Concrete Slab Calculator
Whether you are pouring a patio, garage floor, or driveway, this calculator gives you the exact concrete volume and rebar grid layout for a rectangular slab. Enter your dimensions and thickness, choose a rebar spacing, and you get cubic yards for ordering ready-mix, bag counts for smaller pours, and a complete rebar bill of materials.
Slab Volume Formula
Volume = Length x Width x Thickness
Thickness is entered in inches and converted to feet. Results include both the raw volume and the volume with your chosen waste factor applied.
When to Use Bags vs Ready-Mix
| Volume | Best Option |
|---|---|
| Under 0.5 yd³ | 80-lb bags (hand mixing) |
| 0.5-1.0 yd³ | 60-lb or 80-lb bags + mixer |
| 1.0-2.0 yd³ | Consider ready-mix delivery |
| Over 2.0 yd³ | Ready-mix truck (saves hours) |
Rebar Grid Basics
A rebar grid adds tensile strength to prevent cracking. The grid sits in the lower third of the slab:
- 12" on center -- Standard for driveways and structural slabs. Each bar is 12 inches from the next in both directions.
- 18" on center -- Lighter duty patios and walkways.
- 24" on center -- Minimum reinforcement for basic slabs.
Rebar should have 3 inches of concrete cover from the bottom and edges. Use rebar chairs or dobies to hold the grid at the right height during the pour.
Thickness Recommendations
- Sidewalks: 4 inches
- Patios: 4 inches
- Driveways (cars): 4-5 inches
- Driveways (trucks): 5-6 inches
- Garage floors: 4-6 inches (thicker at the apron)
- Workshops with heavy equipment: 6+ inches