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Concrete Slab Calculator

Cubic yards, bags, and cost for any concrete pour. Calculate concrete volume in cubic yards and cubic feet, number of 60 lb or 80 lb bags required, and total material cost for slabs, footings, and columns.

Slab Dimensions

ft
ft
in
$

Bag Weight

Yield: 0.6 ft³/bag — 10% waste factor applied automatically

Volume (ft³)

66.67 ft³

Volume (yd³)

2.47 yd³

With 10% Waste

2.72 yd³

80 lb Bags Needed

123 bags

Total Material Cost

$922.50

Effective Rate

$337.50/yd³

Formula: V (ft³) = L × W × (T ÷ 12) → ÷ 27 for yd³. Bags = ⌈(V × 1.10) ÷ 0.6⌉. Always order full bags — concrete cannot be mixed in partial batches without waste.

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Solution Methodology

01

Enter Dimensions

Input slab length and width in feet, and thickness in inches.

02

Convert & Compute Volume

Convert thickness to feet (÷ 12), multiply L × W × T for cubic feet, divide by 27 for cubic yards.

03

Apply Waste Factor

Add 10% overage: Total yd³ = Volume × 1.10.

04

Count Bags

Divide total cubic feet by bag yield (0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb) and round up.

05

Estimate Cost

Multiply bag count by cost per bag to get total material cost.

Common Questions

How do I calculate cubic yards of concrete?
Convert all dimensions to feet (1 inch = 0.0833 ft), multiply Length × Width × Thickness to get cubic feet, then divide by 27 to convert to cubic yards (since 1 yd³ = 27 ft³). Always add a 10% overage for waste, spillage, and uneven subgrade.
How many 80 lb bags of concrete make a cubic yard?
One 80 lb bag of premixed concrete yields approximately 0.60 cubic feet when mixed. Since one cubic yard = 27 cubic feet, you need 27 ÷ 0.60 ≈ 45 bags per cubic yard. For 60 lb bags (yield 0.45 ft³ each), you need about 60 bags per cubic yard.
What thickness should a concrete slab be?
Standard residential driveway slabs are 4 inches thick (6 inches for heavy vehicles). Sidewalks and patios are typically 4 inches. Structural slabs and garage floors range from 4 to 6 inches. Footings are typically 8–12 inches thick depending on load and local frost depth requirements.
Why add a 10% waste factor to concrete orders?
Concrete volume calculations assume perfectly level subgrades and exact form dimensions. In practice, subgrade irregularities, form flex under pressure, spillage during pouring, and pump line residuals typically consume 5–10% additional material. Ordering short is costly — running out mid-pour creates cold joints that weaken the slab.