Concrete Slab Calculator
Calculate concrete for slabs — get cubic yards, square feet and 80/60/40lb bags with waste & cost. Enter length, width, thickness in feet, inches, yards or meters — instant results for US customary & metric.
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TOP VIEW — Length × Width
SECTION — Thickness 3in
How it works
- Sq ft = Length × Width × number of slabs
- Cubic feet = Sq ft × thickness (in feet)
- Cubic yards = Cubic feet ÷ 27 (waste multiplies after)
- Bags = ceil(cubic feet with waste ÷ coverage per bag)
Coverages: 80-lb = 0.60 ft³, 60-lb = 0.45 ft³, 40-lb = 0.30 ft³
Example
Patio 12 ft × 10 ft × 4 in, 1 slab, 10% waste:
| OUTPUT | VALUE |
|---|---|
| Sq ft | 120 |
| Cubic ft | 40.00 |
| Cubic yd | 1.48 |
| With waste | 1.63 yd³ (44 ft³) |
| 80-lb bags | 74 |
Assumptions
- Rectangular slabs. L-shapes: calculate as two rectangles.
- No subbase gravel — add via gravel calculator if needed.
- Mix coverage assumes proper water ratio — add one extra bag per 10 for safety.
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Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × thickness (all in feet) and divide by 27 for cubic yards. Example: 10×10 ft at 4 inches (0.333 ft) = 33.3 ft³ = 1.23 yd³. Add 5–10% for waste and uneven subgrade, then use the calculator — it handles inches-to-feet and shows both yards and bags. For a base layer, add gravel via the Gravel Calculator.
How many 80-lb bags are in a yard of concrete?
About 45 bags of 80-lb (0.60 ft³ each), 60 bags of 60-lb (0.45 ft³), or 90 bags of 40-lb (0.30 ft³) make one cubic yard (27 ft³). For a 10×10 ft slab at 4 inches you need about 56 bags of 80-lb (1.23 yd³ × 45). The calculator rounds up — you can’t buy part of a bag.
How thick should a concrete slab be?
Patios, walkways and shed pads: 3.5–4 inches. Driveways, garage floors and RV pads: 5–6 inches, often 4,000 psi mix. Sidewalks: 4 inches. Commercial: 6–8 inches. Thicker adds volume fast — a 6-inch slab uses 50% more than a 4-inch over the same footprint. Confirm with local code and your engineer for loads.
Should I order ready-mix or bags?
Under ~0.75 yd³ (about 30–40 bags of 80-lb) bags can be practical. Over ~1–1.5 yd³, ready-mix delivery is usually cheaper and far faster. A 12×10 ft patio at 4 inches is 1.48 yd³ — that’s 74 bags of 80-lb with waste, so a truck wins. The calculator shows both so you can compare instantly.
How much extra concrete should I order for waste?
Add 5–10% for spillage, over-excavation and slight thickness variation — 10% is safe for most residential forms. Keep the subgrade well-compacted and evenly graded to the right depth, and keep an extra bag per 10 as a buffer. The calculator applies waste after volume, then rounds bags up.
How do I calculate concrete for multiple slabs or an L-shape?
Use Number of slabs for identical rectangles, or run the calculator twice for different sizes and sum the waste-adjusted yards. For an L-shape, split it into two rectangles (e.g., 12×10 plus 8×6) and add the cubic yards. Coverage per bag already assumes proper water ratio — don’t thin the mix to stretch it.
How much does a concrete slab cost?
Ready-mix is often $140–$180 per yard plus $100–$200 delivery/minimum in 2026; bags are $4–$7 per 80-lb. A 10×10 ft 4-inch slab (~1.35 yd³ with waste) is roughly $200–$300 in ready-mix versus $280–$380 in bags before tax. Enter your price per yard or per bag — the calculator multiplies waste-adjusted yards or the rounded bag count for a material-only total.
About Calculator Street
Built by Vipul Otari for US homeowners & contractors. Calculator inputs are processed in your browser. Estimates depend on your measurements, product specifications and the assumptions shown on this page. Confirm order quantities with your supplier and structural requirements with your project designer. Page updated: 2026-08-29.