Concrete is the one material where running short mid-job is genuinely costly. Once a pour begins it has to finish in one continuous go — stop partway and you create a cold joint, a weak seam where fresh concrete meets concrete that has already started to set. That makes an accurate volume calculation, plus a sensible overage, more important here than for almost any other material. This guide walks through the volume maths, the bag-versus-ready-mix decision, and the figures that keep a slab from cracking.
The concrete volume formula
Cubic yards = cubic feet ÷ 27
Bags needed = (cubic feet × overage) ÷ yield per bag, rounded up
The only trap is units: thickness is usually given in inches, so it must be converted to feet (divide by 12) before multiplying. A 4 inch slab is 0.333 ft thick, not 4. The calculator handles this conversion for you, in both imperial and metric.
Cubic feet, cubic yards and bag yields
Ready-mix concrete is sold by the cubic yard; bagged concrete is sold by weight, and each bag yields a known volume once mixed:
| Bag size | Yield (mixed) | Bags per cubic yard |
|---|---|---|
| 80 lb | ≈ 0.60 cu ft | ≈ 45 |
| 60 lb | ≈ 0.45 cu ft | ≈ 60 |
| 40 lb | ≈ 0.30 cu ft | ≈ 90 |
That bags-per-yard column explains why bagged concrete stops making sense on bigger jobs: filling a single cubic yard means mixing 45 to 90 bags by hand. Most people tap out around half a cubic yard.
How thick should the slab be?
Thickness drives both the volume and the strength of the slab. Common figures:
- Walkways and patios: 4 inches is standard.
- Driveways: 4 inches for cars, 5–6 inches for trucks, RVs or heavy loads.
- Garage and shed floors: 4 inches, more if storing heavy equipment.
- Structural and footing work: follow an engineer's specification and local code.
Don't forget the base: most slabs sit on 4 inches of compacted gravel, which you calculate separately. The gravel matters as much as the concrete for preventing settling and cracking.
A worked example
A 12 × 10 ft patio at 4 inches thick, with a 10% overage:
- Thickness in feet = 4 ÷ 12 = 0.333 ft
- Volume = 12 × 10 × 0.333 = 40 cu ft
- With 10% overage = 44 cu ft = 1.63 cu yd
- In 80 lb bags = 44 ÷ 0.60 = 74 bags
Seventy-four bags is well past the practical hand-mixing limit, so this patio is a clear ready-mix job: ordering about 1.75 cubic yards (rounding up to the supplier's increment) is faster, cheaper and gives a single continuous pour.
Why the overage allowance matters
Even a perfectly measured slab uses slightly more concrete than the maths predicts. Subgrades are never perfectly level, forms bow outward under the weight of wet concrete, and some is always lost to spillage and the wheelbarrow. A 5–10% overage covers this. On a ready-mix order it's better to round up to the next quarter-yard than to be caught short with the truck already on site.
Once your concrete volume is set, the related calculators help with the gravel base beneath the slab and the rebar grid inside it.
Frequently asked questions
How many bags of concrete do I need for a slab?
It depends on the slab volume and the bag size. As a guide, an 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft. The calculator divides your slab volume by the chosen bag yield and rounds up.
How do I calculate concrete in cubic yards?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 to get cubic yards. A 10×10 ft slab at 4 in thick is about 1.23 cu yd. Ready-mix is ordered by the cubic yard.
When should I order ready-mix instead of bags?
Bagged concrete is practical up to roughly 1 cubic yard (around 45–60 bags). Beyond that, mixing by hand is exhausting and ready-mix delivery is usually cheaper and far faster. Most suppliers have a minimum order of about 1 cubic yard.
How much extra concrete should I order?
Add about 5–10% for spillage, uneven subgrade and over-excavation. Running short mid-pour creates a cold joint, so it is better to have a little extra than to stop.
How thick should a concrete slab be?
Typical thicknesses are 4 in for patios and walkways, 4–6 in for driveways (6 in for heavier vehicles), and as specified by an engineer for structural slabs. Always check local building codes.