Concrete Calculation

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Concrete Volume & Bag Calculator

Calculation Results

Total Volume Required: 0 Cubic Yards

Total Volume in Cubic Feet: 0 ft³

80lb Bags Needed: 0

60lb Bags Needed: 0

Estimated Material Cost: $0.00

function calculateConcreteVolume() { var length = parseFloat(document.getElementById('conc_length').value); var width = parseFloat(document.getElementById('conc_width').value); var thickness = parseFloat(document.getElementById('conc_thickness').value); var wastePercent = parseFloat(document.getElementById('conc_waste').value) || 0; var bagPrice = parseFloat(document.getElementById('conc_price').value); if (isNaN(length) || isNaN(width) || isNaN(thickness) || length <= 0 || width <= 0 || thickness 0) { var totalCost = bags80 * bagPrice; document.getElementById('cost_display').style.display = 'block'; document.getElementById('res_cost').innerText = totalCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}); } else { document.getElementById('cost_display').style.display = 'none'; } }

How to Calculate Concrete Volume

Planning a patio, driveway, or walkway requires an accurate concrete estimate to avoid multiple trips to the hardware store or high delivery fees for short-loads. Our concrete calculator helps you determine exactly how many cubic yards or bags of pre-mix you need for any rectangular project.

The Concrete Calculation Formula

To calculate the volume of a slab manually, you use the standard volume formula for a rectangular prism:

Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft)

Because most people measure thickness in inches, you must divide the inch measurement by 12 before multiplying. Finally, because concrete is sold by the Cubic Yard, you divide your total cubic feet by 27.

Example Calculation

Suppose you are pouring a standard patio that is 12 feet long, 10 feet wide, and 4 inches thick:

  • Step 1: Convert thickness to feet: 4″ ÷ 12 = 0.333 ft.
  • Step 2: Multiply Dimensions: 12′ × 10′ × 0.333′ = 40 Cubic Feet.
  • Step 3: Convert to Yards: 40 ÷ 27 = 1.48 Cubic Yards.
  • Step 4: Account for Waste: Adding 10% for spillage (1.48 × 1.10) gives you a final order of 1.63 Cubic Yards.

Bagged Mix vs. Ready-Mix

For small projects under 1 cubic yard (roughly 45 bags of 80lb mix), buying bags at a local home center is usually most cost-effective. For larger projects, ordering a "ready-mix" truck is more efficient, though you should be aware of "short-load" fees if ordering less than 3–5 yards.

Bag Size Cubic Feet Yield Bags per Cubic Yard
80lb (36kg) 0.60 ft³ 45 Bags
60lb (27kg) 0.45 ft³ 60 Bags
40lb (18kg) 0.30 ft³ 90 Bags

Pro Tips for a Successful Pour

  1. Order Extra: Always add 5–10% extra for variations in subgrade depth or spillage. It is better to have half a bag left over than to be short 1 inch on your slab.
  2. Subgrade Preparation: Ensure the ground is compacted. Soft soil can settle under the weight of concrete, leading to cracks later.
  3. Forms: Use 2×4 lumber for a 3.5″ to 4″ slab. Ensure your forms are braced well, as wet concrete is extremely heavy and can "blow out" weak forms.
  4. Reinforcement: For driveways or heavy-use areas, use rebar or wire mesh to increase the tensile strength of the concrete.

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