Concrete Slab Calculator
Pouring a patio, garage pad, shed base, or driveway slab? Enter the slab length, width, and thickness to see exactly how much concrete it takes — in cubic yards for ready-mix and in bags by size for smaller pours — with a waste factor built in.
Total volume required
2.61Cubic Yards
Cubic feet
70.40
Cubic meters
1.99
Bags (80 lb)
118
Bags by size
40 lb
235
50 lb
188
60 lb
157
80 lb
118
Estimate includes your waste factor. Figures are estimates, not a quote.
How to use this calculator
- 1Measure the slab length and width in feet and the thickness in inches (4 inches is standard for patios and walkways; 5–6 inches for driveways and heavy loads).
- 2Enter the three dimensions and set how many identical slabs you are pouring with Quantity.
- 3Keep a 10% waste factor for an uneven subgrade, or lower it for a well-prepared form.
- 4Read cubic yards for ordering ready-mix, or the bag count if you are mixing on site.
Formula & rounding
A slab is a rectangular volume: length × width × thickness (all in feet). Cubic feet ÷ 27 gives cubic yards. Thickness entered in inches is converted to feet (inches ÷ 12). Bag counts always round up.
16 ft × 12 ft patio slab, 4 inches thick
- Volume = 16 × 12 × (4 ÷ 12) = 64 cubic feet
- With 10% waste: 64 × 1.10 = 70.4 cubic feet
- Cubic yards = 70.4 ÷ 27 = 2.61 cubic yards
- 80 lb bags = 70.4 ÷ 0.60, rounded up = 118 bags
= ≈ 2.61 cubic yards (order ready-mix at this size)
Same slab with a 12 in × 12 in thickened edge
- Center slab = 16 × 12 × (4 ÷ 12) = 64 cubic feet
- Perimeter edge length = 2 × (16 + 12) = 56 linear feet
- Extra edge concrete = 56 × 1 ft wide × (8 ÷ 12) extra depth = 37.3 cubic feet
- Total with 10% waste = (64 + 37.3) × 1.10 = 111.5 cubic feet
= ≈ 4.13 cubic yards with the thickened edge
Slab thickness quick guide
Thickness changes volume directly: a 6 in slab uses 50% more concrete than a 4 in slab over the same footprint. These are planning ranges only; load-bearing slabs may need an engineer or local code review.
| Thickness | Common use | Concrete per 100 sq ft before waste | Planning note |
|---|---|---|---|
| 4 in | Patios, walks, shed pads | ≈1.23 yd³ | Common light-duty default |
| 5 in | Light driveway / heavier pad | ≈1.54 yd³ | Uses 25% more than 4 in |
| 6 in | Driveways, equipment, thicker edges | ≈1.85 yd³ | Uses 50% more than 4 in |
Estimate only. Reinforcement, base preparation, soil, and vehicle loads are separate design decisions.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × thickness in feet for cubic feet, then divide by 27 for cubic yards. A 16 ft × 12 ft slab at 4 inches needs about 2.6 cubic yards with waste. The calculator does this for any size.
How many bags of concrete for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches is about 33 cubic feet (≈37 with 10% waste). That is roughly 62 × 80 lb bags or 82 × 60 lb bags. Below about 1 cubic yard, bags are convenient; above it, ready-mix is usually cheaper.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed bases. Use 5–6 inches for driveways or anything carrying vehicles or heavy equipment. Thicker slabs use proportionally more concrete, so set the thickness before you read the result.
Should I add a waste factor for a slab?
Yes. Subgrade is rarely perfectly level and some concrete is always lost. 10% is a safe default; tighten it only if your forms and base are very precise.
Assumptions & sources
- Volume formula
- length × width × thickness (all in feet); cubic feet ÷ 27 = cubic yards.
- Thickness
- 4 in is typical for patios/walkways, 5–6 in for driveways and loads; thickness drives volume directly.
- Bag yields
- ≈0.60 cu ft per 80 lb bag (0.45 / 0.375 / 0.30 for 60 / 50 / 40 lb); rounded up.
- Waste factor
- Default 10% for spillage, sub-grade variation, and thickened edges.
- Scope
- Material volume only — this page does not estimate price, labor, delivery, reinforcement, or installed cost.
See the methodology & sources for how these values, formulas, and rounding are chosen.
Helpful concrete guides
- How to calculate concrete — walks through cubic feet, cubic yards, waste factor, and bag conversion for slabs and other pours.
This is an estimate, not a quote. Concrete quantities, bag yields, block coverage, and prices vary with product, brand, mix, region, supplier, tax, delivery, and on-site conditions. Always confirm with your supplier and round up for safety. For structural or code-related work, consult a qualified professional or your local building authority.