Concrete Slab Calculator
Estimate concrete in cubic yards, bags, and total cost — with adjustable slab thickness and waste factor.
Your Concrete Estimate
- Choose units — Imperial (feet, inches, cubic yards) or Metric (meters, mm, cubic metres).
- Enter dimensions — length and width of the slab. Use preset chips for common thicknesses.
- Check thickness — 4 inches is standard for patios. Use 5–6 inches for driveways and heavy loads.
- Set waste factor — 10% is standard. Go higher for uneven subgrade or irregular shapes.
- Review estimate — compare bag count vs. ready-mix pricing with your local supplier.
How Much Concrete Do I Need for a Slab?
Planning a patio, garage floor, or shed foundation starts with one question: how many cubic yards of concrete do you actually need? The short answer — multiply your slab's length × width × thickness (all in the same unit), then divide by 27 to convert cubic feet into cubic yards. A standard 10 × 10 ft slab poured at 4 inches thick needs roughly 1.24 cubic yards before waste.
Most suppliers sell ready-mix concrete by the cubic yard and bagged concrete in 40 lb, 60 lb, and 80 lb sacks. An 80 lb bag covers about 0.6 cubic feet, so that same 10 × 10 slab would need around 56 bags if you go the DIY route. For anything over 2 cubic yards, ordering a ready-mix truck is usually cheaper and faster.
Always add 5–10% for waste — uneven subgrade, form leaks, and spillage eat into your total. The calculator above factors this in automatically.
Common Slab Sizes at 4-Inch Thickness
| Slab Size (ft) | Cubic Yards | 80 lb Bags | Estimated Cost |
|---|---|---|---|
| 8 × 8 | 0.79 | 45 | $120 – $160 |
| 10 × 10 | 1.23 | 56 | $185 – $250 |
| 10 × 20 | 2.47 | 112 | $370 – $500 |
| 12 × 12 | 1.78 | 80 | $265 – $355 |
| 12 × 24 | 3.56 | 160 | $530 – $710 |
| 20 × 20 | 4.94 | 222 | $740 – $990 |
| 20 × 24 | 5.93 | 267 | $890 – $1,190 |
| 24 × 24 | 7.11 | 320 | $1,065 – $1,420 |
Cost range based on $150–$200 per cubic yard for ready-mix delivery (2024–2025 national average). Bag prices vary by retailer.
Recommended Slab Thickness by Project
Slab thickness depends on load. Pouring too thin risks cracking; pouring too thick wastes material and money. Here's what contractors typically specify:
| Project Type | Recommended Thickness | Notes |
|---|---|---|
| Sidewalks & garden paths | 3.5–4 in | Residential foot traffic only |
| Patios & pool decks | 4 in | Standard residential pour |
| Garage floors (cars) | 4–5 in | 4 in minimum; 5 in if heavy vehicles |
| Driveways | 5–6 in | 6 in at the apron where it meets the street |
| Workshops / RV pads | 6 in | Heavier point loads require thicker slabs |
| Commercial / heavy equip. | 6–8 in | Engineer-specified; often with rebar or fiber mesh |
For any load-bearing slab, a compacted gravel base (4–6 inches) underneath improves drainage and reduces cracking.
Formulas Used in This Calculator
Every result in the calculator above uses standard volumetric math:
Cubic Yards = Volume (ft³) ÷ 27
Bags (80 lb) = Volume (ft³) ÷ 0.6
With Waste = Cubic Yards × (1 + Waste %)
The calculator converts all metric inputs into imperial for the core calculation, then converts results back when metric mode is selected.
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Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 ft slab requires approximately 1.23 cubic yards of concrete — that's about 56 bags of 80 lb mix or 74 bags of 60 lb mix. Add 10% waste if you're working with uneven ground.
Is it cheaper to mix concrete yourself or order ready-mix?
For small pours under 1 cubic yard, bags are usually more economical. Once you're past 2 cubic yards, a ready-mix truck saves time and labor — and the per-yard cost drops significantly. Get quotes from local batch plants before deciding.
Do I need rebar in a concrete slab?
For standard 4-inch residential slabs like patios and sidewalks, welded wire mesh or fiber reinforcement is usually sufficient. Driveways, garage floors, and any slab carrying vehicle or structural loads should include #3 or #4 rebar on 18–24 inch centers. Always check local building codes.
How thick should a concrete slab be?
A standard residential slab (patio, garage floor, sidewalk) is 4 inches thick. Slabs supporting heavy loads like RVs or heavy equipment should be 5–6 inches. A 4-inch slab on 4 inches of compacted gravel outperforms a 6-inch slab on soft soil — sub-base preparation matters as much as slab thickness.
Should I use rebar or wire mesh for a slab?
Wire mesh helps control surface cracking on light-duty slabs, while rebar provides stronger reinforcement for thicker slabs and higher loads. Many pours use both: mesh in the field and rebar at edges or high-stress areas. Use this calculator for planning quantities and follow your project specs for reinforcement choices.