Concrete Slab Calculator
Volume (Cubic Yards)
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Volume (Cubic Feet)
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Volume (Cubic Meters)
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Bags Needed (+10% waste)
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Est. Cost (ready-mix)
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Rebar Spacing
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How Concrete Slab Calculation Works
A concrete slab calculator is a tool that estimates the volume of concrete needed for flat pours such as patios, driveways, sidewalks, garage floors, and building foundations. The calculation multiplies length by width by thickness to determine the volume in cubic feet, then converts to cubic yards (the standard ordering unit for ready-mix concrete in the United States). According to the Portland Cement Association, residential concrete slabs account for the majority of concrete used in home construction, with the average US home requiring 7-10 cubic yards of concrete for the foundation slab alone.
This calculator handles three slab shapes: rectangular (the most common), circular (for round pads and fire pits), and cylindrical columns (for fence posts, piers, and sonotubes). It also estimates bag counts for smaller projects and recommends rebar spacing based on slab thickness. For general concrete volume calculations, see our concrete calculator. For reinforcement planning, use the rebar calculator.
The Concrete Slab Volume Formula
The formulas for slab volume depend on the shape:
Rectangular slab: Volume (cu ft) = Length (ft) x Width (ft) x Thickness (inches) / 12
Circular slab: Volume (cu ft) = pi x (Diameter / 2)^2 x Thickness (inches) / 12
Convert to cubic yards: Cubic yards = Cubic feet / 27
Worked example: A 20 x 20 foot patio slab at 4 inches thick: Volume = 20 x 20 x (4/12) = 133.3 cubic feet = 4.94 cubic yards. With 10% waste factor: 5.43 cubic yards. At the 2025 national average of $150 per cubic yard for ready-mix delivery, this project costs approximately $815 for concrete materials. The American Concrete Institute (ACI) recommends always ordering 5-10% more than calculated to account for subgrade irregularities and spillage.
Key Terms You Should Know
- Subgrade -- the compacted soil layer beneath the slab. Must be uniformly compacted to prevent differential settlement. Poor subgrade preparation is the leading cause of slab cracking.
- Subbase -- a layer of gravel (typically 4-6 inches of crushed stone) placed on top of the subgrade for drainage and uniform support.
- Control joints -- grooves cut into the slab surface (1/4 of slab depth) that create planned crack lines. Spacing should not exceed 24-36 times the slab thickness per ACI 360R guidelines.
- Vapor barrier -- a polyethylene sheet (6-10 mil thickness) placed under the slab to prevent ground moisture from migrating up through the concrete. Required under all interior slabs per IRC Section R506.2.3.
- Rebar -- steel reinforcing bars placed within the slab to resist tensile cracking. Standard residential slabs use #3 or #4 rebar in a grid pattern.
Recommended Slab Thickness by Application
The International Residential Code (IRC) sets minimum thickness requirements based on the intended use and load-bearing capacity of the slab.
| Application | Min Thickness | PSI | Rebar |
|---|---|---|---|
| Sidewalk / garden path | 4 inches | 3,000 | Wire mesh or none |
| Patio | 4 inches | 3,000-3,500 | #3 rebar, 18" grid |
| Residential driveway | 4-5 inches | 3,500-4,000 | #4 rebar, 12" grid |
| Garage floor | 4-6 inches | 3,500-4,000 | #4 rebar, 12-18" grid |
| Foundation slab | 6-8 inches | 3,500-4,500 | #4 rebar, 12" grid |
| Heavy-vehicle driveway | 5-6 inches | 4,000+ | #4 rebar, 12" grid |
Practical Examples
Backyard patio (12 x 16 ft, 4 inches thick): Volume = 12 x 16 x 0.333 = 64 cu ft = 2.37 cu yd. With 10% waste: 2.61 cu yd. This is right at the threshold for ready-mix -- you would need about 48 bags of 80-lb mix ($288 at $6/bag) versus approximately $392 for ready-mix with a short-load fee. For a project this size, ready-mix is faster but bags give more control over timing.
Two-car garage slab (24 x 24 ft, 5 inches thick): Volume = 24 x 24 x 0.417 = 240 cu ft = 8.89 cu yd. With waste: 9.78 cu yd. Ready-mix cost: approximately $1,467 for materials. This project requires #4 rebar at 12-inch spacing, a 4-inch gravel base, and a vapor barrier. Use the concrete block calculator if you are also building stem walls.
Circular fire pit pad (8 ft diameter, 4 inches thick): Volume = 3.14159 x 16 x 0.333 = 16.76 cu ft = 0.62 cu yd. With waste: 0.68 cu yd. This is a bag project -- approximately 31 bags of 80-lb mix at $186 total.
Shed foundation (10 x 12 ft, 4 inches thick): Volume = 10 x 12 x 0.333 = 40 cu ft = 1.48 cu yd. With 10% waste: 1.63 cu yd. This is at the crossover point between bags and ready-mix. Approximately 73 bags of 80-lb mix ($438) versus ready-mix at $150/yd + $75 short-load fee ($320). Ready-mix is more economical and significantly faster. Consider adding fiber mesh to the mix for extra crack resistance in a shed slab that may see point loads from equipment.
