Concrete Calculator
Work out exactly how much concrete your slab, footing or post holes need — in cubic yards and meters, plus how many 80-lb, 60-lb or 40-lb bags to buy.
How much concrete do I need?
Concrete is sold by volume. For a slab or footing, the volume is simply length × width × thickness — but the three numbers are usually measured in different units, which is where most mistakes happen. This calculator converts everything for you and answers in the units concrete is actually bought in.
Volume (cubic feet) = length (ft) × width (ft) × thickness (in) ÷ 12
Cubic yards = cubic feet ÷ 27 · Cubic meters = cubic feet × 0.0283
Example: a 10 × 10 ft patio slab, 4 inches thick, is 10 × 10 × 4 ÷ 12 = 33.3 cubic feet = 1.23 cubic yards. Mixed from bags instead, that is 56 × 80-lb bags before any waste allowance — 62 bags with the standard 10% extra.
Bag yields — what one bag actually makes
| Bag size | Yield per bag | Bags per cubic yard |
|---|
| 80 lb (36 kg) | 0.60 cu ft | 45 |
| 60 lb (27 kg) | 0.45 cu ft | 60 |
| 40 lb (18 kg) | 0.30 cu ft | 90 |
As a rule of thumb, bagged concrete stops making sense much above half a cubic yard: the mixing work grows faster than the savings. Beyond that, a ready-mix truck — usually with a minimum order around 1 cubic yard — is normally the smarter buy.
Why the 10% waste allowance?
Ground is never perfectly level, forms bow out slightly, and some concrete stays in the wheelbarrow. Ordering exactly the geometric volume is the classic way to run short with a half-finished slab. Pros order about 10% extra; the calculator includes it in the buying figures and shows it openly, so you can lower it for a neat, well-formed job.
Frequently asked questions
How thick should a concrete slab be?
4 inches (10 cm) is standard for patios, walkways and sheds. Driveways that carry cars are usually 4–5 inches over a compacted gravel base, and garage floors 5–6 inches. Footings are a different calculation — they follow your local code depth below the frost line.
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 ft slab is 1.23 cubic yards. That is 56 × 80-lb bags exactly, or 62 bags with the usual 10% waste allowance. At that size most people order ready-mix instead.
Is it cheaper to mix bags or order ready-mix?
For small jobs (post holes, a small pad) bags win — no minimums, no truck scheduling. Past roughly half a cubic yard, ready-mix usually wins on both price per yard and your back.
What does a cubic yard of concrete weigh?
About 4,000 lb (roughly 1.8 tonnes) freshly mixed. It matters when you are hauling bags: 45 × 80-lb bags for one yard is 3,600 lb of bags alone.
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