Concrete Slab Calculator
Enter your slab length, width, and thickness to get cubic yards, bag count, truck loads, and material cost — with waste factor included.
Concrete Slab
Calculate concrete volume for any rectangular slab
Inputs
Typical: 4" garage, 6" driveway, 8" industrial
Typically 8–12% for slabs
Delivered, inc. PSI spec — leave $0 for volume only
Results
How to Use the Concrete Slab Calculator
Enter the slab length, width, and thickness in the fields above. The calculator outputs volume in cubic yards and cubic metres, adjusts for your chosen waste factor, shows the equivalent bag count (60 lb and 80 lb), and calculates how many ready-mix truck loads are required. Add a unit cost per cubic yard to see total material cost.
How It Calculates
The formula converts your slab dimensions into cubic yards, the standard unit for ordering ready-mix concrete:
Adjusted Volume = Volume × (1 + Waste% / 100)
Bag counts use standard yields: one 60 lb bag ≈ 0.0167 cu yd; one 80 lb bag ≈ 0.022 cu yd. Truck loads assume 9 cu yd per load.
What to Know Before You Calculate
- Thickness in inches: Most residential slabs are 4 in. Driveways are 4–6 in. Heavy equipment areas should be 6–8 in.
- Waste factor: Always add at least 5–10%. Uneven subgrades, formwork gaps, and over-excavation all consume concrete.
- Ready-mix vs. bags: Bagged concrete is practical under 1 cu yd. Above that, ready-mix is more economical and consistent.
- Unit cost: Ready-mix prices vary by region and mix design. Get a local quote before using the cost output for bidding.
Common Mistakes
- Forgetting the waste factor. Ordering exactly what the formula says means you will run short. Always add 5–10%.
- Using nominal dimensions. Measure the actual pour area after forming, not the property or building footprint.
- Mixing metric and imperial. This calculator uses feet and inches. Entering metric values will give wrong results.
- Ignoring subgrade variation. A subgrade that is not perfectly flat can add significant volume. Verify depth at multiple points.
Frequently Asked Questions
How do I calculate how much concrete I need for a slab?
Multiply the length (ft) × width (ft) × thickness (in) ÷ 12 to get cubic feet, then divide by 27 to convert to cubic yards. Always add 5–10% waste. For a 20×30 ft slab at 4 inches: 20 × 30 × (4/12) / 27 = 7.41 cu yd.
What thickness should a concrete slab be?
Residential slabs (garage floors, patios, basement floors) are typically 4 inches. Driveways are usually 4–6 inches. Industrial or heavy vehicle areas should be 6–8 inches or more.
How many bags of concrete do I need for a 10x10 slab at 4 inches?
A 10×10 ft slab at 4 inches thick requires about 1.23 cubic yards. That is approximately 37 bags of 80 lb concrete mix or 50 bags of 60 lb mix, before accounting for waste.
How many cubic yards fit in a concrete truck?
A standard ready-mix concrete truck holds 9–10 cubic yards. For smaller pours (under 1 cu yd), bagged concrete is more practical. For volumes over 1–2 yards, ready-mix is faster and more consistent.
Related Calculators
Disclaimer: Results are estimates based on standard formulas and are intended as a professional reference only. Quantities may vary due to site conditions, subgrade depth variation, and material characteristics. Verify all quantities before procurement.
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