What this calculator does
It works out how much concrete fills a rectangular form (slab, footing, wall, step) or a round column or tube form, and reports the volume in cubic yards (how ready-mix is usually ordered in the US), cubic feet and cubic meters. If you enter the yield printed on a bag product, it also counts bags.
Formulas
- Rectangular:
V = L × W × D × quantity - Round column:
V = π × (diameter ÷ 2)² × H × quantity - With waste:
V_order = V × (1 + waste% ÷ 100) - Bags:
⌈V_order ÷ bag yield⌉(rounded up to a whole bag)
Each dimension is converted to meters first, so feet and inches can be mixed. 1 yd³ = 27 ft³ ≈ 0.764555 m³.
Worked examples
Patio slab: 12 ft × 12 ft × 4 in
- Thickness 4 in = 4 ÷ 12 ≈ 0.3333 ft.
- V = 12 × 12 × 0.3333 = 48 ft³.
- 48 ÷ 27 ≈ 1.778 yd³ (≈ 1.359 m³). With a 10% allowance: 52.8 ft³ ≈ 1.956 yd³.
- If your bag says it yields 0.60 ft³: 52.8 ÷ 0.60 = 88 bags.
Six round columns: 12 in diameter, 4 ft tall
- Radius = 0.5 ft. One column = π × 0.5² × 4 ≈ 3.1416 ft³.
- Six columns ≈ 18.85 ft³ ≈ 0.698 yd³.
Footing: 20 ft × 16 in × 12 in
20 × 1.3333 × 1 ≈ 26.67 ft³ ≈ 0.988 yd³.
Practical use and interpretation
Ready-mix suppliers sell by the cubic yard or cubic meter and may have a minimum load; tell them the volume including your allowance. For small jobs, bagged mix is common: check the yield on the exact product, because different products and bag sizes yield different volumes.
Common mistakes
- Entering slab thickness in feet when it was measured in inches (4 ft instead of 4 in makes the answer 12 times too big). Use the unit selector.
- Dividing cubic feet by 3 instead of 27 to get cubic yards.
- Using a diameter as a radius for round forms.
- Assuming a bag yield instead of reading it from the package.
Assumptions and limitations
Forms are treated as perfect rectangles or cylinders with level bases. The calculator does not cover reinforcement, mix design, strength, or how thick a slab should be. Follow your local building requirements and supplier guidance for those decisions.
Related: cubic yards calculator for gravel, soil and mulch; rectangular prism volume calculator; length converter.
Frequently asked questions
How many bags of concrete do I need?
Divide the volume (including waste) by the yield printed on the bag, then round up. Bag yields differ between products and bag sizes, so this calculator only counts bags when you enter the yield from your product.
How much waste allowance should I add?
There is no single correct figure. Uneven subgrade, form flex and spillage all use extra concrete. Many people add a small percentage; ask your supplier what they recommend for your job.
How many cubic feet are in a cubic yard?
Exactly 27, because a cubic yard is 3 ft × 3 ft × 3 ft.