Concrete Slab Calculator
Plan a uniform rectangular slab and see both placed volume and budget-level ready-mix cost. Structural design and local prices remain separate decisions.
What does the concrete slab calculator calculate?
A slab’s volume equals its length times width times thickness. Convert thickness from inches to feet and cubic feet to cubic yards, then add placement allowance. This estimate assumes uniform thickness; calculate thickened edges, turn-downs, grade beams, piers, or steps as separate volumes.
Uniform slab volume and concrete subtotal
The calculator converts uniform slab thickness to feet, finds cubic feet, and then converts to cubic yards. The displayed subtotal multiplies order volume by your local per-yard material price only.
How to measure for an accurate estimate
- Use the slab dimensions shown on approved plans or measured inside finished forms.
- Inspect subgrade elevation on a grid so low spots do not silently increase volume.
- Separate thickened edges and beams from the uniform field of the slab.
- Do not reduce specified thickness to meet a material budget; revise design professionally.
Garage-size slab
A 24 ft by 20 ft slab is 4 inches thick, with 7% extra and concrete at $175 per yard.
Volume is 5.93 cu yd; order estimate is 6.34 cu yd; material subtotal is about $1,110.Before you purchase material
- Get a written quote covering concrete, delivery, fuel, environmental, short-load, and waiting fees.
- Coordinate reinforcement supports, vapor retarder, base, forms, joints, and embedded items before pouring.
- Confirm truck or pump access and provide an approved concrete washout location.
- Schedule for suitable weather and follow the specified curing method after finishing.
Common questions
How many yards of concrete are in a 20-by-20 slab?
At four inches thick, a 20-by-20-foot uniform slab contains about 4.94 cubic yards before waste. At six inches, it contains about 7.41 cubic yards. Add thickened edges separately and verify the design thickness.
How much does concrete for a slab cost?
Multiply order-ready cubic yards by a current local ready-mix quote, then add delivery, short-load, pumping, reinforcement, forms, base, finishing, tax, and labor. National averages are less useful than a quote matched to mix and location.
Does a slab calculator include footings and thickened edges?
This calculator covers only the uniform rectangular slab field. Calculate perimeter thickening, grade beams, isolated footings, piers, and steps independently, add their exact volumes, and then apply one placement allowance to the combined order.
What thickness should my concrete slab be?
Thickness depends on intended loads, soil, frost, base preparation, reinforcement, joints, and local building requirements. Common practice is not a substitute for design. Use approved plans or obtain guidance from a qualified engineer or building authority.
Sources and reference standards
- International Code Council — 2024 International Residential CodePrimary model-code source for residential foundations, reinforcement, masonry, and stair safety requirements.
- Concrete Reinforcing Steel InstituteIndustry source explaining reinforced-concrete materials and placement fundamentals.