Concrete Calculator
Calculate concrete volume and bags needed for slabs, footings, columns and stairs.
About Concrete Calculator
Concrete volume is calculated in cubic feet for the selected shape. A waste factor (typically 5–10%) is added to account for spillage, overfill and uneven sub-grade. The total is divided by 27 to get cubic yards — the standard unit used when ordering ready-mix. For bag mixes, dividing the cubic-foot total by the published per-bag yield gives the number of bags. Bags are always rounded up because you cannot use a partial bag.
Formulas
Slab / Footing
Volume (cu ft) = Length × Width × (Thickness_inches ÷ 12)
- L
- Length (ft)
- W
- Width (ft)
- T_{in}
- Thickness (inches)
Column (cylinder)
Volume (cu ft) = π × (diameter_in / 24)² × height_ft
Stairs
Volume = width_ft × tread_ft × rise_ft × n(n+1)/2
- n
- Number of steps
- r_{ft}
- Rise in feet
- t_{ft}
- Tread in feet
- w_{ft}
- Width in feet
Bags
Bags = ceiling(volume with waste ÷ per-bag yield)
Worked example
With the default values below, Concrete Calculator returns Cubic Yards of 1.36. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Shape / Pour Typeone of Slab (rectangular), Footing (rectangular trench), Column / Cylinder, Stairs
- Slab (rectangular)
- Lengthminimum 0.01 ft
- 10 ft
- Widthminimum 0.01 ft
- 10 ft
- Thickness / Depthminimum 0.1 in
- 4 in
- Diameterminimum 0.1 in
- 12 in
- Heightminimum 0.01 ft
- 4 ft
- Rise per Stepminimum 1 in
- 7 in
- Run (Tread depth)minimum 1 in
- 11 in
- Stair Widthminimum 1 in
- 36 in
- Number of Stepsminimum 1
- 5
- Waste Factoraccepts 0–50 %
- 10 %
- Bag Sizeone of 40 lb (0.45 cu ft yield), 60 lb (0.45 cu ft yield), 80 lb (0.60 cu ft yield)
- 80 lb (0.60 cu ft yield)
- Number of Identical Areasminimum 1
- 1
Results
- Cubic Yards
- 1.36
- Cubic Feet (with waste)
- 36.67
- Cubic Feet (net)
- 33.33
- Bags NeededRounded up to whole bags.
- 62
- Bag Size Used
- 80 lb
Step by step
- 1Net volume= 33.333 cu ft
- 2Add 10% waste= 36.667 cu ft
- 3Convert to cubic yards= 1.36 yd³
- 4Bags needed (80 lb, 0.6 cu ft yield)= 62 bags
Assumptions and limits
- The result is derived only from the 13 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How many cubic yards do I need for a 10×10 slab at 4 inches thick?
10 × 10 × (4/12) = 33.33 cu ft. With 10% waste: 36.67 cu ft ÷ 27 = 1.36 cubic yards. The calculator gives you this instantly — just set Length 10 ft, Width 10 ft, Thickness 4 in.
Why is 80 lb a better choice than 60 lb?
An 80 lb bag yields 0.60 cu ft vs 0.45 cu ft for a 60 lb bag. You mix fewer bags for the same volume, reducing labor. On small pours, the weight difference is manageable.
What waste factor should I use?
5% for well-prepared, flat sub-grade. 10% for typical residential work. 15% for rough or sloped terrain. The default 10% is a safe choice for most jobs.
When should I order ready-mix instead of using bags?
Ready-mix is typically more economical above 1 cubic yard. Below that, bags are practical. The calculator shows cubic yards so you can make that call directly.