How to Estimate Concrete for a Slab
Calculating the correct amount of concrete is crucial for any construction project, whether you are pouring a patio, a driveway, or a shed foundation. Ordering too little can result in a catastrophic "cold joint" where new concrete fails to bond with the hardening material, while ordering too much is a waste of money.
The core formula for concrete volume involves three dimensions: length, width, and thickness. Since concrete is typically sold by the Cubic Yard (for truck delivery) or by the Bag (for DIY projects), accurate conversion is key.
The Concrete Formula
To determine the volume manually, use the following steps:
- Step 1: Measure the Length and Width in feet.
- Step 2: Measure the Thickness in inches and convert it to feet (divide by 12).
- Step 3: Multiply Length × Width × Thickness (in feet) to get Cubic Feet.
- Step 4: Divide Cubic Feet by 27 to get Cubic Yards.
Pre-Mix Concrete Bags (60lb vs 80lb)
For smaller projects where a ready-mix truck isn't necessary, pre-mixed bags (like Quikrete or Sakrete) are the standard. The calculation for bags depends on the yield of the specific mix, but generally:
- One 80lb bag yields approximately 0.60 cubic feet.
- One 60lb bag yields approximately 0.45 cubic feet.
Our calculator above uses these standard yields to tell you exactly how many bags to buy from the hardware store.
Why Factor in Waste?
Professional contractors never order the exact amount of concrete calculated mathematically. Site conditions vary, and you must account for:
- Uneven Sub-base: If the ground isn't perfectly flat, some areas of the slab will be thicker than 4 inches.
- Spillage: Concrete is heavy and messy; some will inevitably be lost during the pour.
- Form Bowing: Wooden forms may bow outward slightly under the weight of the wet concrete, increasing the volume.
We recommend adding a 5% safety margin for standard flatwork and up to 10% for complex shapes or uneven ground.
Standard Slab Thicknesses
Not sure how thick your slab should be? Here are common standards:
- 4 Inches: Standard for sidewalks, patios, and residential shed floors.
- 5-6 Inches: Recommended for driveways and garages hosting standard vehicles.
- 6+ Inches: Required for heavy equipment or RV parking pads.