Concrete Slab Calculator
This slab calculator is purpose-built for rectangular concrete slabs — patios, driveways, shed pads, garage floors and sidewalks. Enter length, width and thickness to get the concrete volume in cubic yards, the number of 80-lb pre-mix bags, and how many rolls of 6×6 welded wire mesh you need for reinforcement. It is the fastest way to size a flat-work order without doing the yardage math by hand.
4 in for patios and walkways; 5–6 in for driveways.
Mesh is sold in 5 ft × 150 ft rolls (750 ft² each).
Concrete needed
4.07yd³
- Volume (cubic feet)
- 110ft³
- 80-lb bagsEach 80-lb bag yields ≈ 0.60 ft³.
- 184
How it was calculated
- Slab area: 20 ft × 15 ft = 300.0 ft².
- Gross volume: 300.0 ft² × 0.333 ft = 100.00 ft³.
- With 10% waste: 100.00 × 1.10 = 110.00 ft³ (4.07 yd³).
- 80-lb bags: 110.00 ÷ 0.60 = 183.3 → 184 bags (rounded up).
How it works
A slab is a rectangular prism: volume = length × width × thickness. Thickness entered in inches is divided by 12 to get feet before multiplying.
Divide cubic feet by 27 to get cubic yards — the unit ready-mix is sold in.
The waste factor scales the volume up (× 1.10 for 10%) to cover subgrade unevenness, form deflection and spillage.
Mesh rolls are sized from slab area alone: rolls = ceiling(area ÷ 750), since each standard roll covers 750 ft² before overlaps.
Worked examples
20 × 15 ft patio at 4 inches with mesh
Area = 300 ft²; volume = 300 × (4 ÷ 12) = 100 ft³. With 10% waste: 110 ft³ = 4.07 yd³. That is 184 eighty-pound bags (110 ÷ 0.60 = 183.3, rounded up), and the 300 ft² of mesh fits in a single 750 ft² roll.
12 × 12 ft shed pad at 6 inches, no mesh
Volume = 144 × 0.5 = 72 ft³. With 5% waste: 75.6 ft³ = 2.80 yd³. Bags: 75.6 ÷ 0.60 = 126 exactly, so 126 eighty-pound bags with no rounding loss.
How to measure
- Measure the inside of the forms after they are staked — boards shift during setting.
- Check thickness in at least four places; dig down and probe the subgrade if you are unsure.
- For a driveway, measure the full apron including the flare at the street — it is often forgotten.
- Add 6 inches of length and width when sizing mesh rolls so sheets can lap at the edges.
- If the slab meets an existing structure, measure from the structure face, not the siding.
Assumptions & limitations
Planning estimate
- Slab is assumed perfectly rectangular with uniform thickness; L-shaped or notched slabs should be split into rectangles and summed.
- The 80-lb bag yield of ≈ 0.60 ft³ is a manufacturer rule of thumb for standard pre-mix.
- Mesh roll coverage (750 ft²) assumes 6-inch laps between sheets — a rule of thumb; windy sites or heavy mesh may need wider laps.
- This is a planning estimate only — not a substitute for code compliance or professional design. Driveway thickness, base prep and joint spacing should follow local practice.
Frequently asked questions
Patios, walkways and shed pads are typically 4 inches thick. Driveways and garage floors that carry vehicles should be 5–6 inches. Anything supporting a structure needs an engineered design and code-approved footings.
For a 4-inch patio on well-compacted base, mesh or fiber reinforcement helps control cracking but is not always required. Driveways and larger slabs benefit from 6×6 W1.4 or W2.9 welded wire mesh. Check local practice — some municipalities require it.
At 4 inches thick: 10 × 10 × 0.333 = 33.33 ft³, plus 10% waste = 36.67 ft³. Divide by 0.60 ft³ per bag = 61.1, so 62 bags. For anything over about a yard, price ready-mix — it is usually cheaper and far less work.
6×6 is the grid spacing in inches. W1.4 and W2.9 are the wire sizes — W2.9 is heavier wire for driveways and heavier loads, while W1.4 suits patios and walkways. Both come in 5 × 150 ft rolls.
Subgrade is never perfectly flat, forms flex under the weight of wet concrete, and some spills during placement. The industry standard is 5–10% for slabs; 10% is the safe default for a hand-graded base.