The short answer
To find how much concrete you need, multiply length × width × thickness, all in feet, then divide by 27 to get cubic yards. Add 5–10% for waste. A 10 × 10 ft slab that is 4 inches thick needs 1.23 cubic yards, or about 1.36 cubic yards with 10% extra. That is roughly 62 bags of 80 lb concrete, which is why most pours over one cubic yard use ready-mix.
How the calculation works
Concrete is ordered by volume. In the US that means cubic yards, and in most other countries cubic metres. The calculator works out the volume of the shape you are pouring, adds your waste allowance, then converts that volume into bags.
| Shape | Formula |
|---|---|
| Slab, patio or driveway | length × width × thickness |
| Wall footing | length × footing width × footing depth |
| Round column or post hole | π × (diameter ÷ 2)² × height × number of holes |
Keep your units consistent. If the length and width are in feet, convert the thickness from inches to feet (divide by 12) before multiplying. One cubic yard is 27 cubic feet, and one cubic metre is 1.308 cubic yards.
Worked example
A 12 × 16 ft patio poured 4 inches thick:
- 12 × 16 × (4 ÷ 12) = 64 cubic feet
- 64 ÷ 27 = 2.37 cubic yards
- Add 10% for waste: 2.37 × 1.10 = 2.61 cubic yards
At that size, a ready-mix delivery is the practical choice. The same job would take about 118 bags of 80 lb concrete, including the 10% extra.
How many bags you need
Pre-mixed bags list their yield on the label. These are the typical yields for standard concrete mix:
| Bag size | Yield per bag | Bags per cubic yard | Bags per cubic metre |
|---|---|---|---|
| 40 lb (18 kg) | 0.30 cu ft | 90 | 118 |
| 50 lb (23 kg) | 0.375 cu ft | 72 | 95 |
| 60 lb (27 kg) | 0.45 cu ft | 60 | 79 |
| 80 lb (36 kg) | 0.60 cu ft | 45 | 59 |
These counts are before waste. Yields vary a little between brands and mix types, so check the bag you are buying. For metric bags the calculator uses the same yield per kilogram as a standard 80 lb bag, about 0.47 litres per kilogram of dry mix.
Concrete needed for common slab sizes
Volumes at 4 inches thick, before and after a 10% waste allowance:
| Slab size | Cubic yards | With 10% extra | 80 lb bags (with extra) |
|---|---|---|---|
| 4 × 4 ft | 0.20 | 0.22 | 10 |
| 10 × 10 ft | 1.23 | 1.36 | 62 |
| 12 × 12 ft | 1.78 | 1.96 | 88 |
| 10 × 20 ft | 2.47 | 2.72 | 123 |
| 20 × 20 ft | 4.94 | 5.43 | 245 |
| 24 × 24 ft (two-car garage) | 7.11 | 7.82 | 352 |
Tips before you order
- Measure the actual depth in several places. A slab planned at 4 inches often averages closer to 4.5 inches once the subgrade is dug, which adds about 12% to the volume.
- Round up when ordering ready-mix. Most suppliers sell in quarter- or half-yard increments, and a short load costs more than a little leftover.
- Ask about short-load fees. Many plants add a fee for orders under a few yards. Compare that against the cost of bags.
- Have the base ready. A compacted gravel base, typically 4 inches, helps the slab drain and resists cracking. Use the gravel calculator to size it.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet, so a cubic yard (27 cubic feet) takes 45 bags before any allowance for waste. With 10% extra, plan on about 50 bags.
How many 60 lb bags make a cubic yard?
A 60 lb bag yields about 0.45 cubic feet, so you need 60 bags per cubic yard, or about 66 with 10% extra.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, sidewalks and most garage floors. Driveways that carry heavier vehicles are commonly poured 5 to 6 inches thick. Check your local building code for structural slabs.
How much extra concrete should I order?
Order 5–10% more than the calculated volume. Ground is rarely perfectly flat, forms bow slightly, and running short in the middle of a pour creates a weak cold joint.
Is it cheaper to buy bags or ready-mix concrete?
Bags make sense for small jobs such as setting posts or pouring a small pad. For anything over about one cubic yard, ready-mix is usually cheaper per yard and far less labour, though many suppliers charge a short-load fee for small orders.
How do I calculate concrete for a round post hole?
Multiply π (3.1416) by the radius squared by the depth. A 12-inch-diameter hole that is 4 feet deep holds about 3.14 cubic feet, a little over five 80 lb bags.
Sources
Related calculators
About this calculator. Every formula on this page is tested against worked examples from the sources above before publishing. Results are estimates for planning; confirm with your supplier or local code.
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