What actually affects how much concrete you need
Concrete volume math is exact, but real-world pours rarely match the math exactly. Sub-grade that isn't perfectly level eats extra concrete in the low spots, which is why most contractors budget 5–10% over the calculated volume rather than ordering to the cubic inch. Post holes waste even more predictably: hand-digging rarely produces a perfectly round, straight-sided hole, so the actual volume tends to run higher than the cylinder formula suggests — 10% is a safer minimum there. Weather affects more than pour timing: concrete cures slower in cold temperatures and can be damaged by frost within the first 24–48 hours, while hot, dry weather causes surface water to evaporate too fast, weakening the top layer if it isn't kept damp. Bagged mix works fine for small pours, but once you're past roughly a cubic yard, ready-mix delivery is usually cheaper and far less physically demanding.
How to calculate how much concrete you need
Concrete is a volume calculation. Slabs use L × W × thickness; post holes use the cylinder formula:
slab: cu ft = length × width × (thickness_in ÷ 12) post hole: cu ft = π × (diameter_in ÷ 24)² × depth cu yd = total cu ft × (1 + waste%) ÷ 27 bags = cu ft with waste ÷ yield per bag
An 80 lb bag yields 0.6 cu ft; a 60 lb bag yields 0.45 cu ft. Once the job exceeds about 1 cubic yard, ordering ready-mix is almost always faster and cheaper — the calculator flags that automatically.
Common mistakes
- Mixing dozens of bags when ready-mix is cheaper — flag appears above 1 cu yd.
- Under-estimating thickness — a 6 in slab uses 50% more concrete than a 4 in slab.
- Not allowing enough cure time — driving on concrete too soon cracks the surface.
- Skipping reinforcement required by local building code for structural work.