Concrete slab calculator

How It Is Calculated

This calculator estimates the amount of concrete needed for a rectangular slab and its material cost. The calculations follow these steps:

1. Slab area: Multiply length by width.

   Slab area (ft²) = L × W

   where L is length in feet and W is width in feet.
2. Volume in cubic feet: Multiply slab area by thickness converted to feet.

   Volume (ft³) = Slab area × (T / 12)

   where T is thickness in inches.
3. Volume in cubic yards: Divide cubic feet by 27.

   Volume (yd³) = Volume (ft³) / 27

4. Concrete needed with waste: Add the waste percentage.

   Concrete needed (yd³) = Volume (yd³) × (1 + W / 100)

   where W is the waste factor in percent.
5. Material cost: Multiply concrete needed by price per cubic yard.

   Cost ($) = Concrete needed (yd³) × P

   where P is the price per cubic yard in dollars.

Assumptions

• The slab is rectangular with uniform thickness.
• Concrete is measured in cubic yards.
• The waste factor accounts for spillage, over-excavation, and rounding up.
• Price per cubic yard is for standard concrete mix and does not include delivery or labor.
• All calculations are rounded to two decimal places.

Worked Example

For a slab with Length = 10 ft, Width = 10 ft, Thickness = 4 in, Waste factor = 10%, and Concrete price = 150 $/yd³:

1. Slab area = 10 ft × 10 ft = 100 ft²
2. Volume (ft³) = 100 ft² × (4 in / 12) = 33.33 ft³
3. Volume (yd³) = 33.33 ft³ / 27 = 1.23 yd³
4. Concrete needed = 1.23 yd³ × (1 + 10/100) = 1.35 yd³
5. Material cost = 1.35 yd³ × 150 $/yd³ = 202.50 $

Results: Slab area = 100 ft², Volume (cubic feet) = 33.33 ft³, Volume (cubic yards) = 1.23 yd³, Concrete needed = 1.35 yd³, Concrete material cost = 202.50 $.

Practical Tips

• Always order a little extra concrete (waste factor) to account for spillage and uneven subgrades.
• Verify your slab dimensions and thickness before ordering; consider using a concrete calculator for other shapes.
• For projects involving multiple slabs or footings, use the concrete calculator to sum requirements.
• Check with your supplier for minimum order quantities and delivery fees.
• If you need to estimate materials for a related project, try the brick calculator for walls or paint calculator for finishing.