What This Calculator Does
Compute the volume of concrete (in cubic feet, yards, and meters) plus weight and number of pre-mix bags needed for three common shapes: rectangular slabs, round columns, and circular tubes. Choose your shape, enter dimensions, and the calculator handles the geometry and unit conversions.
Volume Formulas
- Slab / Wall / Footing (rectangular): V = Length × Width × Thickness
- Column / Hole (round): V = π × (Diameter/2)² × Depth
- Tube / Annular Cylinder: V = π × [(Outer/2)² − (Inner/2)²] × Depth
Multiply by quantity if you're pouring multiple identical pieces.
Worked Example
A 5 ft × 2.5 ft slab, 5 inches thick:
- V = 5 × 2.5 × (5 / 12) = 5.208 cu ft
- = 5.208 / 27 = 0.193 cu yd
- Weight ≈ 5.208 × 133 lbs/cu ft = 693 lbs
- 60-lb bags needed: ⌈5.208 / 0.45⌉ = 12 bags
- 80-lb bags needed: ⌈5.208 / 0.60⌉ = 9 bags
Bag Coverage
Pre-mix concrete bags cover approximately:
- 40-lb bag: ~0.30 cu ft
- 60-lb bag: ~0.45 cu ft
- 80-lb bag: ~0.60 cu ft
Bag yield varies slightly by manufacturer (Quikrete, Sakrete, etc.) — always check the label. Above ~1 cubic yard, ordering ready-mix from a truck becomes more economical than bagged.
Concrete Density & Weight
Standard reinforced concrete weighs ~133 lbs/cu ft (~2,130 kg/m³) when cured. The fresh wet weight is slightly higher. Account for weight when planning truck delivery, formwork strength, or lifting equipment for precast pieces.
Order More Than the Math Says
Real pours always need a buffer:
- Forms aren't perfectly flat — concrete settles into low spots.
- Subgrade absorbs water and slightly more concrete than expected.
- Spillage at the chute, and concrete left in the truck or wheelbarrow.
Standard practice: order 5–10% more than the calculated volume. For ready-mix delivery, round UP to the next quarter-yard increment.
When to Bag vs. Order Truck Delivery
Bagged concrete is cost-effective up to about 1 cubic yard (around 60 bags of 60-lb mix). Above that, ready-mix delivery from a truck is cheaper, faster, and more consistent. Truck delivery typically has a 1-yard minimum and a per-yard price plus delivery surcharge.