Choose the Right Concrete Shape
Choose the geometry that matches the space you will fill. This tool estimates solid concrete sections. It does not include hollow blocks, internal voids or stepped profiles automatically. If a structure contains several different shapes, estimate each one separately and add their net volumes.
| Shape | Measurements | Net volume in feet |
|---|---|---|
| Rectangular slab | Length, width, thickness | L × W × T |
| Rectangular footing | Length, width, depth | L × W × D |
| Round column | Diameter, height | π × (diameter ÷ 2)² × H |
| Wall | Length, height, thickness | L × H × T |
| Circular slab | Diameter, thickness | π × (diameter ÷ 2)² × T |
The thickness field starts in inches. Length, width, diameter and height start in feet. Switch to metric to enter length dimensions in meters and thickness in centimeters. The calculator converts current entries when you switch; it does not silently reinterpret feet as meters.
Concrete for Footings
A straight rectangular footing is a long box. Measure its actual length, width and depth, rather than using the wall dimensions above it. Enter depth in the thickness field. A footing 20 feet long, 2 feet wide and 12 inches deep contains 40 cubic feet, which is 1.4815 cubic yards.
40 × 1.10 ÷ 27 = 1.63 yd³ with 10% allowance
For connected footings, avoid counting corner intersections twice. A deeper pad under a column is a separate shape: include only the additional depth if its top portion is already counted in the continuous footing. Use actual project dimensions rather than a nominal trench size.
Concrete for Columns
For a round column, divide diameter by two to find the radius. Square the radius, multiply by π and then by height. Diameter and radius are not interchangeable: using the diameter as the radius makes the volume four times too large.
Worked example: a 12-inch-diameter column, 3 feet tall
2.35619449 ÷ 27 = 0.08726646 yd³ ≈ 0.09 yd³
With 10% allowance, the column requires 2.5918 cubic feet, or 5 bags yielding 0.60 cubic feet each. Enter 1 foot for a 12-inch diameter. Use quantity for identical columns and apply bag rounding after the combined volume has been calculated.
Concrete for Walls
A solid rectangular wall uses length multiplied by height multiplied by thickness. For a 20-foot-long wall that is 4 feet tall and 8 inches thick, the net volume is 53.3333 cubic feet, or 1.9753 cubic yards. Adding 10% brings the estimate to 2.1728 cubic yards.
The wall mode does not subtract doors, windows or penetrations. Calculate the opening volume separately and subtract it from the gross wall volume before applying waste. Do not subtract an opening that will actually be filled with concrete. Footings and any thickened ends should be counted as their own sections.
From Net Volume to an Order
All five shapes share the same final conversion. Divide net cubic feet by 27 for cubic yards, multiply by the allowance factor and divide by the selected bag’s yield. The result panel shows both net volume and the quantity including allowance.
| Bag size | Assumed yield | Bags per 1 yd³, no waste |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
A 10% allowance means multiplying by 1.10, not adding ten bags. When multiple sections share one order, add their volumes first. Rounding every small section separately can overstate the combined bag count, especially for small posts and repairs.
Check the supplier’s product yield and order increments. The calculator does not determine mix strength, reinforcing requirements or whether a footing is large enough for the intended load. Those choices come from the project specification. Concrete mix product data provides a reference for the approximate bag yields used here.
Frequently Asked Questions
How do I calculate concrete for a slab?
Select rectangular slab and enter length, width and thickness. The calculator converts thickness into the same unit as the other dimensions, multiplies the three values and applies quantity and waste. For a 10 × 10 ft slab at 4 inches, the net volume is 1.23 cubic yards, or 1.36 cubic yards with 10% extra.
How many bags of concrete do I need for a 10×10 slab?
At a thickness of 4 inches, a 10 × 10 ft slab contains 33.3333 cubic feet. At the assumed yields, the no-waste counts are 112 bags of 40 lb mix, 75 bags of 60 lb mix, or 56 bags of 80 lb mix. Choose one size; do not add the three alternatives together.
How much concrete do I need for a 4×8 slab?
At 4 inches thick, a 4 × 8 ft rectangular slab needs approximately 0.3951 cubic yards of concrete before waste. A 10% allowance increases the estimate to 0.4346 cubic yards. Using 80 lb bags yielding 0.60 cubic feet each, the purchase estimate is 20 bags. Different thicknesses produce proportionally different volumes.
How thick should a concrete slab be?
Use the thickness specified for your actual project. This calculator can compare volume at several depths, but it cannot assess bearing capacity, soil support or reinforcement. A slab, footing and wall perform different jobs, so a dimension suitable for one shape should not be reused as a construction recommendation for another.