Concrete Slab Calculator
Size a slab pour: enter the dimensions, waste allowance and (optionally) your local ready-mix price per cubic yard to get the order volume and material cost. For bag counts instead of a truck, use the bag calculator in the related tools.
Slab volume and cost
A slab is a rectangular prism: length × width × thickness (in feet) gives cubic feet; divide by 27 for the cubic yards ready-mix is sold in. Order with 5–10% extra — forms flex, subgrade is uneven, and a short pour means a cold joint.
Cost is simply order volume × your supplier's price per cubic yard. We don't guess prices — they swing widely by region and PSI mix; grab the number from a local supplier quote. Watch for short-load fees on orders under a full truck (usually under 8–10 yards).
Worked examples
Example 1: 20 × 20 ft garage slab
A 20 × 20 ft slab, 4 in thick, 10% waste.
- Volume in cubic feet.20 × 20 × 0.333 = 133.3 cu ft
- To cubic yards.133.3 ÷ 27 = 4.94 cu yd
- Add waste.4.94 × 1.10 ≈ 5.43 → order 5.5 cu yd
Order about 5.5 cubic yards.
Example 2: slab cost at a quoted price
The same slab with ready-mix quoted at $165 per cubic yard.
- Multiply the order volume by the quote.5.43 × $165 ≈ $896
- Confirm extras with the supplier.+ possible short-load / fuel fees
Roughly $900 of concrete, before delivery fees and labor.
Example 3: three footing pads in one pour
Three 3 × 3 ft pads, 6 in thick, 5% waste.
- One pad.3 × 3 × 0.5 = 4.5 cu ft
- Times three, with waste, to yards.13.5 × 1.05 ÷ 27 = 0.53 cu yd
About half a yard — bag territory, or piggyback on another pour.
FAQ
How thick should my slab be?+
4 inches covers patios, walkways and shed floors; driveways want 4–6 in (6 where trucks park); garage slabs commonly 4–6 in over a proper gravel base. Structural slabs and footings are a code/engineer question.
What does a cubic yard of concrete cost?+
It varies too much to hardcode — by region, PSI rating, and month. US quotes in recent years have commonly run between roughly $130 and $200 per yard delivered; get a local quote and type it into the price field.
Truck or bags for a slab?+
Over about half a cubic yard, ready-mix wins on consistency and your back. A 20×20 slab would take ~245 80-lb bags — that's a truck. Our QUIKRETE bag calculator covers the small-pour side.
Do I need rebar or mesh in a slab?+
Driveways and garage slabs typically get #3/#4 rebar grid or welded wire mesh plus control joints every 8–12 ft; patios often just get fiber mix and joints. Local code and soil conditions decide — joints matter as much as steel.
Related calculators
- Concrete CalculatorWork out how much concrete you need for a slab: cubic yards, 60 lb and 80 lb bag counts, and waste allowance — with the math shown step by step.
- QUIKRETE Bag CalculatorHow many QUIKRETE bags for your pour: enter the slab size and bag weight (40–90 lb) to get the bag count from the manufacturer's published yields.
- Cubic Feet CalculatorCalculate cubic feet from length, width and height — each in inches or feet — with cubic yards and cubic inches, and the conversion steps shown.