Concrete Slab Calculator
Enter your slab dimensions to instantly find cubic yards, pre-mix bag counts, and ready-mix cost range — with a built-in 10% waste factor.
How to Calculate Concrete for a Slab
Concrete volume is calculated in cubic yards — the standard unit for ordering ready-mix. The formula is straightforward:
- Cubic feet = Length (ft) × Width (ft) × Thickness (ft)
- Thickness in feet = Thickness (inches) ÷ 12
- Cubic yards = Cubic feet ÷ 27
Always add at least 10% for waste — concrete can't be returned once it's mixed, and running short mid-pour forces a costly cold joint that weakens the slab.
Pre-mix bags vs. ready-mix truck
For small slabs under about 1 cubic yard (roughly a 10×10 at 4"), pre-mix bags are often more practical. Above that, a ready-mix truck is almost always cheaper per yard and saves hours of labor. The calculator gives you both options.
Bags per cubic yard
| Bag size | Volume per bag | Bags per cubic yard |
|---|---|---|
| 60 lb | 0.45 cu ft | ≈ 60 bags |
| 80 lb | 0.60 cu ft | ≈ 45 bags |
Concrete Cost Estimates (2026)
Prices vary by region, season, and mix design. The ranges below are for standard 3,000–4,000 PSI concrete in the continental US.
| Option | Cost | Best for |
|---|---|---|
| Ready-mix truck | $125–$175 / cubic yard | Any slab over 1 cu yd |
| Short-load fee | $50–$200 extra | Orders under ~5 yards |
| 80 lb pre-mix bag | $6–$8 / bag | Repairs, small footings |
| 60 lb pre-mix bag | $5–$7 / bag | Same as above |
| Fiber reinforcement | $5–$10 / yard add-on | Driveways, high-traffic slabs |
Always order 10% more than calculated — most ready-mix suppliers charge a significant fee for a second small load. If you're pouring a driveway or garage slab, call your local batch plant and ask about their minimum load and short-load surcharge before you finalize the order. In many markets, that surcharge alone is $150–$250, which changes the math on pre-mix vs. ready-mix at smaller volumes.
Choosing the Right Slab Thickness
| Thickness | Application | Notes |
|---|---|---|
| 3–3.5 inches | Sidewalks, overlays | Minimum for foot traffic only |
| 4 inches | Patios, garage floors, interior slabs | Residential standard |
| 6 inches | Driveways, areas with vehicle traffic | Add rebar or wire mesh |
| 8 inches | Heavy equipment pads, commercial | Rebar required, often fiber-reinforced |
If you're in a freeze-thaw climate and pouring a driveway, don't skip rebar just to save money — the savings will disappear the first winter the slab heaves and cracks. A standard grid of #4 rebar at 18" on center adds roughly $0.50–$1.00 per square foot but can double the slab's service life.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab at 4 inches?
A 10×10 slab at 4 inches is approximately 1.23 cubic yards (33.3 cubic feet). With 10% waste, you'd need about 37 cubic feet of concrete — roughly 62 bags of 80 lb pre-mix or 83 bags of 60 lb. At this volume a ready-mix truck is worth considering if available in your area.
How much does a yard of concrete cost?
Ready-mix concrete typically costs $125–$175 per cubic yard in 2026, depending on your region and mix design. Higher-strength mixes (5,000+ PSI) run $150–$200+. Most suppliers also charge a short-load fee for orders under 5–7 yards.
How thick should a concrete slab be?
For most residential applications — patios, garage floors, shed pads — 4 inches is the standard. Bump to 6 inches for driveways and anywhere passenger vehicles will park. Commercial or heavy-equipment pads typically require 8 inches with rebar.