Concrete Slab Calculator
Get the cubic yards and bags of concrete needed to pour a slab.
What you'll need to order
- ready-mix concrete
- rebar or wire mesh
- vapour barrier
- expansion joint
- compacted base stone
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
cubic yards = length × width × (thickness ÷ 12) ÷ 27 × 1.10
Concrete is sold by the cubic yard and trucks deliver in quarter-yard increments, so round up. Running short mid-pour creates a cold joint that never fully bonds — over-ordering is far cheaper than the alternative.
Worked example
With length 10 ft, width 10 ft, thickness 4 in, this calculator returns 1.36 cu yd.
Waste allowance
The headline figure includes a 10% allowance for waste and over-ordering. The exact requirement is shown alongside it.
Concrete volume by slab thickness
Figures below assume length 10 ft × width 10 ft. Change any measurement in the calculator to get your own numbers.
| Thickness | Concrete to order (cu yd) | 80 lb bags (small pours only) (bags) |
|---|---|---|
| 3 in | 1.02 | 41.67 |
| 4 in | 1.36 | 55.56 |
| 5 in | 1.70 | 69.44 |
| 6 in | 2.04 | 83.33 |
| 8 in | 2.72 | 111.1 |
| 12 in | 4.07 | 166.7 |
Per 100 square feet, including the 10% over-order allowance. Bag counts are for 80 lb bags of ready-mix.
How much concrete for a shed, patio or garage slab?
| Job | Size | Concrete to order | Material cost |
|---|---|---|---|
| Shed base | 10 ft × 12 ft × 4 in | 1.63 cu yd | $228 – $326 |
| Patio | 16 ft × 12 ft × 4 in | 2.61 cu yd | $365 – $521 |
| Single garage | 12 ft × 22 ft × 4 in | 3.59 cu yd | $502 – $717 |
| Two-car garage | 24 ft × 24 ft × 4 in | 7.82 cu yd | $1,095 – $1,564 |
| Driveway apron thicker for vehicle loads |
20 ft × 10 ft × 6 in | 4.07 cu yd | $570 – $815 |
| Workshop floor | 30 ft × 40 ft × 6 in | 24.44 cu yd | $3,422 – $4,889 |
Anything above about 1 cubic yard is cheaper and far easier as a ready-mix delivery than as bags.
Cost range uses $140–$200 per cubic yard for material only.
What does a concrete slab cost?
Material typically runs $140 to $200 per cubic yard. Ready-mix delivered, material only. Placed and finished by a contractor runs $6 to $12 per square foot including forms, reinforcement and labour.
What moves the price:
- Short-load fees below about 3 cubic yards, often $100 to $200 per load
- Strength: 4,000 psi costs more than the 3,000 psi typical of residential slabs
- Admixtures for cold weather, fibre reinforcement or accelerated set
- Pump hire when the truck cannot reach the pour
Prices are US averages and move with fuel, season and haul distance. Always get two local quotes before budgeting.
Slab thickness by use
| Type | Thickness (in) | Notes |
|---|---|---|
| Footpath, garden shed | 3.50 | Light foot traffic only |
| Patio, standard garage floor | 4.00 | The residential default |
| Driveway, RV or boat parking | 5.00 | Add rebar on a compacted base |
| Workshop, light truck traffic | 6.00 | Rebar mat, not just mesh |
| Heavy equipment pad | 8.00 | Engineered design; check with your local code |
Thickness is only half the job. A 6 inch slab on poor subgrade cracks sooner than a 4 inch slab on a properly compacted base.
Concrete slab questions
How many cubic yards of concrete do I need for a 20x20 slab?
At 4 inches thick, about 4.9 cubic yards, or 5.4 once you allow 10%. At 6 inches it rises to 7.4 yards, or 8.1 with the allowance.
How many 80 lb bags make a cubic yard?
Forty-five bags, since each yields 0.6 cubic feet. Beyond about one cubic yard, bagged concrete stops making sense — the cost, mixing time and joint risk all favour ready-mix.
How thick should a concrete slab be?
Four inches for patios, walkways and shed floors. Five to six inches for driveways, and six inches with rebar anywhere a heavy vehicle parks. Thickness matters less than a properly compacted base.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, 1.23 cubic yards exactly, or 1.36 with the 10% allowance. That is right at the point where ready-mix beats bags — 45 bags of 80 lb would be needed instead.
How do I calculate concrete in cubic yards?
Multiply length by width by thickness, all in feet, then divide by 27. For thickness in inches, divide the inches by 12 first. A 20 by 20 ft slab at 4 inches is 20 × 20 × 0.333 ÷ 27 = 4.94 cubic yards.
Do I need rebar in a concrete slab?
Wire mesh is enough for a patio or shed floor. Use #4 rebar on an 18 inch grid for driveways, garages and anywhere a vehicle parks. Reinforcement does not stop cracks forming — it holds the two sides together when they do.
How much gravel goes under a concrete slab?
Four inches of compacted crushed stone is standard, six inches on clay or poorly drained soil. The base matters more than slab thickness: most residential slab failures start underneath.
How long before you can walk on a new slab?
About 24 to 48 hours for foot traffic, seven days before vehicles, and 28 days to reach full design strength. Cold weather extends all three considerably.
Why order 10% extra concrete?
Subgrade is never perfectly flat, forms flex, and some concrete is lost in the chute and wheelbarrow. Ten percent covers it. Running short means a cold joint, which is a permanent weakness in the slab.