How to Calculate Concrete
Every concrete estimate starts with volume. The core formula is simple: Volume = Length × Width × Thickness. The challenge lies in unit discipline. Mixing feet and inches without converting first is the most common source of error. This calculator handles all conversions for you, but if you are working by hand, convert all dimensions to the same unit before multiplying.
Once you have volume in cubic feet, divide by 27 to get cubic yards — the unit most suppliers quote. Multiply cubic yards by 0.7646 to get cubic meters.
Formulas by Shape
Slab & Footing
Volume = Length × Width × Thickness
Example: 10 ft × 10 ft × 4 in slab
Convert thickness: 4 in ÷ 12 = 0.333 ft
Volume = 10 × 10 × 0.333 = 33.33 ft³ = 1.23 yd³
Wall with Openings
Volume = (Length × Height − openings area) × Thickness
Example: 20 ft × 8 ft wall, 8 in thick, one 3 ft × 7 ft door opening
Wall area = 20 × 8 = 160 ft²; Opening = 3 × 7 = 21 ft²
Net area = 139 ft²; Thickness = 8/12 = 0.667 ft
Volume = 139 × 0.667 = 92.7 ft³ = 3.43 yd³
Circular Column or Pier
Volume = π × r² × Height (r = diameter ÷ 2)
Example: 12 in diameter × 10 ft tall column
r = 6 in = 0.5 ft
Volume = 3.14159 × 0.5² × 10 = 7.85 ft³ = 0.29 yd³
Stairs
Each step adds a rectangular block of concrete equal to its width × tread depth × riser height. The staircase volume is the sum of all steps, building from the bottom up (step 1 = 1 riser, step 2 = 2 risers, etc.).
Curb & Gutter
Volume = (Curb cross-section area + Gutter cross-section area) × Run length, where curb cross-section = curb width × curb height and gutter cross-section = gutter width × gutter thickness.
Bags vs Ready-Mix
Pre-mixed bags are ideal for volumes below roughly one cubic yard (27 ft³). They are available at any hardware store, and no minimum order applies. Ready-mix concrete is delivered by truck and becomes economical above one yard because delivery charges and minimum loads (usually 1 yd³) offset the higher per-unit bag cost.
| Bag size | Yield per bag | Bags per yd³ |
|---|
| 40 lb | 0.30 ft³ | 90 bags |
| 50 lb | 0.375 ft³ | 72 bags |
| 60 lb | 0.45 ft³ | 60 bags |
| 80 lb | 0.60 ft³ | 45 bags |
Concrete Mix Ratios
The most common general-purpose mix is 1:2:3 — one part Portland cement, two parts sand and three parts coarse aggregate by volume. By weight this corresponds to roughly 22% cement, 33% sand and 45% aggregate of the total concrete volume. A higher cement ratio (such as 1:1.5:3) produces stronger concrete suitable for structural members, while a lean mix (1:3:6) is used for mass fills where high strength is not required.
How Much Does Concrete Weigh?
Normal-weight concrete weighs approximately 150 lb per cubic foot, which is 4,050 lb (about 2 short tons or 1,837 kg) per cubic yard. Reinforced concrete with embedded rebar typically runs 155–160 lb/ft³. Lightweight concrete — made with expanded shale or pumice aggregate — weighs 90–115 lb/ft³ and is used where dead load must be minimised.
Why Add a Waste Allowance?
Even the most careful pour loses material to spills, uneven sub-base, overfill at form edges and volume rounding. Industry practice is to order 5–10% more than the calculated volume. Increase this to 10–15% if the ground surface is rough or uneven. Running short mid-pour is far more costly and disruptive than having a little left over, since a cold joint weakens the finished slab.
Choosing Slab Thickness
Thickness is the single biggest driver of concrete volume. Use these guidelines as a starting point:
- Sidewalks and patios: 4 inches is standard. Add wire mesh or fiber for crack resistance.
- Residential driveways: 4–6 inches. Use 6 inches if heavy vehicles (trucks, SUVs) will park on it regularly.
- Garage floors: 4–6 inches with a 3,000–4,000 psi mix and vapor barrier below.
- Heavy industrial slabs: 6 inches minimum, usually with rebar and a higher-strength mix.
- Footings: Depth should meet local frost-line requirements (typically 12–48 inches depending on climate).
Concrete Curing Times
Curing is the process of maintaining moisture and temperature in fresh concrete so it reaches its design strength. The strength gain schedule for a standard mix is roughly:
- 24–48 hours: Light foot traffic is usually safe.
- 7 days: About 70% of design strength — safe for passenger vehicles on a driveway.
- 28 days: Near full design strength. Structural loads and heavy vehicles can be applied.
Keep the surface moist by covering with wet burlap or a plastic sheet, especially in hot or windy conditions. Avoid placing concrete below 4°C (40°F) without cold-weather precautions.
Tips for an Accurate Estimate
- Measure twice. A 1-inch error in thickness on a 10 × 10 ft slab changes the volume by about 0.31 ft³ — nearly one 60-lb bag.
- Account for every piece separately: footings, columns and the slab above are different pours.
- Ask your supplier about the minimum load charge before deciding between bags and ready-mix.
- Use the waste slider. Start at 5% for formed work and bump to 10% for open-top slabs on grade.
- Round up to the nearest quarter or half yard when ordering ready-mix; suppliers cannot easily provide fractional adjustments on site.