Concrete Calculator
Project geometry
Choose a shape and enter dimensions
Result
Calculation summary
Enter values to see the result
Visual guide
Concrete slab dimensions
How to use this calculator
- 1Choose a slab, strip footing, cylindrical column, post hole, or a combined project.
- 2Enter the dimensions and quantity for the selected shape. Combined projects can mix up to 10 areas with independent units.
- 3Set the allowance and result unit. To estimate bags, enter the volume yield printed on one bag of your selected product.
Formula
Required volume = Σ(shape volume × quantity) × (1 + allowance ÷ 100)
Rectangular shapes use length × width × depth. Cylinders use π × radius² × height, and post holes subtract the cylindrical post volume.
Calculation steps
- Validate the dimensions for every selected shape, including that a post is narrower than its hole.
- Convert all horizontal and vertical dimensions to meters.
- Calculate each rectangular or cylindrical volume and multiply it by its quantity.
- Combine all area volumes, add the selected allowance, and convert the total into the requested unit.
- When a package yield is provided, divide the adjusted volume by that yield and round up to a whole bag.
Worked example
A 5 m × 4 m × 10 cm slab needs 2 m³. A 0.4 m diameter, 3 m high column adds about 0.377 m³. Together they need about 2.615 m³ after a 10% allowance.
Assumptions
- Slabs and footings are rectangular prisms; columns and holes are circular cylinders with uniform dimensions.
- The optional post diameter is subtracted from a post hole; use zero to calculate a completely filled hole.
- The allowance is applied once to the complete base volume and is controlled by the user.
- Bag estimates use the package volume yield supplied by the user, not bag weight.
- Actual requirements can vary with excavation, subgrade, formwork, compaction, spillage, and product yield.
Sources
Frequently asked questions
How do I calculate concrete for a rectangular slab?
Multiply length by width by thickness after converting all three measurements to compatible units. Then multiply by the number of identical slabs.
Why is thickness allowed to use a different unit?
Slabs are often measured in feet or meters for length and width but inches or centimeters for thickness. The calculator converts them before multiplication.
How should I choose a waste allowance?
Use the allowance appropriate for your measurements and job conditions. The calculator does not treat any percentage as universally correct.
Why does the bag estimate ask for package yield instead of bag weight?
Products with the same weight can produce different concrete volumes. Using the volume yield printed by the manufacturer gives a more defensible estimate.
Can this calculator handle footings, columns, or post holes?
Yes. Select the corresponding shape, or use Combined areas to total slabs, strip footings, cylindrical columns, and post holes in one project.