Concrete Slab Calculator
Calculate exactly how much concrete you need for any rectangular slab — patios, driveways, garage floors, sidewalks, and shed pads. Enter your dimensions and get instant results with bag counts and cost estimates.
Concrete Calculator
Select your project type, enter dimensions, and get instant results with bag counts, weight, and cost.
Zone 1
Results
Total Volume
0.00 cu ft
0.00 cu yd
60 lb Bags
0
80 lb Bags
0
Total Weight
0 lbs
(0.00 tons)
💡 Ready-Mix Recommended
At 1+ cubic yard, truck delivery ($125–$175/yd) is usually cheaper and easier than mixing bags by hand.
Reinforcement Estimate
Cost Estimate
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Project Details
| Zone | Type | Dimensions | Qty | Volume (cu ft) |
|---|
Results
Total Volume — | Cubic Yards — | Total Weight — |
60 lb Bags — | 80 lb Bags — | Est. Cost — |
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How to Calculate Concrete for a Slab
The formula for a rectangular concrete slab is simple: Volume = Length (ft) × Width (ft) × Thickness (in) ÷ 12. The result is in cubic feet. Divide by 27 to convert to cubic yards.
Standard Slab Thicknesses
- 4 inches — patios, walkways, shed pads (pedestrian traffic only)
- 5 inches — residential driveways, carports (light vehicle traffic)
- 6 inches — heavy-duty driveways, garage floors, RV pads, hot tub bases
- 8 inches — commercial applications, heavy equipment pads
Quick Reference: Common Slab Sizes
| Size | Cu Yd (4") | 80lb Bags | Use Case |
|---|---|---|---|
| 8×10 | 0.99 | 49 | Garden shed |
| 10×10 | 1.23 | 62 | Small patio |
| 10×12 | 1.48 | 74 | Medium patio |
| 12×12 | 1.78 | 89 | Large patio |
| 16×16 | 3.16 | 158 | Entertainment area |
| 20×20 | 4.94 | 245 | 2-car driveway pad |
| 24×24 | 7.11 | 356 | Garage floor |
Tips for Pouring a Concrete Slab
- Prepare a 4-6 inch compacted gravel base for drainage
- Use 2×4 or 2×6 lumber for forms — stake every 3-4 feet
- Add wire mesh or rebar for slabs over 100 sq ft
- Pour in one continuous session to avoid cold joints
- Finish with a broom texture for outdoor slabs (prevents slipping)
- Cure for at least 7 days — keep moist or cover with plastic
Related Calculators
- Footing Calculator — for deck and wall footings
- Rebar Calculator — estimate reinforcement for your slab
- Cost Calculator — estimate total material cost
- Unit Converter — convert between ft, in, m, cu ft, cu yd
Slab Calculator FAQ
How thick should a concrete slab be?
For pedestrian traffic (patios, walkways), 4 inches is standard. For vehicle traffic (driveways, garages), use 5-6 inches. For heavy loads (RVs, commercial), 6-8 inches is recommended.
Do I need gravel under a concrete slab?
Yes. A 4-6 inch layer of compacted gravel (crushed stone, not pea gravel) improves drainage, prevents moisture from wicking up through the slab, and reduces cracking from soil movement. This is standard practice for all slab projects.
How long does a concrete slab take to cure?
Initial set occurs in 24-48 hours (you can walk on it). Full strength (4000 PSI) takes 28 days. You can drive on a driveway slab after 7 days. Keep the surface moist for the first week for best results.
How many bags of concrete for a 10×10 slab?
A 10×10 foot slab at 4 inches thick needs approximately 1.23 cubic yards of concrete — that's about 62 bags of 80lb mix or 82 bags of 60lb mix (with 10% waste factor). At this volume, a ready-mix truck delivery is worth considering.
Should I use bags or order ready-mix for my slab?
For slabs under 1 cubic yard (roughly a 9×9 at 4"), bags are practical. Above 1 yard, ready-mix truck delivery ($125-$175/yard) is cheaper and faster than mixing 45+ bags by hand. A 10×10 slab at 1.23 yards is right at the crossover point — consider ready-mix.