Quick answer: cubic yards = length × width × thickness (all in feet) ÷ 27. A 10 × 12 ft slab at 4 inches is 40 ft³ = 1.48 yards. One cubic yard takes 45 × 80 lb bags, 60 × 60 lb bags or 90 × 40 lb bags — and weighs about 4,050 lb. Order 5–10% extra.
Concrete needed for common slab sizes
Cubic yards before waste allowance. Add 5–10% to whatever you order:
| Slab size | 4 in thick | 5 in thick | 6 in thick | 80 lb bags (4 in) |
|---|---|---|---|---|
| 4 × 4 ft | 0.20 yd³ | 0.25 | 0.30 | 9 |
| 8 × 8 ft | 0.79 yd³ | 0.99 | 1.19 | 36 |
| 10 × 10 ft | 1.23 yd³ | 1.54 | 1.85 | 56 |
| 10 × 12 ft | 1.48 yd³ | 1.85 | 2.22 | 67 |
| 12 × 12 ft | 1.78 yd³ | 2.22 | 2.67 | 80 |
| 12 × 20 ft | 2.96 yd³ | 3.70 | 4.44 | 134 |
| 16 × 20 ft (2-car) | 3.95 yd³ | 4.94 | 5.93 | 178 |
| 20 × 20 ft | 4.94 yd³ | 6.17 | 7.41 | 223 |
| 24 × 24 ft | 7.11 yd³ | 8.89 | 10.67 | 320 |
Look at the bag column and the crossover becomes obvious. A 10 × 10 patio means mixing 56 bags by hand — realistically a full day with a mixer and a helper. The same pour is a 25-minute ready-mix delivery. Bags win on small jobs; past about a yard they stop making sense.
The formula for each pour type
Round column (sonotube): V = π × (diameter/2)² × height
Stairs: V = width × rise × run × N(N+1)/2
The stairs formula is the one worth explaining. Seen from the side, a solid staircase is a stack of rectangles: the bottom step runs the full length, the next is one tread shorter, and so on to the top. Adding those up gives the triangular number N(N+1)/2. A 4-step flight is therefore 10 rectangles' worth of concrete, not 4 — which is why stairs consume far more than people expect.
Worked example — a 10 × 12 ft patio at 4 inches:
- Convert the thickness: 4 in ÷ 12 = 0.333 ft
- Volume: 10 × 12 × 0.333 = 40 ft³
- Cubic yards: 40 ÷ 27 = 1.48 yd³
- With 10% waste: 1.63 yd³ → order 1.75 yards (suppliers sell in quarter-yard steps)
- Weight: 40 × 150 = 6,000 lb of concrete
Bag yields and what they cost you in effort
| Bag size | Yield | Bags per yd³ | Bags per ft³ |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 3.34 |
| 50 lb | 0.375 ft³ | 72 | 2.67 |
| 60 lb | 0.45 ft³ | 60 | 2.23 |
| 80 lb | 0.60 ft³ | 45 | 1.67 |
A cubic yard as 80 lb bags is 45 bags weighing 3,600 lb — two pallets, and about 90 gallons of water to mix in. Ready-mix trucks typically carry 8 to 11 yards, most suppliers set a minimum of 1 yard, and short loads under about 3 yards usually attract a fee. Factor that into the comparison rather than just the per-yard price.
How thick, and how much rebar
| Application | Thickness | Typical reinforcement |
|---|---|---|
| Walkway / path | 4 in | Fibre mesh or none |
| Patio | 4 in | Wire mesh or fibre |
| Shed / garden base | 4 in | Wire mesh |
| Car driveway | 4–6 in | #3 rebar at 18 in centres |
| Garage floor | 4–6 in | #3 rebar or mesh |
| RV / truck parking | 6–8 in | #4 rebar at 12–18 in |
| Footings (frost depth) | Per local code | #4 rebar, continuous |
These are common practice, not code. Footing depth in particular is set by your local frost line and soil type, and anything structural should be confirmed against your building department's requirements — this calculator sizes the pour, it does not design it.
Common mistakes & pro tips
- Leaving thickness in inches. Multiplying 10 × 12 × 4 gives 480, not 40. Convert inches to feet by dividing by 12 — or use the per-field unit selectors above.
- Dividing by 9 instead of 27. A cubic yard is 3 × 3 × 3 = 27 cubic feet. Nine is square yards.
- Ordering the exact amount. Subgrade is never flat, and forms bow. Five to ten percent extra is cheap insurance against a cold joint.
- Forgetting the thickened edges. Many slabs are deepened around the perimeter, which can add 10–15% over the flat-slab figure.
- Pro tip — pour continuously. Concrete that sets in stages forms a cold joint, a permanent weak plane. Have enough people and equipment lined up before the truck arrives.
- Pro tip — check truck access first. A loaded mixer is over 30 tons and needs solid ground and roughly 12 ft of overhead clearance. Discovering the driveway will not take it on pour day is an expensive lesson.
How to use this calculator
- Pick the pour type. Slab, footing, round column, stairs or wall. Each has its own geometry and its own labels.
- Enter the dimensions. Thickness is usually entered in inches while length and width are in feet — the calculator handles the mixed units.
- Add a waste allowance. Order 5 to 10% extra for spillage, uneven subgrade and over-excavation. The calculator adds this for you.
- Read yards or bags. Under about half a cubic yard, bagged mix is usually cheaper. Above roughly one yard, ready-mix delivery wins on both cost and effort.
Frequently asked questions
How do I calculate cubic yards of concrete?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 10 × 12 ft slab poured 4 inches thick is 10 × 12 × 0.333 = 40 ft³, which is 40 ÷ 27 ≈ 1.48 cubic yards. Order about 10% more than that to cover waste and an uneven subgrade.
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. An 80 lb bag yields about 0.60 cubic feet of mixed concrete, and 27 ÷ 0.60 = 45. A 60 lb bag yields 0.45 ft³ so you need 60 bags, and a 40 lb bag yields 0.30 ft³ so you need 90.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, a 10 × 10 ft slab needs 33.3 cubic feet, or 1.23 cubic yards — about 56 × 80 lb bags. At 6 inches thick it rises to 50 ft³ or 1.85 cubic yards. Add 10% and order 1.4 or 2.0 yards respectively.
Should I use bags or ready-mix?
Bags make sense below about half a cubic yard — roughly 23 × 80 lb bags. Between half a yard and a yard it is a judgement call. Above one cubic yard, ready-mix is almost always cheaper per yard, far less work, and gives a single continuous pour, which matters for strength since cold joints form when batches set at different times.
How much does a cubic yard of concrete weigh?
About 4,050 pounds, just over two tons, at a typical density of 150 lb per cubic foot. That is why a full 10-yard ready-mix truck weighs upwards of 20 tons loaded and why driveways and site access need thinking about before the truck arrives.
How thick should a concrete slab be?
Four inches suits patios, walkways and shed bases. Driveways carrying cars want 4 to 6 inches, and 6 inches or more where trucks or RVs will park. Garage floors are typically 4 to 6 inches. Always check your local building code, which governs for anything structural.
How much extra concrete should I order?
Five to ten percent is standard. The subgrade is never perfectly level, forms bow slightly under load, and some concrete is always lost to spillage. Running short mid-pour is far worse than a little waste — it creates a cold joint and can mean a second delivery charge.