Tips for Pouring a Concrete Slab
- Compact the subgrade thoroughly. Use a plate compactor on the native soil, then add and compact gravel in 2-inch lifts. Soft spots under the slab cause uneven settling and cracking that cannot be repaired without demolition.
- Set forms level and braced. Use 2x4 or 2x6 lumber staked every 2-3 feet. The top of the form defines the slab surface, so accuracy here determines the final result. Check for level across the entire area before pouring.
- Plan for drainage slope. Exterior slabs should slope at least 1/8 inch per foot (1%) away from the house to prevent water pooling against the foundation.
- Work fast once the truck arrives. Ready-mix concrete begins setting within 60-90 minutes. Have enough workers and tools staged before the truck arrives. A 4-person crew is recommended for slabs over 3 cubic yards.
- Cure the concrete properly. Keep the surface moist for at least 7 days using wet burlap, plastic sheeting, or spray-on curing compound. Concrete that dries too quickly loses strength and develops surface cracks. According to ACI, proper curing can increase 28-day strength by 40% compared to uncured concrete.
- Cut control joints within 24 hours. Use a concrete saw to cut joints 1/4 of the slab depth at spacing equal to 24-36 times the slab thickness (for a 4-inch slab, joints every 8-12 feet). Control joints create intentional weak points where cracks will form in a straight line rather than randomly across the surface.
Concrete Slab Cost Breakdown
Understanding the full cost of a concrete slab helps with budgeting. According to 2025 pricing data, the major cost components for a residential slab pour are:
| Cost Component | Cost per Sq Ft | Notes |
|---|---|---|
| Site prep and excavation | $1.00-$2.50 | Grading, compaction, gravel base |
| Forms and labor | $0.50-$1.50 | Lumber, stakes, setup and removal |
| Rebar / wire mesh | $0.30-$0.80 | Material plus labor to tie and place |
| Concrete material | $2.50-$4.50 | Ready-mix at $125-175 per cubic yard |
| Finishing and curing | $1.00-$2.50 | Screeding, floating, brooming, curing compound |
| Total (basic slab) | $6-$12 | Standard gray broom finish |
Decorative options like stamped concrete ($12-$18/sq ft), exposed aggregate ($10-$16/sq ft), and colored concrete ($8-$14/sq ft) cost more but add significant curb appeal. For estimating materials for painted concrete surfaces, see our paint calculator.
Frequently Asked Questions
How do I calculate concrete for a slab?
Multiply the slab length (in feet) by the width (in feet) by the thickness (converted to feet by dividing inches by 12) to get cubic feet. Divide by 27 to convert to cubic yards. For example, a 10 x 10 foot slab at 4 inches thick = 10 x 10 x 0.333 = 33.3 cubic feet = 1.23 cubic yards. Always add 5-10% for waste from spillage, uneven subgrade, and form imperfections.
How many bags of concrete do I need for a slab?
An 80-lb bag yields 0.60 cubic feet and a 60-lb bag yields 0.45 cubic feet. Divide your total cubic feet (including waste) by the per-bag yield. For a 10x10 foot slab at 4 inches thick (33.3 cu ft + 10% waste = 36.6 cu ft), you need approximately 61 bags of 80-lb mix or 82 bags of 60-lb mix. For projects over 1 cubic yard, ready-mix delivery is more economical and practical.
How thick should a concrete slab be?
Thickness depends on the intended use. Sidewalks and patios require 4 inches minimum per the International Residential Code. Residential driveways need 4-5 inches for passenger vehicles and 5-6 inches for heavier vehicles (trucks, RVs). Garage floors typically require 4-6 inches. Foundation slabs need 6-8 inches or more depending on soil conditions and building loads. Always check local building codes for your specific jurisdiction.
What rebar spacing should I use for a concrete slab?
For 4-inch patio and sidewalk slabs, #3 rebar at 18-inch spacing or welded wire mesh (6x6 W1.4/W1.4) provides adequate crack control. For 4-5 inch driveways, use #4 rebar at 12-inch spacing in a grid pattern. Position rebar at one-third of the slab depth from the bottom using rebar chairs. For foundation slabs, an engineer should specify the reinforcement based on soil conditions and structural loads.
How much does a concrete slab cost per square foot?
Total installed cost for a standard 4-inch concrete slab ranges from $6 to $12 per square foot as of 2025, including excavation, gravel base, forms, reinforcement, concrete, finishing, and cleanup. A basic 10x10 patio slab costs approximately $600-$1,200 total. A 24x24 garage slab runs $3,500-$7,000. Material costs alone (concrete only) average $4-$6 per square foot. Stamped or decorative concrete adds $3-$8 per square foot for surface treatments.
Do I need a gravel base under a concrete slab?
Yes, a 4-6 inch layer of compacted gravel (crushed stone or road base) is recommended under all concrete slabs. The gravel provides uniform support, improves drainage, and prevents the slab from sitting directly on expansive clay or organic soil that can heave and settle. The IRC requires a 4-inch minimum granular base for slab-on-grade foundations. Compact the gravel in 2-inch lifts with a plate compactor to achieve 95% compaction density.