Concrete slab calculator — cubic yards, bags & cost
A slab’s volume is length × width × thickness. A 10 × 10 ft slab at 4 in thick needs 1.23 cubic yards — about 56 80-lb bags or a short ready-mix delivery. Enter your dimensions for exact quantities, bag counts, and a cost estimate at August 2026 national-average prices.
Your estimate
At this size a short ready-mix load and bags are both realistic — compare the two estimates above, and watch for short-load fees ($50–$150) on small truck orders.
Common slab sizes, worked out
Every row uses the same math as the calculator (4 in thick unless noted). Open any size in the calculator to adjust thickness or add cost.
| Slab | Cubic yards (exact) | 80-lb bags (exact) | Typical order* | |
|---|---|---|---|---|
| 10 × 10 ft · 4 in | 1.23 yd³ | 56 | 1.5 yd³ | Open › |
| 10 × 12 ft · 4 in | 1.48 yd³ | 67 | 1.75 yd³ | Open › |
| 12 × 12 ft · 4 in | 1.78 yd³ | 80 | 2 yd³ | Open › |
| 12 × 16 ft · 4 in | 2.37 yd³ | 107 | 2.75 yd³ | Open › |
| 16 × 16 ft · 4 in | 3.16 yd³ | 143 | 3.5 yd³ | Open › |
| 20 × 20 ft · 4 in | 4.94 yd³ | — | 5.5 yd³ | Open › |
| 24 × 24 ft · 4 in | 7.11 yd³ | — | 8 yd³ | Open › |
| 12 × 24 ft · 5 in (parking pad) | 4.44 yd³ | — | 5 yd³ | Open › |
| 24 × 24 ft · 6 in (garage) | 10.67 yd³ | — | 11.75 yd³ | Open › |
| 30 × 30 ft · 5 in | 13.89 yd³ | — | 15.5 yd³ | Open › |
| 40 × 60 ft · 6 in (shop) | 44.44 yd³ | — | 49 yd³ | Open › |
*Typical order = exact volume + 10% waste, rounded up to the quarter yard most suppliers sell. Bags shown where bag-mixing is realistic (under ~150 bags).
How this calculator works
Volume: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet; ÷ 27 = cubic yards. Bags: an 80-lb bag yields 0.60 ft³ and a 60-lb bag 0.45 ft³ (manufacturer yield specs), so a cubic yard is 45 × 80-lb or 60 × 60-lb bags. Waste: we recommend 10% for typical slabs (uneven subgrade, spillage, over-excavation); 5% suits formed pours on a screeded base.
Prices: ready-mix $145–$195/yd³ delivered (avg ~$160) for 3,000–4,000 PSI within ~20 miles of the plant; 3,000–4,000 PSI mix, delivered within ~20 miles. Short-load fees of $50–$150 are common on orders under ~4 yd³. 80-lb bags priced at $6.50 (Quikrete 80-lb $6.47 at Home Depot, Aug 2026 ($6.15/bag at 42+)). Sources: Concrete Network and retailer listings. National averages — your local quote decides.
Common questions
How much concrete do I need for a 20×20 slab?
A 20 × 20 ft slab at 4 inches thick needs 4.94 cubic yards (133 cubic feet) of concrete. Order about 5.5 yards to cover waste and an uneven subgrade. At the August 2026 national average of ~$160/yd delivered, that is roughly $880 of ready-mix.
How many bags of concrete are in a cubic yard?
One cubic yard = 45 80-lb bags, 60 60-lb bags, 72 50-lb bags, or 90 40-lb bags. An 80-lb bag yields about 0.60 cubic feet of mixed concrete.
How thick should a concrete slab be?
Common practice: patios, walkways, and shed pads 4 in; car driveways 4–5 in (6 in where trucks or RVs park); garage floors 5–6 in. Thickened edges and footings are governed by local code — always check before pouring.
Should I mix bags or order a ready-mix truck?
Above roughly 1 cubic yard (~45 80-lb bags), a truck is usually less work and competitive on price once bag costs add up — but orders under ~4 yards often carry a $50–$150 short-load fee. Bags win for small pours and repairs; trucks win from about a 10 × 10 slab upward if the site has access.
Do I need rebar or wire mesh?
Typical practice (not structural advice): 4-in patios use fiber mix or 6×6 welded wire mesh; driveways use mesh or #3–#4 rebar on a 12–18 in grid. Anything load-bearing or code-governed (footings, garage slabs) should follow your local building department’s requirements.