How to use the concrete slab cost calculator
- Enter the slab length and width, then the thickness. A 4 in slab is typical for patios and walkways; driveways and garage floors are often 5-6 in. Check local requirements for your project.
- Set a waste allowance to cover spillage, form deflection and uneven subgrade.
- Enter the price per cubic yard from your supplier. The default is only an example. Concrete prices vary widely by region, mix strength and date.
- Optionally add delivery or short-load fees, labor per square foot, and other costs such as forms, reinforcement or finishing.
How the calculation works
The calculator multiplies length × width × thickness after converting every dimension to the same unit, giving the slab volume. It adds the waste allowance, converts the result to cubic yards (27 cubic feet per cubic yard), and multiplies by your price. Labor is charged on the slab area, and fixed fees are added once.
It also shows the order rounded up to the next quarter yard, since ready-mix trucks are dispatched in fixed increments.
Formula
- L, W
- Slab length and width in feet
- T
- Thickness (inches ÷ 12 gives feet)
- w
- Waste allowance as a decimal
- p
- Price per cubic yard
Worked examples
20 × 12 ft patio, 4 in thick
Volume = 20 × 12 × (4 ÷ 12) = 80 ft³ = 2.963 yd³. With 10% waste: 2.963 × 1.10 = 3.26 yd³.
At an example price of $165/yd³ the concrete costs 3.26 × 165 = $537.78. A supplier would likely deliver 3.5 yd³ (rounded up to the next ¼ yd³).
Adding labor and delivery
The same 240 sq ft slab with $6.00/sq ft labor and a $90 short-load fee: 537.78 + 6 × 240 + 90 = $2,067.78, or about $8.62 per square foot.
Assumptions and limitations
- The calculator does not supply current or local concrete prices. Prices depend on location, mix strength, additives, season and order size, so always use a written quote.
- Volume assumes a flat slab of uniform thickness. Thickened edges, footings under the slab and sloped subgrade add concrete; use the footing calculator for those parts.
- Base material (gravel), vapor barrier, reinforcement, permits, excavation and finishing are only included if you enter them as “other” costs.
- This is a cost and quantity estimate, not a structural design. Slab thickness, reinforcement and base requirements depend on loads, soil and local codes.
Frequently asked questions
How many cubic yards of concrete do I need for a 10 × 10 slab?
At 4 in thick, a 10 × 10 ft slab is 33.3 ft³, or 1.23 yd³ before waste. With 10% extra, order about 1.36 yd³ (in practice, 1.5 yd³).
How much waste should I allow for concrete?
Five to ten percent is a common allowance. Use the higher end when the subgrade is uneven, forms are long or flexible, or the pour is being placed by wheelbarrow.
Is it cheaper to use bags or ready-mix?
For very small pours (under about a cubic yard) bagged concrete can be practical. For larger slabs, ready-mix is usually cheaper and gives a more consistent pour. The footing calculator shows bag counts for small volumes.
Why is my supplier quoting more concrete than this calculator?
Suppliers round up to their delivery increments and may add their own overage. They may also have measured thickened edges or slab variations that a simple rectangle does not capture.