🧱 SLAB SIZE CALCULATOR
Slab Thickness by Use Case
How thick should your concrete slab be? Code-recommended dimensions for any project
| Application | Min Thickness | Recommended | Reinforcement |
|---|---|---|---|
| Sidewalk / Walkway | 3.5" | 4" | Optional fiber |
| Patio | 4" | 4" | Mesh recommended |
| Residential Driveway | 4" | 5" | Mesh or rebar grid |
| Garage Floor | 4" | 5–6" | Rebar grid required |
| RV/Truck Pad | 6" | 6–8" | Heavy rebar grid |
| Shed Foundation (small) | 4" | 4" | Mesh |
| Pool Deck | 4" | 4–5" | Mesh + control joints |
| Hot Tub Pad | 4" | 6" | Rebar grid |
For freeze-thaw climates (Northeast, Midwest, Mountain West), add 1 inch to the recommended thickness. Always check your local building code — some jurisdictions require 6" minimum for driveways regardless of use.
Slab Pour Checklist
A repeatable order-of-operations that prevents the most common DIY slab mistakes
- Excavate to depth. Slab thickness + base depth + 2" for forms = total dig depth. For a 4" slab on 4" base, dig 10" below final grade.
- Compact subgrade. Native soil should be compacted to 95% before adding base. Don't skip this on clay or recently disturbed soil.
- Lay 4" of crushed stone or roadbase. Compact in 2" lifts. 95% compaction is the target.
- Set forms. 2x4 lumber for 3.5" slabs, 2x6 for 5" slabs. Stake every 4 feet. Check for level and proper slope (¼" per foot away from buildings).
- Place vapor barrier and reinforcement. 6 mil poly under interior slabs. Wire mesh or rebar grid raised 1.5–2" off the base on chairs.
- Pour and screed. Pour from far end working back. Screed level with the forms using a 2x4. Don't over-work the surface.
- Float, edge, and joint. Bull float once water sheen disappears. Edge the perimeter with an edging tool. Cut control joints every 8–10 feet.
- Final finish and cure. Broom finish for traction or trowel for smooth. Cover with plastic for 7 days minimum, lightly mist daily.
- Wait before loading. Foot traffic 24–48 hours. Vehicles 7 days. Heavy equipment 28 days.
Concrete by Slab Size — Complete Reference Table
All values include 10% waste factor. Use 80lb bags or ready-mix for projects over 1 yard.
| Slab Size | Sq Ft | 4" Thick (yards) | 4" — 80lb Bags | 6" Thick (yards) | 6" — 80lb Bags | Est. Ready-Mix Cost |
|---|---|---|---|---|---|---|
| 4 × 4 ft | 16 | 0.22 | 7 bags | 0.33 | 10 bags | bags recommended |
| 4 × 8 ft | 32 | 0.43 | 13 bags | 0.65 | 20 bags | bags recommended |
| 10 × 10 ft | 100 | 1.37 | 41 bags | 2.06 | 62 bags | $165–$247 |
| 12 × 12 ft | 144 | 1.98 | 59 bags | 2.97 | 89 bags | $238–$356 |
| 12 × 20 ft | 240 | 3.30 | 99 bags | 4.94 | 148 bags | $396–$594 |
| 20 × 20 ft | 400 | 5.49 | 164 bags | 8.23 | 247 bags | $659–$988 |
| 20 × 30 ft (driveway) | 600 | 8.23 | ready-mix | 12.35 | ready-mix | $988–$1,481 |
| 24 × 24 ft (garage) | 576 | 7.91 | ready-mix | 11.87 | ready-mix | $949–$1,424 |
| 30 × 30 ft | 900 | 12.35 | ready-mix | 18.52 | ready-mix | $1,482–$2,224 |
| 40 × 60 ft (large pad) | 2,400 | 32.93 | ready-mix | 49.38 | ready-mix | $3,951–$5,927 |
All values include 10% waste. Ready-mix cost based on $120–$180/yard. For projects under 1 yard, bags are always cheaper (no short-load fee).
Bags vs Ready-Mix: Which to Use by Slab Size
The break-even point is around 1.5 cubic yards — here's the rule of thumb
✓ Use BAGS for:
- Projects under 1.5 cubic yards (~60 sq ft at 4")
- Fence posts, footings, mailbox bases
- Tight spaces trucks cannot access
- Weekend projects with flexible timing
- Areas with no good truck access
✓ Use READY-MIX for:
- Projects over 1.5 cubic yards
- Driveways, garage floors, large patios
- Time-sensitive pours (stays workable longer)
- Projects requiring consistent mix quality
- When labor cost of mixing bags outweighs delivery
Common Slab Questions with Specific Answers
The answers Google users are looking for — with exact numbers
How much concrete for a 10x10 slab?
At 4" thick: 1.37 cubic yards or 41 bags of 80lb concrete. At 6" thick: 2.06 yards or 62 bags.
How much concrete for a 12x12 slab?
At 4" thick: 1.98 cubic yards or 59 bags of 80lb concrete. At 6" thick: 2.97 yards or 89 bags.
How much concrete for a 20x20 slab?
At 4" thick: 5.49 cubic yards. Order ready-mix — mixing 164 bags by hand is not practical for one person.
How much concrete for a 4x8 slab?
At 4" thick: 0.43 cubic yards or 13 bags of 80lb concrete. Small project — bags are the right choice.
Concrete Slab FAQ
Common questions about calculating and pouring concrete slabs