How Much Concrete Do I Need?
Ordering too little concrete leaves you with a cold joint, and ordering too much wastes money. This concrete calculator works out the exact volume for four common shapes: rectangular slabs, wall footings, round columns (such as Sonotube piers) and circular slabs. You get the result in cubic feet, cubic yards and cubic meters, plus the number of 60 lb and 80 lb pre-mix bags. Turn on the optional 10% waste factor to allow for spillage, uneven ground and form flex.
Concrete Volume Formulas
- Slab: length (ft) × width (ft) × thickness (in ÷ 12).
- Footing: length (ft) × width (in ÷ 12) × depth (in ÷ 12).
- Column: π × (diameter in ÷ 24)² × height (ft).
- Circular slab: π × (diameter ft ÷ 2)² × thickness (in ÷ 12).
- Cubic yards = cubic feet ÷ 27. One 80 lb bag yields about 0.6 cubic feet; one 60 lb bag yields about 0.45 cubic feet.
Example: A 10 × 10 Patio Slab
A 10 ft × 10 ft slab that is 4 inches thick needs 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or about 1.23 cubic yards. That is about 56 bags of 80 lb mix or 75 bags of 60 lb mix (always round bag counts up). With 10% added for waste, plan on about 1.36 cubic yards.
As a rule of thumb, ready-mix delivery from a truck is usually more cost-effective than bags once a job needs more than about 1 cubic yard.
Frequently Asked Questions
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways and garage floors that carry vehicles are commonly 5–6 inches. Check local building codes for your project.
Should I add extra concrete for waste?
Yes. Most contractors add 5–10% to cover spillage, uneven subgrade and form movement. Running short in the middle of a pour is far more costly than having a little left over.
How many 80 lb bags of concrete make a cubic yard?
About 45 bags. A cubic yard is 27 cubic feet, and each 80 lb bag yields about 0.6 cubic feet. With 60 lb bags, you need about 60 bags per cubic yard.