How to calculate concrete
Concrete is ordered by volume, in cubic yards. The formula is:
Cubic yards = length × width × thickness ÷ 27 (all measurements in feet)
Because thickness is usually given in inches, divide it by 12 to convert to feet first. The calculator does the conversion for you, adds your waste allowance, and also shows how many bags it would take if you're mixing by hand.
Common slab thicknesses
| Project | Typical thickness |
|---|---|
| Walkways and patios | 4 in |
| Driveways (cars) | 4–5 in |
| Driveways (heavy vehicles) | 6 in |
| Shed and garage floors | 4–6 in |
How many bags of concrete do you need?
A cubic yard is 27 cubic feet, and each bag yields only a fraction of that — which is why bags add up so fast on anything but a small job:
| Bag size | Yield | Bags per cubic yard | Covers at 4 in |
|---|---|---|---|
| 40 lb | ~0.30 cu ft | ~90 | ~0.9 sq ft/bag |
| 60 lb | ~0.45 cu ft | ~60 | ~1.35 sq ft/bag |
| 80 lb | ~0.60 cu ft | ~45 | ~1.8 sq ft/bag |
Bags vs. ready-mix
- Small jobs. For posts and small pads, bagged mix is convenient — but it adds up fast, at about 45 80-lb bags per cubic yard.
- Larger pours. At roughly a cubic yard or more, ready-mix delivery is usually cheaper and dramatically less work.
- Order extra. Add 5–10% — you can't pause a pour to run to the store.
Planning a slab? Read our guide on how much concrete you need for a slab, or compare bags vs. ready-mix concrete.
Don't forget the base and reinforcement
A lasting slab is more than concrete. Plan for a compacted gravel sub-base (usually about 4 inches of crusher run) for drainage and support, and reinforcement — rebar or welded wire mesh — for any slab that will carry vehicles or heavy loads. Cut or tool control joints to steer the inevitable cracking, and on garage or living-space slabs add a poly vapor barrier underneath.
How much does concrete cost?
Ready-mix concrete runs about $120–$170 per cubic yard delivered, but watch for a short-load fee on small orders (typically under 4–5 yards) that can add $50–$150. Bagged mix works out far more expensive per yard once you tally the bags, so the crossover to ordering a truck comes quickly — usually around a cubic yard.
Pouring and finishing a slab
- Form and prep. Build level forms, compact a gravel base, add a vapor barrier and reinforcement, and dampen the subgrade so it doesn't pull water from the mix.
- Pour and spread. Place concrete from the far corner back, working it into the edges so there are no voids.
- Screed. Drag a straight board across the forms to level the surface, filling low spots as you go.
- Float and edge. Bull-float to smooth, run an edger along the forms, then cut or tool control joints about every 8–10 ft to steer the cracking.
- Finish and cure. Broom the surface for grip once the water sheen is gone, then keep it damp (or cover it) for several days — slow curing is what makes concrete strong.