To calculate concrete for a slab, use this formula:

Volume (ft³) = Length (ft) × Width (ft) × Thickness (in) ÷ 12

Then divide by 27 to get cubic yards. For example, a 10×10 ft slab at 4 inches thick:

10 × 10 × (4÷12) = 33.33 ft³ ÷ 27 = 1.23 cubic yards

Always add 10% for waste, so order 1.35 cubic yards minimum. Use our free calculator above to skip the math!

There are three common formulas depending on your unit system:

Imperial: Volume (ft³) = L (ft) × W (ft) × T (in) ÷ 12

To cubic yards: Volume (yd³) = Volume (ft³) ÷ 27

Metric: Volume (m³) = L (m) × W (m) × T (m)

For circular slabs: Volume = π × r² × Thickness where r is the radius.

For triangular slabs: Volume = 0.5 × Base × Height × Thickness

A 20×20 ft slab at different thicknesses requires:

• 4 inches thick: 1.48 cubic yards (net) / 1.63 yd³ with 10% waste

• 6 inches thick: 2.22 cubic yards (net) / 2.44 yd³ with 10% waste

• 8 inches thick: 2.96 cubic yards (net) / 3.26 yd³ with 10% waste

A 20×20 garage floor at 4" thick needs approximately 89 bags of 80 lb concrete or 1.48 yd³ of ready-mix.

For a circular concrete slab: Volume = π × (Diameter ÷ 2)² × Thickness (ft)

Example: 10 ft diameter circular patio, 4 inches thick:

3.14159 × (10÷2)² × (4÷12) = 3.14159 × 25 × 0.333 = 26.18 ft³ ÷ 27 = 0.97 cubic yards

Select "Circular" in our calculator and enter just the diameter — it handles the formula automatically.

A 10×10 ft slab at 4 inches thick requires 33.33 cubic feet of concrete.

• 80 lb bags (0.60 ft³ yield): 56 bags — order 62 bags with 10% waste

• 60 lb bags (0.45 ft³ yield): 74 bags — order 82 bags with 10% waste

• 40 lb bags (0.30 ft³ yield): 112 bags — order 123 bags with 10% waste

For this size, consider ordering 1.35 cubic yards of ready-mix concrete instead, which will likely be faster and more cost-effective.

Each 80 lb bag of concrete mix yields approximately 0.60 cubic feet when mixed.

Since 1 cubic yard = 27 cubic feet: 27 ÷ 0.60 = 45 bags of 80 lb concrete per cubic yard.

• 40 lb bags (0.30 ft³): 90 bags per cubic yard

• 60 lb bags (0.45 ft³): 60 bags per cubic yard

• 80 lb bags (0.60 ft³): 45 bags per cubic yard

The decision depends on project size and budget:

• Under 0.5 cubic yards (<14 ft³): Use bags — convenient and no minimum order

• 0.5–1 cubic yard: Either works; bags give you more control over timing

• Over 1 cubic yard: Ready-mix is almost always more economical and gives better consistency

Ready-mix concrete is typically delivered in a minimum of 1 cubic yard. For large slabs, it also ensures a uniform mix strength throughout the entire pour.

Waste accounts for concrete lost during:

• Mixing (some material clings to equipment)

• Pouring (spillage at the formwork edges)

• Variations in sub-base depth (ground is rarely perfectly level)

• Settling and compression of the sub-base

Standard industry recommendation: add 10% for flat slabs and 15% for irregular shapes or rough sub-bases. Running out of concrete mid-pour is a serious issue — the cold joint created is a structural weak point.

Recommended concrete slab thickness by application:

• 3 inches: Foot paths, decorative paths (light residential only)

• 4 inches: Patios, walkways, shed floors, standard residential driveways

• 5–6 inches: Garage floors, heavy-use driveways, car parks

• 6–8 inches: RV pads, commercial light-duty areas, heavy vehicle access

• 8–12 inches: Strip footings, foundations, industrial floors

Always check with your local building code for minimum thickness requirements, especially for foundations and load-bearing slabs.

Concrete curing timeline:

• 24–48 hours: Light foot traffic allowed

• 7 days: Approximately 70% design strength — passenger vehicles allowed

• 14 days: Approximately 85% strength

• 28 days: Full design strength — heavy vehicles allowed

Concrete continues gaining strength for years after pouring. During the first 7 days, keep the slab moist (curing compound or wet burlap) to prevent surface cracking and ensure proper hydration.

Yes! Our concrete slab calculator works for any rectangular, circular, or triangular concrete project, including:

• Driveways (single and double car)

• Patios and outdoor living spaces

• Garage floors and shop floors

• Sidewalks and footpaths

• Shed bases and outbuilding floors

• Pool surrounds and decks

• Foundations and footings (use it for individual pads)

Simply enter your dimensions and the correct slab thickness for your application.

Yes! Our Concrete Calculator for Slab is 100% free — no sign-up, no registration, no download, and no hidden fees. Simply visit the homepage, enter your dimensions, and get instant results.

The calculator works on all devices: desktop computers, laptops, tablets, and smartphones. It is optimized for use on construction sites via mobile browsers.

Yes. Our calculator supports both Imperial (feet/inches) and Metric (meters/centimeters) units.

Click the "Metric" button above the calculator inputs to switch. In metric mode, enter length and width in meters and thickness in centimeters. Results show volume in cubic meters (m³) alongside cubic yards and cubic feet for reference.