Volume for a slab, footing or column in cubic yards and cubic metres, plus how many bags of each size that actually means.
Concrete is sold by volume — cubic yards in the US, cubic metres almost everywhere else. For a slab that is simply length × width × thickness, with the thickness converted to the same units as the other two. The most common error is leaving thickness in inches while length and width are in feet, which produces an answer twelve times too large.
A cubic yard is 27 cubic feet, which is worth memorising because it is the unit ready-mix is ordered in. A 10 ft × 10 ft slab at 4 inches thick is 33.33 ft³, or 1.23 cubic yards.
Bag yields are set by the manufacturer and governed by ASTM C387. An 80 lb bag yields about 0.60 ft³, a 60 lb bag 0.45 ft³, and a 40 lb bag 0.30 ft³. So a single cubic yard takes 45 bags at 80 lb — that is 3,600 lb of material to buy, carry and mix.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
The practical threshold is around one cubic yard. Below that, bags make sense. Above it, ready-mix delivery is cheaper per unit, dramatically less work, and gives a more consistent mix — batch-to-batch variation across 45 hand-mixed bags is a real quality problem, not a theoretical one. Most suppliers have a minimum order and a short-load fee below it, so ask before assuming.
Subgrade is never perfectly level, forms bow slightly under the weight of wet concrete, and a slab poured over gravel loses some into the voids. Ten percent is the standard allowance and it is not padding — running short mid-pour is far worse than having a little left over, because a cold joint forms where fresh concrete meets concrete that has already begun to set, and that joint is a permanent weakness.
| Application | Typical thickness |
|---|---|
| Footpath, patio | 4 in (100 mm) |
| Driveway, cars only | 4–5 in (100–125 mm) |
| Driveway with heavy vehicles | 6 in (150 mm) |
| Garage or shed floor | 4–6 in (100–150 mm) |
| Footings | Per structural design — not a rule of thumb |
Concrete is strong in compression and weak in tension, which is why reinforcement exists. Steel mesh or rebar does not stop a slab cracking — concrete shrinks as it cures and it will crack. What reinforcement does is hold the crack tight so it stays a hairline rather than opening and allowing the two sides to move independently. Control joints do the complementary job: they create a deliberate weak line so the inevitable crack happens where you chose rather than diagonally across the middle. Cut them to a quarter of the slab depth, in panels no more than about 30 times the thickness across.
Concrete does not dry out to gain strength — it hydrates, a chemical reaction between cement and water that continues for weeks. Letting it dry too fast stops that reaction and permanently reduces strength. Keeping a fresh slab damp for the first several days, under plastic or with periodic misting, makes a genuine difference. Most of the strength develops in the first 28 days, which is why concrete is specified by its 28-day compressive strength.
Weather matters too. Below about 5 °C hydration slows sharply and freezing before the concrete has set can destroy it outright. Above about 30 °C the surface can set before the body does, causing plastic shrinkage cracking.
Forty-five 80 lb bags, sixty 60 lb bags, or ninety 40 lb bags. Those follow from the yields on the packaging — 0.60, 0.45 and 0.30 cubic feet respectively — divided into the 27 cubic feet in a yard. Forty-five 80 lb bags is 3,600 lb of material, which is the point at which most people conclude ready-mix delivery is worth the call.
Ten percent is standard. Subgrade is never perfectly flat, forms flex under the weight of wet concrete, and some is lost into gravel beneath the slab. Running short mid-pour is genuinely bad — a cold joint forms where fresh concrete meets partly-set concrete and that line stays a permanent weakness. Having a wheelbarrow left over is a much cheaper mistake.
For a footpath or small patio at 4 inches, often not, provided the subgrade is well compacted and control joints are cut. For driveways, garage floors, anything carrying vehicle loads, and anything structural, yes. Reinforcement does not prevent cracking — concrete shrinks as it cures and will crack regardless. It holds cracks tight so they stay hairline instead of opening up and letting the slab move differentially.
Four inches is adequate for cars on a well-compacted base. Go to five or six inches if delivery lorries, RVs or heavy trucks will use it, or if the subgrade is soft clay. The base matters as much as the thickness: four inches of well-compacted gravel under a four-inch slab outperforms six inches of concrete poured on poorly prepared ground.
Roughly 24 to 48 hours for foot traffic and about seven days for vehicles, with full strength at 28 days. Those are guidelines and cold weather stretches them considerably. Keep the surface damp during the first several days — concrete gains strength by hydration, not by drying, and letting it dry out early permanently costs strength.
Related: Square Footage Calculator · Gravel Calculator · Volume Calculator · Stair Calculator Finishing rather than pouring? The Tile Calculator works out tile and box counts, and the Mulch Calculator covers landscaping beds. Finishing the space? The Paint Calculator covers the decorating side.