Concrete Calculator
Enter your slab's length, width and thickness to get the volume in cubic yards and the number of 60 lb or 80 lb bags, including a waste allowance. The steps under the result show the exact math, so you can re-check it against your measurements before ordering.
Concrete volume formula
Concrete for a rectangular slab is plain volume math: multiply length by width by thickness in the same unit (feet), then convert to cubic yards — the unit ready-mix is sold in — by dividing by 27.
Always order a little extra: 5–10% waste covers spillage, an uneven subgrade and slight over-digging. For bagged concrete, divide the volume (with waste) by the bag yield and round up: a 60 lb bag yields about 0.45 cu ft and an 80 lb bag about 0.60 cu ft (QUIKRETE's published yields).
Worked examples
Example 1: 10 × 10 ft patio slab, 4 inches thick
How many cubic yards for a 10 ft × 10 ft slab, 4 in thick, with 10% waste?
- Convert thickness to feet.4 ÷ 12 = 0.333 ft
- Multiply length × width × thickness.10 × 10 × 0.333 = 33.3 cu ft
- Convert to cubic yards.33.3 ÷ 27 = 1.23 cu yd
- Add 10% waste.1.23 × 1.10 ≈ 1.36 cu yd
Order about 1.4 cubic yards of ready-mix (most suppliers sell in quarter-yard increments).
Example 2: bags for a 4 × 4 ft pad, 4 inches thick
How many 80 lb bags for a small 4 ft × 4 ft equipment pad, 4 in thick, 10% waste?
- Volume in cubic feet.4 × 4 × (4 ÷ 12) = 5.33 cu ft
- Add 10% waste.5.33 × 1.10 = 5.87 cu ft
- Divide by the 80 lb bag yield and round up.5.87 ÷ 0.60 = 9.8 → 10 bags
10 bags of 80 lb concrete mix (or 14 bags of 60 lb: 5.87 ÷ 0.45 = 13.1 → 14).
Example 3: sidewalk 20 ft × 3 ft, 4 inches thick
Cubic yards for a 20 ft sidewalk section, 3 ft wide, 4 in thick, 5% waste.
- Volume in cubic feet.20 × 3 × 0.333 = 20 cu ft
- Convert to cubic yards.20 ÷ 27 = 0.74 cu yd
- Add 5% waste.0.74 × 1.05 ≈ 0.78 cu yd
About 0.8 cubic yards — below most ready-mix minimums, so bags or a short-load delivery are the practical options.
FAQ
How many 80 lb bags of concrete make a cubic yard?+
A cubic yard is 27 cu ft and an 80 lb bag yields about 0.60 cu ft, so you need about 45 bags (27 ÷ 0.60). With 60 lb bags it's 60 bags. At that scale ready-mix delivery is usually easier and cheaper per yard.
How thick should a concrete slab be?+
Common US practice: 4 inches for patios, sidewalks and shed slabs; 4–6 inches for driveways (6 where heavier vehicles park). Check local code for footings and anything structural — frost depth requirements vary by region.
How much extra concrete should I order?+
Add 5–10% to the computed volume. Forms are rarely perfect, subgrade is rarely flat, and running short mid-pour costs far more than a little extra — a cold joint between two pours weakens the slab.
When should I use bags instead of ready-mix delivery?+
Bags make sense up to roughly half a cubic yard (20–25 of the 80 lb bags) — fence posts, small pads, repairs. Above that, mixing by hand gets slow and inconsistent; ready-mix or a short-load service gives a uniform pour.
Does this calculator work for metric measurements?+
The inputs are feet and inches, the US standard. For metric: volume in m³ = length × width × thickness in metres; 1 m³ ≈ 1.31 cu yd. A metric mode is on the roadmap.
Related calculators
- Concrete Slab CalculatorConcrete for a slab: cubic yards with waste allowance and material cost at your local ready-mix price — multiple slabs supported, steps shown.
- QUIKRETE Bag CalculatorHow many QUIKRETE bags for your pour: enter the slab size and bag weight (40–90 lb) to get the bag count from the manufacturer's published yields.
- Cubic Feet CalculatorCalculate cubic feet from length, width and height — each in inches or feet — with cubic yards and cubic inches, and the conversion steps shown.
- Square Footage CalculatorCalculate square footage from length and width, convert to square yards, meters and acres, and estimate material cost from a price per square foot.