UNDERSTAND YOUR ESTIMATE
How to use the concrete calculator
Concrete is ordered by volume, so all three dimensions must use a consistent unit before multiplication. This calculator converts mixed measurements to meters internally, then shows cubic yards and cubic meters. For a slab, thickness is the concrete thickness, not the depth of the excavation or subbase.
Formula
Rectangle volume = length × width × thickness Circle volume = π × (diameter ÷ 2)² × thickness Triangle volume = ½ × base × perpendicular height × thickness Order volume = net volume × (1 + allowance/100)
Assumptions & scope
- Thickness is uniform; thickened edges, beams, steps and footings must be calculated separately and added.
- Waste is an ordering allowance, not reinforcement, strength or mix design advice.
- Bagged concrete yield depends on the actual product. Do not use these yields for mortar, grout or cement alone.
A calculation you can check
A 12 ft × 10 ft slab at 4 in thick uses 40 ft³, or 1.481 yd³ net. A 10% allowance produces 44 ft³, or 1.630 yd³ to order, and 74 bags at 0.60 ft³ per bag.
Quick reference
| 100 ft² slab thickness | Net volume | Net cubic yards |
|---|---|---|
| 3 in | 25.00 ft³ | 0.926 yd³ |
| 4 in | 33.33 ft³ | 1.235 yd³ |
| 5 in | 41.67 ft³ | 1.543 yd³ |
| 6 in | 50.00 ft³ | 1.852 yd³ |
Calculation methodology
Standard prism and cylinder formulas with exact length conversions. Whole bags are rounded up only after combining net volume and the chosen allowance.
Length conversions use the NIST SI conversion factors. Concrete-mix yield examples follow the QUIKRETE Concrete Mix technical data sheet. See our calculation and editorial standards for precision, validation and limitations.
Common questions
How many 80 lb bags make a cubic yard?
At an approximate yield of 0.60 ft³ per bag, 27 ÷ 0.60 = 45 bags per cubic yard before allowance. Check the product label.
What waste allowance should I use?
The default 10% is an editable estimating allowance. Actual needs depend on excavation accuracy, formwork, spills and supplier practices. Review the quantity with your supplier before ordering.
Does the calculator choose slab thickness?
No. Thickness is an input determined by the project design. The calculator does not account for structural capacity, reinforcement or subgrade suitability.