 4 (where 4 is inches, not feet) gives 960 cu ft — 12× too much. Always divide inch measurements by 12 before the multiplication.
2. Forgetting the waste factor entirely. A 2% short-pour on a 5 yd³ slab means finishing crews are scrambling. Always add a minimum of 10% and round up to the nearest quarter-yard or 0.25 m³.
3. Treating the slab area as if the subgrade is perfectly flat. Even a 1 inch (25 mm) average low spot across 400 sq ft (37 m²) adds 0.62 cu ft (0.018 m³) — about a tenth of a yard. Survey your subgrade before calculating.
4. Using manufacturer-stated bag yield without accounting for mixing losses. Bags are rated at full dry yield. Real mixed yield after water addition and mixing loss runs 3–5% below the stated figure. Use 0.43 cu ft per 60 lb bag (not 0.45) for hand-mixed work.
Related calculators you might need
Once you have your concrete volume, the next step depends on how you’re sourcing the material. If you’re mixing on-site, the Concrete Bags Calculator converts cubic yards into 40 lb, 60 lb, and 80 lb bag counts — and shows you the cost difference between bag sizes. If you’re ordering ready-mix, the Concrete Truck Load Calculator tells you how many full and partial truck loads your volume requires. Short loads below about 5 yd³ (3.8 m³) typically trigger a short load surcharge — the Short Load Fee Estimator quantifies that cost so you can decide whether to add a neighbouring project or pay the premium. For total budget planning, the Full Concrete Project Estimator combines material volume, delivery, labour, and finishing into a single project cost figure.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft (3 × 3 m) slab at 4 inches (100 mm) thick requires 100 sq ft × 0.333 ft = 33.3 cu ft = 1.23 cubic yards (0.94 m³). With a 10% waste allowance, order 1.4 cubic yards (1.04 m³). At 5 inches / 125 mm thick for vehicle-rated slabs, that becomes 1.73 yd³ including waste. A single ready-mix truck minimum load is typically 1 yd³, so this project qualifies for truck delivery.
How many bags of concrete do I need for 1 cubic yard?
One cubic yard of concrete (27 cu ft) requires approximately 45 × 60 lb bags, 34 × 80 lb bags, or 60 × 40 lb bags. These figures assume standard bag yield (0.45 cu ft per 60 lb bag) before waste. For hand-mixed work, use a 15% buffer — bump to 52 × 60 lb bags per cubic yard. The Cubic Yards to Bags Converter handles these conversions for any bag size.
What is the standard concrete thickness for a driveway?
Residential driveways in the US should be 4 inches / 100 mm minimum for passenger vehicles, and 5 inches / 125 mm if SUVs, pickup trucks, or light delivery vehicles will use it. For RV parking or areas that will see occasional 3-axle trucks, 6 inches / 150 mm with added rebar is the standard. In freeze-thaw climates (Hardiness Zone 5 and colder), thicker slabs — 5 to 6 inches — reduce frost heave damage.
How do I calculate concrete for a round footing?
Volume of a cylinder = π × radius² × depth. For a 12-inch / 300 mm diameter tube footing 36 inches / 900 mm deep: π × (0.5 ft)² × 3 ft = 2.36 cu ft = 0.087 yd³ (0.067 m³). Multiply by the number of footings and add 10% waste. For multiple sonotube footings on a deck or post frame, the Concrete Tube/Sonotube Calculator calculates all footings simultaneously.
Is it cheaper to use bags or ready-mix concrete?
For volumes under 0.5 cubic yards (0.38 m³), bagged concrete is usually cheaper once you account for ready-mix minimum order charges and delivery fees. Above 1 cubic yard (0.76 m³), ready-mix is typically 15–30% less expensive per cubic yard when all costs are included. Labour is the deciding factor — mixing by hand above 1 yd³ is impractical. The Ready-Mix vs Bagged Concrete Cost Calculator compares both options at your local prices.
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