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Concrete Volume, Weight, and Bag Formulas

Review formulas for concrete slabs, cylinders, hollow tubes, curbs, gutters, stairs, waste allowance, weight, unit conversion, and whole bags.

How do the Concrete Calculator formulas work?

This formula page defines the variables used by the Concrete Calculator, shows how those values are combined, and explains the order of operations behind the result. Use the worked steps to verify an estimate, compare scenarios, and understand which assumptions or rounding rules can affect the displayed answer.

Formula Overview

Every entered dimension is converted to feet before the selected solid-geometry formula is applied. Quantity multiplies the net shape volume. Waste changes only the order volume, while density and package yield convert that order into weight and bags.

Variable Definitions

  • V: net concrete volume
  • L, W, T, H: length, width, thickness, and height
  • D, Do, Di: diameter, outer diameter, and inner diameter
  • r: stair run, h: stair rise, n: riser count
  • p: top-platform depth
  • q: quantity, w: waste percentage

Rectangular Slab, Footing, or Wall

V=LWTqV=LWTq

Use inside length, width, and thickness or height. All three dimensions must use compatible linear units before multiplication.

Cylinder

V=π(D2)2HqV=\pi\left(\frac{D}{2}\right)^2Hq

The diameter is divided by two to obtain radius before calculating the circular area.

Hollow Circular Slab or Tube

V=π[(Do2)2−(Di2)2]HqV=\pi\left[\left(\frac{D_o}{2}\right)^2-\left(\frac{D_i}{2}\right)^2\right]Hq

The inner cylinder is removed from the outer cylinder, so the inner diameter must be smaller.

Curb and Gutter

V=(dchc+wgtg)LqV=\left(d_ch_c+w_gt_g\right)Lq

The model treats the curb and gutter as two nonoverlapping rectangular cross-sections.

Solid Concrete Stairs

V=W[rhn(n+1)2+phn]qV=W\left[rh\frac{n(n+1)}{2}+phn\right]q

The first term sums the increasing rectangular layers beneath the steps. The second adds the top-platform prism.

Waste, Weight, and Bags

Vorder=V(1+w100)V_{order}=V\left(1+\frac{w}{100}\right)
Weight=Vorder×DensityWeight=V_{order}\times Density
Bags=⌈VorderYield per bag⌉Bags=\left\lceil\frac{V_{order}}{Yield\ per\ bag}\right\rceil

Bag count rounds up to a whole package. Density and bag yield are user-editable assumptions, not universal constants.

Unit Conversions

The engine uses 12 inches per foot, 3 feet per yard, 0.3048 meters per foot, 27 cubic feet per cubic yard, and 0.028316846592 cubic meters per cubic foot. Display values round only after calculations are complete.

Worked Slab Calculation

For a 5 ft × 2.5 ft slab that is 5 in thick:

V=5×2.5×512=5.2083 ft3V=5\times2.5\times\frac{5}{12}=5.2083\ ft^3

Dividing by 27 gives approximately 0.1929 yd³. At a 0.45 ft³ yield, the no-waste 60 lb bag estimate is ceiling(5.2083 ÷ 0.45) = 12 bags.

Assumptions and Boundaries

  • Dimensions describe the inside fill space.
  • Each modeled shape is geometrically uniform.
  • Waste is applied uniformly after net volume is calculated.
  • Weight depends on the entered density.
  • Bag counts depend on the entered product yield.
  • Structural and site-specific requirements are excluded.

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