Concrete Slab Calculator

How this calculator works

This calculator multiplies the slab's length × width × thickness (converted from inches to feet) to get the volume in cubic feet, then divides by 27 to get cubic yards. The recommended order adds a 10% waste allowance so you don't run short during the pour.

Bag counts assume a 60 lb bag yields approximately 0.45 cubic feet of cured concrete and an 80 lb bag yields approximately 0.60 cubic feet. Both counts include the 10% waste factor and are rounded up to the nearest whole bag.

For slabs, common thicknesses are: 4 inches for patios and walkways, 5 inches for residential driveways, and 6 inches for heavy-duty or commercial driveways.

Formula

volume_cuft = length × width × (thickness / 12)
volume_cuyd = volume_cuft / 27
recommended = volume_cuyd × 1.10  (10% waste)
bags_60lb = ceil(volume_cuft / 0.45 × 1.10)
bags_80lb = ceil(volume_cuft / 0.60 × 1.10)

Example

For a 10 ft × 10 ft slab that is 4 inches thick (a common patio pour):

• volume_cuft = 10 × 10 × (4/12) = 33.33 cu ft
• volume_cuyd = 33.33 / 27 ≈ 1.2346 cu yd
• recommended order ≈ 1.358 cu yd (with 10% waste)
• 60 lb bags (with waste): ~82
• 80 lb bags (with waste): ~62

For ready-mix delivery, round up to 1.5 cubic yards.

Important assumptions

• Inputs are positive numbers in imperial units (feet and inches).
• 10% waste factor is added automatically — adjust manually for irregular shapes or first-time pours.
• Bag yields follow manufacturer averages. Actual yield varies by mix design and water content.
• For ready-mix orders, round up to the nearest 0.25 cubic yard (most suppliers have a minimum 1 yard delivery).
• This calculator assumes a single rectangular pour. For L-shaped slabs, calculate each rectangle separately and add the results.