Rebar Calculator

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Calculator / Last modified by KurtHeckman on 2017/08/07 14:21
Rebar Calculator
Length of Rebar in Grid
Weight of Rebar in Grid
Weight of Length of Rebar
Area of Concrete Slab
Volume of Concrete Slab
Weight of Concrete Slab
Rebar - Irregular Shaped Slab
Rebar - Circular Slab
Volume of a Circular Slab
Wall - Rebar, Concrete and Forms
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The Rebar Calculator contains equations and data used in working with reinforcement bars (rebar) in concrete slabs.  The formulas calculate materials needed for rectangular, circular and irregular shaped slabs.  slabRebar.png(para español, HAGA CLIC AQUÍ) .

Reinforcement Bar (rebar) Grid

  • Length of Rebar in Grid
  • Weight of Rebar in Grid
  • Weight of a Length of Rebar
  • Area of a Concrete Slab
  • Volume of Concrete Slab
  • Weight of Concrete Slab
  • Rebar in an Irregular Shaped Slab
  • Rebar in a Circular Slab (New!)
  • Volume of Circular Slab
  • Wall - Rebar, Concrete and Forms

slabs and rebar

The rebar and concrete slab equations include:

  • Length of Rebar in Grid:  The user enters the dimensions of the slab (Length and Width), Irregular Rebar.pngRebar in Irregular Shaped slab.Rebar Circular Slab.pngthe onCenter spacing of the rebar and the inset from the edge of the slab for the rebar grid.   The user can also enter lapping factors: length of uncut rebar, rebar size and a lapping factor. The formula returns the total length of rebar needed to build such a grid  (see diagram above).
  • Weight of Rebar in Grid: The user enters the dimensions of the slab as in the Length of Rebar in a Grid formula (above), and selects a size of rebar (see table).  The formula calculates the total length needed for the grid and applies the weight measurement for the specified rebar size to provide the Total Weight of the Rebar Grid.  The calculator assumes a lapping factor of 40 for rebar lengths greater than 40 feet.  For the same Rebar Weight / Length function and the ability to control the lapping factor and max length of solid rebar, CLICK HERE.
  • Weight of a Length of Rebar: This lets the user enter any length and select a size of rebar.  It then returns the weight of that length of rebar.
  • Slab - Surface Area: This lets the user enter the length and width of the slab and returns the surface area of the slab or wall.
  • Slab - Volume: This lets the user enter the length, width and depth of the slab and returns the slab volume of concrete in cubic yards.  Cubic yards can be automatically converted to many other system volume units (e.g. cubic meters).
  • Slab - Weight: This uses the volume of the slab (above) and the mean density of concrete to return the total weight of the concrete slab.
  • Irregular Slab - Rebar Length and Weight:  This formula computes the length or weight of rebar in an irregular shaped slab based on the spacing and the rough area and perimeter of the slab.
  • Rebar in Circular Slab:  This computes the length or weight of rebar in a circular slab based on the spacing and the radius of the slab.
  • Volume of a Circular Slab:  This lets the user enter the radius and depth of the slab and returns the cubic yards of concrete needed for a circular slab.
  • Wall - Rebar, Concrete and Forms:  This lets the user enter the dimensions of a wall and optionally the rebar specifications and computes the length and weight of rebar, the volume of concrete, and the surface area of the forms for a concrete wall.
 Nov-20163,588 95 million feet  28.2 thousand short tons
 Dec-2016 4,944 56 million feet  29.3 thousand short tons
 Jan-2017 6,791 588 million feet  342.7 thousand short tons

Help Us Help You!  If there are suggestions, improvements or corrections you would like to see in this calculator, click on the Contact Us button at the very bottom of this web page and send us a message.  Our team will get back with you quickly.  It was through this process that this calculator now takes into account multiple rebar mats, the lapping of rebar, and rebar for an Irregular shaped slab.  So the process works!  

The slab equations provide the surface area, volume, and rebar lengths for concrete slabs of different shapes.  Furthermore, they also provide weight estimations of the rebar  and slabs based on their volume and density (concrete) and/or length and size of rebar.  Equations also include simple but useful geometric equations for the perimeter of the slabs and the surface areas.  Note: the volume equations have a setting of SYSTEM.  This provides the volume of concrete in standard units like meters cubed or cubic  yard, that can be changed using vCalc's units selection pull-downs.  However, in addition to the standard SYSTEM units, this calculator let the user specify units that are very specific to concrete work, namely the number of 40lb, 60lb or 80lb bags of concrete.  This let's to figure out how many bags of concrete you'd need for your job.RebarSpec.jpg Rebar Specifications 

 Re-enforcement bar (Rebar) length based on slab dimensions (length and width), and rebar inset and spacing.  Polygon slab volume based on number of sides, radius and depth. 

Related Calculators

Video Instructions: 

A guideline for the  depth of concrete is as follows:

    4” - Home basement floors, farm building floors
    4”-5” - Home garage and porches
    5”-6” - Sidewalks, barn and granary floors, small sheds
    6”-8” - Driveways


These formulas are useful in any job that requires reinforced concrete slabs.  Rebar Lapping.png


When the dimensions of your slab or wall exceed the length of a single piece of rebar, it is required to lap bars and tie them to create the added length.  This is called Lapping, and the length of the overlapping rebar is the Lapping Length (CLICK HERE).  There are a few considerations.  First, the length of the lap is specified by a "Lapping Factor (LF)" which is often 40 times the diameter of the rebar. 60 is also a common factor, but the engineering specifications should always be applied.  See these YouTube videos to better understand rebar lap:

The typical length of pre-cut rebar is 40' and 60' in the United States.  For a calculator that computes the Length and/or Weight of Rebar in a Grid using Different Lapping Factor or Greater than 40 foot (40 ft) solid lengths of Rebar,CLICK HERE.

        Plywood Forms for ConcreteForms.png


Concrete forms are a vital part of construction.  The Form Area of Concrete Wall function in this calculator helps compute the amount of plywood or OSB needed to construct the forms.   It give you the choice of specifying whether you want to compute:

  • One Side:  This is just the face of one side of the form.
  • Two Sides: This is the face of both sides of the form.
  • Two Sides and Edges:  This is the most common amount of plywood needed to build a form.  It entirely encloses the volume to be poured.

A survey of plywood prices can be found HERE.


The term rebar is short for reinforcement bars, and refers to bars of steel embedded in concrete to improve tensile strength.

Rebar Calc in Spanish

The Rebar Calculator is now available in Spanish.  It has Spanish language descriptions, labels and diagrams and it also has default units in the metric system (SI).  The lengths are in meters and centimeters, the areas are in square meters, the volumes are in cubic meters, and the weights are in kilograms and metric tons.

Calculadora de Barras de Refuerzo:)

See Also


This calculator, Rebar Calculator, is listed in 2 Collections.