Last updated: May 2026
Get cubic yards, cubic feet, and bags needed — for slabs, footings, columns, and steps.
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Typical Concrete Uses — Thickness Guide
| Use case | Recommended thickness | Notes |
|---|---|---|
| Sidewalks & patios | 4 inches | Light foot traffic |
| Residential driveway | 4 inches | Passenger cars; add rebar for longevity |
| Heavy vehicle traffic | 6 inches | Trucks, RVs, heavy loads |
| Garage floors | 4–6 inches | 6 in if storing heavy equipment |
| Foundation footings | 8–12 inches | Check local building codes |
| Basement floors | 4 inches | Non-load-bearing slab |
Slab: Volume = Length × Width × (Thickness ÷ 12) → cubic feet. Divide by 27 for cubic yards.
Footing / Wall: Volume = Length × Width × Depth (all in feet).
Column / Cylinder: Volume = π × (Diameter ÷ 24)² × Height. Diameter is in inches, converted to ft radius.
Steps: Each step is a rectangular prism. Total volume = Number of steps × average step volume, where each step = Width × (Run ÷ 12) × cumulative rise height. The staircase is modeled as a right-angle wedge: Volume = n × Width × (Rise/12) × (Run/12) × (n+1)/2 approximation.
Waste factor: Multiply result by 1.10 when enabled.
Bags: Bags needed = ceil(Cubic feet ÷ coverage per bag). 40 lb = 0.30 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft.
Cubic meters: Cubic feet × 0.0283168.
Concrete is sold and mixed by volume — cubic yards in the US, cubic meters internationally. The challenge is that your project dimensions are usually in feet and inches, so you need to convert before you can calculate. The formula for a basic slab: Length (ft) × Width (ft) × Thickness (inches ÷ 12) ÷ 27 = cubic yards.
The ÷ 27 converts cubic feet to cubic yards (1 cubic yard = 27 cubic feet). Always add a 10% waste factor for spills, over-excavation, and uneven sub-base. Running short on concrete mid-pour is a serious problem — the batch needs to be placed before it starts setting.
| Project | Typical Volume | Notes |
|---|---|---|
| Sidewalk (4 in thick, 40 ft long) | ~0.5 cu yd | Standard 3-ft wide walk |
| Driveway (4 in thick, 10×20 ft) | ~2.5 cu yd | Increase to 6 in for heavy vehicles |
| Patio (4 in thick, 12×16 ft) | ~3 cu yd | Most common residential size |
| Garage floor (4 in thick, 24×24 ft) | ~7 cu yd | 2-car garage |
| Fence post (10 in dia, 4 ft deep) | ~0.25 cu ft each | 2 bags of 60-lb mix per post |
| Foundation wall (8 in thick, 10 ft tall) | ~1 cu yd per 10 lin ft | Typical basement wall |
How thick should a concrete slab be?
4 inches is the standard residential thickness for patios, sidewalks, and driveways for passenger vehicles. Increase to 5–6 inches for heavy vehicles (trucks, RVs). Garage floors for large trucks or commercial use: 6 inches minimum. Structural slabs and foundations are engineered separately and may require 8–12 inches depending on the load.
How many 80-lb bags of concrete do I need per cubic yard?
One 80-lb bag yields approximately 0.60 cubic feet. One cubic yard = 27 cubic feet. So you need 27 ÷ 0.60 = 45 bags of 80-lb mix per cubic yard. For 60-lb bags (0.45 cu ft each): 60 bags per cubic yard. For 40-lb bags (0.30 cu ft each): 90 bags per cubic yard. Bags are practical for small pours; ready-mix trucks are more economical above 1–1.5 cubic yards.
How long does concrete take to cure?
Concrete reaches initial set in 24–48 hours (enough to walk on), but full strength development takes 28 days. At 7 days, concrete has reached roughly 70% of its 28-day design strength. For foot traffic: wait 24–48 hours. For vehicle traffic: wait 7 days minimum. For full structural loads: 28 days. Curing in cold weather significantly slows these timelines.
What is the minimum concrete order from a ready-mix truck?
Most ready-mix companies have a minimum order of 1 cubic yard, with a "short load" surcharge for orders under 3–5 cubic yards. If your project needs less than 1 yard, bag mix is usually more economical. If you need 1–3 yards, compare the short-load fee against the labor cost of mixing bags — it often tips in favor of ready-mix above 1.5 yards.
Do I need rebar or wire mesh in a concrete slab?
Wire mesh (6×6 W1.4×W1.4) is common in residential slabs and helps control cracking. Rebar (typically #3 or #4) provides higher structural reinforcement and is used in driveways that will bear heavy vehicle loads, any slab on fill, and structural elements. Plain concrete without reinforcement is only acceptable for small, lightly loaded slabs like a small garden path or fence post base.