Slab volume and bag count — enter dimensions in any length unit.
V (cubic yards) = (L × W × T) / 27 V (cubic feet) = L × W × T V (cubic meters) = L_m × W_m × T_m
The volume of concrete required for a rectangular slab is found by multiplying the three dimensions of the slab together. Because concrete is typically ordered in cubic yards in the US, you must convert all measurements to the same unit before multiplying. The formula below works when all three dimensions are expressed in feet; divide the result by 27 to convert from cubic feet to cubic yards (since 1 cubic yard = 27 cubic feet).
V (ft³) = 20 × 12 × 0.3333 = 80 ft³ V (yd³) = 80 ÷ 27 ≈ 2.96 yd³ With 10% waste: 2.96 × 1.10 ≈ 3.26 yd³ → order 3.5 yd³
Result: Approximately **2.96 cubic yards** of concrete are needed for the bare slab; order **3.5 cubic yards** to safely account for waste and spillage.
Your result tells you the minimum volume of concrete required to fill the slab at the specified dimensions. Because ready-mix trucks are typically ordered in quarter- or half-yard increments, always round up to the next practical batch size. For the example above, rounding 3.26 yd³ up to 3.5 yd³ ensures you have enough material without stopping work mid-pour — a scenario that can create cold joints and structural weaknesses.
In the United States, ready-mix concrete is sold by the cubic yard (yd³). One cubic yard equals 27 cubic feet, which weighs roughly 4,000 lb (1,800 kg) for standard 3,000 psi mix. Knowing your required cubic yardage is the single most important number when calling a concrete supplier.
Small changes in thickness have a large impact on volume:
Always check local building codes; many jurisdictions specify minimum slab thickness for permitted projects.
One cubic yard forms a cube measuring 3 ft × 3 ft × 3 ft. A standard ready-mix truck carries 8–10 cubic yards. For small projects under ~1 yd³, consider renting a concrete mixer or purchasing pre-mixed bags.
For very small pours, 60 lb bags of pre-mixed concrete are practical. One 60 lb bag yields approximately 0.45 cubic feet. To find how many bags you need, divide your required cubic feet by 0.45. For 80 lb bags (≈0.60 ft³ each), divide by 0.60.
Concrete volume calculations assume perfectly flat, uniform sub-grades. In reality, ground settling, sub-base irregularities, and formwork imperfections consume extra material. A 5% waste factor is appropriate for well-prepared sites; use 10% for rough or uneven ground.
If you measure in meters, multiply L × W × T directly to get cubic meters (m³). One cubic meter equals approximately 1.308 cubic yards.
As of 2024, ready-mix concrete typically costs **$125–$200 per cubic yard** in the US, depending on region, mix strength (PSI), and delivery distance. Short-load fees may apply for orders under 3–4 cubic yards.
For driveways, a **3,000–4,000 PSI mix** is standard. High-traffic or freeze-thaw climates may call for 4,000–5,000 PSI with air-entrainment additives to resist cracking from freezing water.
Concrete reaches approximately **70% of its design strength in 7 days** and full design strength at **28 days**. Foot traffic is generally safe after 24–48 hours; vehicle traffic should wait at least 7 days.
A **4-inch slab** is sufficient for most residential garages with passenger cars. If you park heavy trucks, SUVs, or store heavy equipment, a **5-to-6-inch slab** with proper rebar reinforcement is advisable.
**Cement** (specifically Portland cement) is one ingredient in concrete — it acts as the binder. **Concrete** is the final mixture of cement, water, sand (fine aggregate), and gravel (coarse aggregate). Ordering 'cement' from a supplier when you mean 'concrete' is a common confusion on job sites.
V = (10 × 10 × 0.333) ÷ 27 = 33.3 ÷ 27 ≈ 1.23 cubic yards. With a 10% waste factor, order about 1.35 yd³ — most suppliers round this up to 1.5 yd³.
Each 60 lb bag yields approximately 0.45 cubic feet. Divide your total cubic feet by 0.45 to get the number of bags. For a slab requiring 33.3 ft³, you would need 33.3 ÷ 0.45 ≈ 74 bags.
Most residential driveways are poured at **4 inches thick** for passenger vehicles. For driveways that accommodate heavier vehicles such as RVs or large trucks, 5–6 inches is recommended.
No. The calculator computes the gross volume of concrete needed to fill the slab dimensions. Rebar and wire mesh displace a negligible volume (typically less than 0.5%) and do not meaningfully change your concrete order.
This calculator is designed for rectangular slabs. For a circular slab (like a round patio), use the formula V = π × r² × T, where r is the radius and T is the thickness in matching units.
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