Concrete Slab Calculator
Calculate concrete volume, bags needed, and rebar estimation for slab projects. Includes waste percentage and ready-mix truck recommendations.
Concrete Slab Calculator
Calculate concrete volume, bags needed, and rebar estimation for slab projects
Thickness Guidelines
- 4" - Sidewalks, patios, garage floors
- 5-6" - Driveways, heavy foot traffic areas
- 6" - Heavy duty, industrial applications
- 8"+ - Foundations, structural slabs
Formula
Volume (cu yd) = (Length × Width × Thickness) / 27Where all dimensions are converted to feet. One cubic yard equals 27 cubic feet. Includes waste percentage for spillage and uneven ground.
Quick Reference: Common Slab Sizes (4" thick, 10% waste)
| Slab Size | Thickness | Cubic Yards | 60 lb Bags | 80 lb Bags |
|---|---|---|---|---|
| 8' × 10' | 4" | 0.99 | 66 | 50 |
| 10' × 10' | 4" | 1.23 | 82 | 62 |
| 10' × 12' | 4" | 1.48 | 99 | 74 |
| 12' × 12' | 4" | 1.78 | 119 | 89 |
| 12' × 16' | 4" | 2.37 | 158 | 119 |
| 16' × 16' | 4" | 3.16 | 211 | 158 |
| 16' × 20' | 4" | 3.95 | 264 | 198 |
| 20' × 20' | 4" | 4.94 | 330 | 247 |
| 20' × 24' | 4" | 5.93 | 395 | 297 |
| 24' × 24' | 4" | 7.11 | 475 | 356 |
Understanding Concrete Slabs
Concrete Volume Formula
Where all dimensions are in feet. One cubic yard equals 27 cubic feet.
Rebar Reinforcement
Rebar (reinforcing bar) strengthens concrete and prevents cracking. It's laid in a grid pattern with pieces running both lengthwise and widthwise. Standard #4 rebar (1/2" diameter) is commonly used for residential slabs. The rebar should be positioned in the lower third of the slab thickness.
Concrete Bag Coverage
- 60 lb bag: Covers approximately 0.45 cubic feet
- 80 lb bag: Covers approximately 0.60 cubic feet
For large projects (over 2-3 cubic yards), ready-mix concrete delivery is more economical and ensures consistent quality throughout the pour.
Slab Thickness Guidelines
4" Thickness
Sidewalks, patios, garage floors, light foot traffic
5" Thickness
Driveways for passenger vehicles
6" Thickness
Heavy-duty driveways, RV parking, industrial floors
8"+ Thickness
Foundations, structural slabs, heavy equipment areas
How to Use
- 1Enter dimensions — Enter the length and width of your slab in feet, inches, or meters.
- 2Select thickness — Choose a preset slab type or enter a custom thickness. Common thicknesses are 4" for patios and 6" for driveways.
- 3Choose rebar spacing — Select rebar spacing if reinforcement is needed. Standard is 16" on center.
- 4Set waste percentage — Add waste percentage (typically 10%) to account for spillage and uneven ground.
- 5View results — See concrete volume, bags needed, rebar pieces, and ready-mix truck recommendations.
Frequently Asked Questions
How do I calculate concrete for a slab?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. For example, a 10' × 12' × 4" slab = 10 × 12 × 0.333 = 40 cubic feet = 1.48 cubic yards.
How thick should a concrete slab be?
Thickness depends on the application: 4" for sidewalks and patios, 4-6" for driveways and garage floors, 6" for heavy-duty applications, and 8"+ for foundations. Always check local building codes.
How much waste should I add to my concrete estimate?
Add 10% for waste to account for spillage, uneven ground, and form variations. For complex shapes or uneven terrain, consider adding 15-20%.
When should I use bagged concrete vs. ready-mix?
Bagged concrete is economical for small projects under 1 cubic yard. For projects over 2-3 cubic yards, ready-mix delivery is more cost-effective and ensures consistent quality.
What rebar spacing should I use?
Standard rebar spacing is 16" on center for most residential slabs. Use 12" spacing for heavy-duty applications like driveways with heavy vehicles. Light-duty patios may use 18-24" spacing or wire mesh instead.
How many bags of concrete do I need?
A 60 lb bag covers about 0.45 cubic feet, and an 80 lb bag covers about 0.60 cubic feet. For a 1 cubic yard project (27 cu ft), you'd need about 60 bags of 60 lb mix or 45 bags of 80 lb mix.