Concrete Calculator – Estimate Concrete Volume & Bags Needed
Our concrete calculator helps contractors, DIYers, and homeowners quickly estimate the volume of concrete needed for any project. Whether you're pouring a driveway slab, a deck footing, or a cylindrical column, simply enter your dimensions and get instant results in cubic feet, cubic yards, and cubic meters. You'll also see exactly how many pre-mixed bags to purchase so you never run short or overbuy.
Select the shape of the concrete structure you are pouring.
Length of the rectangular area (not used for circular shapes).
Width of the rectangular area (not used for circular shapes).
Diameter of the circular or cylindrical shape.
Thickness or depth of the concrete pour in inches.
Select the weight of pre-mixed concrete bags you plan to use.
Add a percentage buffer for waste, spillage, or uneven ground. 10% is typical.
Your results will appear here
How to Use This Calculator
1. Select the shape of your concrete pour — rectangular slab, circular slab, or cylindrical column. 2. Enter the length and width in feet for rectangular shapes, or the diameter in feet for circular and cylindrical shapes. 3. Enter the depth or thickness of the pour in inches (e.g., 4 inches for a standard driveway). 4. Choose your pre-mixed concrete bag size (40 lb, 60 lb, or 80 lb). 5. Adjust the waste factor percentage to account for spillage or uneven ground — 10% is a safe standard. 6. Your results will appear instantly, showing total volume in multiple units and the exact number of bags needed.
How Concrete Volume Is Calculated
Concrete volume depends entirely on the geometry of the area being filled. All dimensions must be converted to the same unit (feet) before multiplying.
Rectangular Slab Formula
For a rectangular slab or footing, the volume is calculated as:
- Volume (cu ft) = Length (ft) × Width (ft) × Depth (ft)
- Depth in inches is divided by 12 to convert to feet before calculation.
Circular Slab or Cylindrical Column Formula
For round shapes, the formula uses the area of a circle:
- Volume (cu ft) = π × r² × Depth (ft)
- Where r = Diameter ÷ 2 (the radius in feet).
Converting to Cubic Yards and Cubic Meters
Concrete is typically ordered from ready-mix suppliers by the cubic yard. The conversions are:
- Cubic Yards = Cubic Feet ÷ 27
- Cubic Meters = Cubic Feet × 0.0283168
Waste Factor
It is always wise to order more concrete than your exact calculation suggests. Losses occur due to spillage, uneven sub-base, forms that aren't perfectly level, and absorption into the ground. A 10% waste buffer is the industry standard for most residential projects. Add up to 20–30% for very large or complex pours.
Pre-Mixed Bag Coverage
Pre-mixed bags come in three standard sizes with the following approximate yields:
- 40 lb bag → approximately 0.30 cubic feet
- 60 lb bag → approximately 0.45 cubic feet
- 80 lb bag → approximately 0.60 cubic feet
Divide the total volume (including waste) by the yield per bag and round up to get the number of bags to purchase. For large pours exceeding 1 cubic yard, ordering ready-mix concrete from a supplier is often more economical than using bags.
When to Order Ready-Mix Concrete
For projects requiring more than about 1 cubic yard (27 cubic feet) of concrete, ordering from a ready-mix supplier is typically faster and more cost-effective than using pre-mixed bags. Use this calculator's cubic yards output to place your order accurately.