Concrete slab calculator
Work out how much concrete a slab needs, in cubic yards and in 40, 60 and 80 lb bags, at the thickness the job calls for.
Concrete slab calculator
Use your supplier's density
You need
- Volume
- Weight
- Area
| Order by | Exact | Order, rounded up for anything sold in whole bags |
|---|
Enter your measurements to see what to order.
One cubic yard of concrete weighs about 2.02 US tons and covers about 82 square feet at 3.9 in.
How to work out how much concrete you need
- Measure the area in feet and multiply length by width.
- Pick a depth in inches and multiply it by the area to get a volume.
- Multiply the volume by the bulk density, 2400 kg per cubic metre for concrete, to get a weight.
- Divide the weight by your supplier's ton or cubic yard to get what to order, and round up. Suppliers do not sell part bags.
- Add 10 percent. Covers over-dig, spillage and a sub-base that is not perfectly level. Concrete cannot be topped up once it has started to go off, so running short is the expensive failure.
How many bags of concrete in a cubic yard?
45 of the 80 lb bags, because a cubic yard is 27 cubic feet and each bag yields 0.6 cubic feet. Sakrete states 45 bags of 80 lb mix to the cubic yard, which is the same figure.
How much does a cubic yard of concrete weigh?
About 2.02 US tons, or 2.4 tonnes per cubic metre. That is the weight of the finished pour, which is what matters for a trailer or a slab's load on the ground below it.
How thick should a concrete slab be?
| Use | Depth | What the supplier says |
|---|---|---|
| Sidewalk | 3.9 in | Quikrete: sidewalks should be at least 4 inches (100mm) in thickness. |
| Patio | 3.9 in | Quikrete: 4 inches for a walk or a light-duty slab on stable ground. |
| Slab or base | 3.9 in to 5.9 in | Quikrete: simple slabs for small foundations and bases are normally 4 to 6 inches (100 to 150mm) thick, depending on the load they must bear. |
| Driveway | 5.9 in | Quikrete: 6 inches is the upper end of its small-slab range, for the heaviest loads it covers. |
How many bags of concrete for a slab?
At 4 inches thick, a cubic yard of concrete covers 81 square feet, and one cubic yard takes 45 of the 80 lb bags. Every row below is this page's own calculator with no waste added, so add 10 percent before you buy.
| Slab | Thickness | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 4 ft x 4 ft | 4 in | 0.2 | 9 | 12 | 18 |
| 4 ft x 4 ft | 6 in | 0.3 | 14 | 18 | 27 |
| 8 ft x 8 ft | 4 in | 0.79 | 36 | 48 | 72 |
| 8 ft x 8 ft | 6 in | 1.19 | 54 | 72 | 107 |
| 10 ft x 10 ft | 4 in | 1.23 | 56 | 75 | 112 |
| 10 ft x 10 ft | 6 in | 1.85 | 84 | 112 | 167 |
| 12 ft x 12 ft | 4 in | 1.78 | 80 | 107 | 160 |
| 12 ft x 12 ft | 6 in | 2.67 | 120 | 160 | 240 |
| 20 ft x 20 ft | 4 in | 4.94 | 223 | 297 | 445 |
| 20 ft x 20 ft | 6 in | 7.41 | 334 | 445 | 667 |
When bagged concrete stops being worth it
A 10 ft by 10 ft slab at 4 inches is 56 bags of 80 lb mix, each of which has to be carried, opened and mixed. Past roughly a cubic yard, ready-mix delivered by truck is usually the cheaper and faster option, and the calculator gives you the cubic yard figure to quote.
How much does concrete weigh?
Between 2373 and 2500 kg per cubic metre, a spread of 5 percent. Normal weight concrete as placed, unreinforced. Four of the five independent sources land within 3 kg of 2,400, which is why the midpoint is the modal value rather than the arithmetic mean. The calculator above uses 2400, and you can replace it with your own supplier's figure.
9 published figures from 8 suppliers, read 2026-09-20.
| Supplier | Published figure | Condition |
|---|---|---|
| Washington State Department of Transportation (WSDOT), Bridge Design Manual M 23-50.24, Chapter 5 Concrete Structures, Section 5.1.1.DUS | 0.150 kcf (= 150 lb/ft3) for normal-weight concrete; 0.155 kcf for prestressed concrete girders | Plain (unreinforced) concrete, hardened, in-place design value |
| Washington State Department of Transportation (WSDOT), Bridge Design Manual M 23-50.24, Chapter 3 Loads, Table 3.8-1 Permanent LoadsUS | 155 lb/ft3 for All Other Normal-Weight Reinforced Concrete including 5 lb/ft3 allowance for reinforcement; 165 lb/ft3 for precast pre-tensioned or post-tensioned spliced girders including 10 lb/ft3 allowance for reinforcement | Reinforced, hardened, in-place design value |
| Cemex USA (Ready Mix Concrete Calculator, FAQ block)US | about two tons (or 4,000 pounds) per cubic yard | Solid, unbroken (i.e. placed and hardened, not broken-out rubble) |
| Heidelberg Materials UK (Ready-mixed concrete pricing guide)UK | around 2.5 tons per cubic metre (= approx. 2,500 kg/m3) | Unstated (as supplied / as placed) |
| Holcim Australia (Glossary, Tools, FAQs and Resources)AU | around 2400 kg per cubic metre | Unstated (design value per AS 3600) |
| The Concrete Society (UK)UK | oven dry density between 2000-2600 kg/m3 | OVEN DRY |
| SMC Minimix (UK ready-mix supplier), Resource Centre FAQsUK | around 2.4 tonnes per cubic metre | Unstated (as delivered / as placed). |
| Wright Mix (UK on-site ready-mix and screed supplier), Concrete Calculator pageUK | an average of 2.4 tonnes per cubic metre | Unstated (average, as delivered) |
| Ohio University, 'Weights Of Building Materials, Agricultural Commodities, and Floor Loads For Buildings' (course handout H-20, hcia350)US | 150 pounds per cubic foot | Unstated (dead-load design value) |
What you can order concrete in
| Pack | Where | Size |
|---|---|---|
| cubic yard | US, CA | 0.765 m3 |
| 80 lb bag | US, CA | 0.017 m3 |
| 60 lb bag | US, CA | 0.013 m3 |
| 40 lb bag | US, CA | 0.008 m3 |
| cubic metre | UK, AU | 1 m3 |
| 20kg bag | UK, AU | 0.01 m3 |