Which tool fits the job
For a whole project, use a planner: the concrete slab planner prices the base, forms, steel and concrete for a patio, the paver patio planner does the same for pavers, and the retaining wall planner draws every block of a wall up to 3 ft. For one material, use the concrete slab, concrete column, post hole concrete, brick, concrete block or paver calculator.
Plan in this order
Work out the volume in cubic feet, then convert to what you buy. A cubic yard is 27 cubic feet, and an 80 lb bag of concrete mix yields about 0.6 cubic feet, so a yard takes 45 bags. A 10 × 10 ft slab 4 in thick is 1.23 cubic yards; with 10% extra that is 62 bags.
Common buying mistakes
- Bags for a big pour. Past about a cubic yard, ready-mix usually costs less and saves hours of mixing.
- Forgetting the mortar. Brick and block need mortar mix: an 80 lb bag lays up to 37 bricks or 13 standard 8 × 8 × 16 in blocks. A 20 × 4 ft block wall takes 95 blocks with 5% extra and about 8 bags.
- No control joints. Groove them about 1 in deep every 6–8 ft in a 4 in slab, or it cracks where it likes.
- A thin paver base. A 12 × 12 ft patio takes 713 pavers with 10% for cuts, on about 2 cubic yards of gravel for a 4 in compacted base and 1 in of sand.
Wet concrete and mortar can cause severe skin burns and eye damage: wear nitrile gloves and eye protection. Call 811 before you dig footings or post holes, and take footing sizes from your plans or building department.