Concrete Calculator
Slabs, footings, fence-post holes and steps. Get cubic yards for a ready-mix order or the number of bags to pick up — and which one makes sense.
Bags or a truck? Drag the volume
An 80-lb bag of concrete mix yields about 0.6 cubic feet. That sounds like plenty until you see how many it takes to make a yard.
The crossover zone. Bags are still cheaper, but you’re lifting over two tons and mixing for hours — a short-load truck or a trailer mixer is worth pricing. Prices assume about $6.50 per bag and $165 per yard delivered with a short-load fee under 4 yards; yours will differ.
Concrete formulas
slab yd³ = L × W × (T ÷ 12) ÷ 27
post hole ft³ = π × (D ÷ 24)² × depth ÷ 12 − post volume
Always convert everything to feet before multiplying — thickness in inches is the classic trap. Order 5–10% extra: subgrades are never perfectly flat, forms bow, and running short in the middle of a pour leaves a cold joint.
Bags per cubic yard
| Bag | Yield | Bags / yd³ | 10×10×4″ slab |
|---|---|---|---|
| 40 lb | 0.3 ft³ | 90 | 120 |
| 50 lb | 0.375 ft³ | 72 | 96 |
| 60 lb | 0.45 ft³ | 60 | 80 |
| 80 lb | 0.6 ft³ | 45 | 60 |
Concrete under pavers?
Pavers are normally laid on compacted gravel, not concrete — the flexible base drains and lets the surface move with frost. Pavers can be mortared or sand-set onto an existing sound slab, but new work should use the paver base calculator. Concrete is the right call for the paver edge restraint haunch, step footings and post holes.
Questions people ask
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. A 10×10 slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cubic yards. Add 5–10% for uneven subgrade and spillage.
How many bags of concrete do I need for 1 cubic yard?
About 45 bags of 80-lb mix (0.6 cu ft each), 60 bags of 60-lb mix (0.45 cu ft) or 90 bags of 40-lb mix (0.3 cu ft). Bag yields are printed on the bag; they vary slightly by brand.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Driveways for passenger cars are usually 4–5 inches, and 6 inches where heavier vehicles park. Pads under hot tubs or equipment often go to 6 inches with reinforcement.
How much concrete for a fence post hole?
Hole volume minus post volume. A 10-inch-diameter hole 30 inches deep holds 1.36 cu ft; a 4×4 post takes up 0.25 cu ft of that, leaving about 1.1 cu ft — roughly two 50-lb bags of fast-setting mix or two 60-lb bags of standard mix.
How much does concrete cost per yard?
Ready-mix concrete commonly runs roughly $140–$200 per cubic yard delivered in the US, plus short-load fees on small orders and extra for fibre, colour or air entrainment. Bagged mix works out to around $250–$350 per yard in materials.