Concrete Slab Calculator (Patio, Garage, Drive)
Size a patio, garage floor, or driveway slab with the right thickness for the load — then get cubic yards, cost, and square footage in one pass.
Concrete Slab Calculator
Enter project dimensions below — results update instantly. Switch units freely.
Estimates assume typical industry density and waste factors. Always verify with your supplier and local building code before purchasing material.
Slab Thickness: Match the Depth to the Load, Not to Tradition
‘4 inches is standard.’ That rule comes from 1940s residential construction and still works for patios and walkways — but it's the wrong answer for anything carrying a truck, trailer, or heavy equipment.
Slab thickness by actual load:
- Patio, walkway — foot traffic only — 3-4 inches
- Residential driveway — passenger vehicles — 4-5 inches
- Garage floor — typical cars + storage — 4-5 inches
- Garage floor with lift / workshop — 5-6 inches
- RV pad, boat trailer — 6 inches
- Heavy equipment, dump truck — 6-8 inches with rebar
Every extra inch adds ~25% material cost but quadruples the point-load capacity. Err thicker on garage floors — the difference between 4 and 5 inches is $200 on a typical slab, and it prevents the spider-web cracks that show up under engine-hoist point loads.
Why Thicker Isn't Always Stronger — Reinforcement Matters More
Plain concrete is strong in compression (crushing) but weak in tension (bending). Slabs crack because something pulls them apart — shrinkage during cure, thermal expansion, settling subgrade, or point loads.
Reinforcement options, cheapest to most expensive:
- Fiber mesh (+$8-12 per yd³) — chopped polypropylene fibers mixed in. Controls shrinkage cracks, does nothing for structural loads.
- Wire mesh (6×6 W1.4/W1.4, +$0.25-0.40 per ft²) — holds cracks together once they form. Minimum spec for any slab >100 ft².
- Rebar grid (#4 at 16–24 in o.c., +$0.50-1.00 per ft²) — structural reinforcement. Spec'd for driveways, garage floors, anything carrying vehicle loads.
- Post-tensioned (+$1.50-3.00 per ft²) — cables tensioned after cure. Commercial and foundation slabs only.
Rule of thumb: residential patios get fiber + wire mesh; driveways and garage floors get #4 rebar at 16 in on-center. Don't let a contractor skip rebar on a driveway to save $200 — the crack repair bill will be $2,000.
| Use | Min. Thickness | Reinforcement | Target PSI |
|---|---|---|---|
| Walkway | 3 in | Fiber mesh | 3,000 psi |
| Patio | 4 in | Fiber + 6×6 wire | 3,000 psi |
| Sidewalk | 4 in | 6×6 wire mesh | 3,500 psi |
| Residential driveway | 5 in | #4 rebar 24 in o.c. | 4,000 psi |
| Garage floor | 5 in | #4 rebar 16 in o.c. | 4,000 psi |
| Workshop / lift area | 6 in | #4 rebar 12 in o.c. | 4,500 psi |
| RV / boat pad | 6 in | #5 rebar 12 in o.c. | 4,500 psi |
| Commercial loading | 8 in+ | Engineered design | 5,000+ psi |
PSI = compressive strength at 28 days. Ready-mix plants quote by PSI; specify when ordering.
| Slab Dimensions | Square Feet | Cubic Yards | Cost @ $165/yd³ |
|---|---|---|---|
| 8 × 10 ft | 80 | 0.99 | $163 |
| 10 × 10 ft | 100 | 1.23 | $203 |
| 12 × 12 ft | 144 | 1.78 | $294 |
| 14 × 14 ft | 196 | 2.42 | $399 |
| 16 × 20 ft | 320 | 3.95 | $652 |
| 20 × 20 ft | 400 | 4.94 | $815 |
| 24 × 24 ft | 576 | 7.11 | $1,173 |
| 30 × 40 ft | 1,200 | 14.81 | $2,444 |
Add 10% waste and 5-10% for thicker sections (turned-down edges, integral footings).
Real-World Example Calculations
Backyard Patio 14 × 14 ft @ 4 in
Stamped concrete patio with decorative border, fiber + wire mesh reinforcement.
- Length × Width
- 14 × 14 ft
- Thickness
- 4 in
- $/yd³
- $170
Takeaway: Add $0.30/ft² for wire mesh ($59) and $300-400 for stamp pattern rental. Total DIY cost ~$900.
Two-Car Garage Slab 24 × 24 ft @ 5 in
Detached garage floor over compacted gravel base with vapor barrier.
- Length × Width
- 24 × 24 ft
- Thickness
- 5 in
- $/yd³
- $165
Takeaway: One ready-mix truck. Add rebar grid $400-550, vapor barrier $90, isolation joints $60. Total ~$2,100.
RV Pad 12 × 45 ft @ 6 in
Heavy-duty concrete pad for 35-ft Class A motorhome.
- Length × Width
- 12 × 45 ft
- Thickness
- 6 in
- $/yd³
- $175
Takeaway: One full-truck delivery. Spec 4,500 psi mix + #5 rebar grid for jack-pad loading.
Frequently Asked Questions
How thick should a concrete slab be?
For patios: 4 inches. Residential driveways: 4-5 in. Garage floors: 5 in. RV pads: 6 in. Always match thickness to the heaviest expected load, not the average load.
Do I need rebar in a concrete slab?
For driveways, garage floors, and anything carrying vehicles: yes, #4 rebar at 16-24 in on center. Patios and walkways can use fiber mesh or wire mesh instead of rebar. Plain concrete (no reinforcement) is only appropriate for unloaded slabs under 100 ft².
What is the best PSI concrete for a slab?
Patios: 3,000-3,500 psi. Driveways and garage floors: 4,000 psi. Heavy loading (RV, commercial): 4,500-5,000 psi. PSI refers to 28-day compressive strength; higher PSI costs more but resists cracking better.
How much does a concrete slab cost installed?
Residential concrete slabs run $6-12 per square foot installed. Breakdown: $2-3 material, $3-5 labor, $1-3 base prep, $1-2 reinforcement. Stamped or colored concrete adds $3-8 per ft².
Should concrete have control joints?
Yes — every slab needs control joints to force cracks into predictable locations. Spacing rule: 30× slab thickness in feet, maximum 15 ft. A 4-inch slab needs joints every 10 feet; a 5-inch slab every 12.5 feet. Cut joints within 12 hours of placement, 1/4 of slab thickness deep.
How long before I can drive on a new concrete slab?
For passenger vehicles: 7 days. For heavy loads (RV, truck): 14 days. Concrete reaches 70% of design strength at 7 days and 100% at 28 days. Foot traffic is OK after 24 hours; bike / light wheelbarrow after 48 hours.
Does a concrete slab need a vapor barrier?
For any slab inside a building (garage, basement, shed floor): yes — 10-mil polyethylene under the slab. Prevents ground moisture from wicking up through the concrete and damaging stored items or flooring. Exterior slabs (patios, driveways) don't need vapor barrier.