slab - total grid rebar weight

vCalc Reviewed
Equation / Last modified by AndrewBudd on 2019/07/23 15:59
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vCalc.slab - total grid rebar weight

The rebar weight calculator computes the total weight slab.pngRebarGridNew.jpg Concrete Slab with one Rebar Grid mat of reinforcement bars (rebar) needed for a concrete slab based on the dimensions of the slab and the spacing and size of the rebar.

INSTRUCTIONS: Choose units and enter the following:

  • (L) The length of the slab
  • (W) The width of the slab 
  • (rS) The size of rebar  which is used to compute the diameter for lapping considerations.
  • (oC) The on-center spacing of the rebar (default: 18")  
  • (i) The inset distance from the side of the slab to the rebar grid (default: 3").
  • (m) The number of mats.  (default is one).
  • (OC) The output choice of weight (default) or length.
  • (rL) The length of the reinforcement bars being used before they are cut.  Common lengths are 20, 30, 40 and 60.
  • (LF)  The lapping factor is the multiple of the diameter of the rebar to specify the lapping distance.  40 (default) and 60 are common.

Weight of Rebar: The calculator returns the total weight of the rebar in pounds.   However, this can be automatically converted into other weight units (e.g. kilograms or tons) via the pull-down menu.   If OC set to length, the length of the rebar in the mat(s) is return in feet, which can be automatically converted to other length units (e.g. meters or yards) via the pull-down menu.

General Information

See the Rebar Calculator for more concrete and rebar functions and features.  See VIDEO INSTRUCTION for a YouTube video on the use of this calculator.

Rebar and Slab Calculators:

Note1: the default units are feet and inches.  However, you can change the input units to metric (SI) or others by clicking on the  units selection button (to the right of the entry fields).   You can also change the output units to metric (SI) units by clicking on the red output units selection button (to the right of the answer box).  In both cases, vCalc will make the automatic conversions.

Note2: The lapping portion of the rebar length calculation assumes that the uncut rebar on site is 40' (feet) in length and that lapping is not necessary under that length.  Furthermore, this calculator assumes that rebar is lapped by a factor of 40 times the diameter of the rebar chosen. For the same calculations with the ability to add a different "uncut" length and/or a different lapping factor CLICK HERE.


RebarSpec.jpg Rebar Specifications 

The rebar weight algorithm calculates the weight of reinforcement steel bars in a concrete slab.   The algorithm calculates the area of the slab, and calculates the length of rebar needed for a rectangular grid within the slab.  The grid is specified by the inset from the edge of the slab and the nominal separation between horizontal and vertical rows of rebar (see diagram).  Once the algorithm calculates the length of rebar, the length is used with the user specified rebar gauge, and density constants from the vCalc library to calculate the total weight of the reinforcement steel bars.

The user specified parameters are:

  • length - length of the slab
  • width - width of the slab
  • OnCenter - the on-center spacing between rebar, which defaults to 18"
  • inset - an offset distance from the side of the slab to the first row the rebar grid,  which has a default of 3"
  • size - this is the size of the rebar.  The standard sizes include: 3-11,14,18
  • mats - (m) this is the number of mats of rebar.
  • Output choice - this lets the user choose between the weight of the rebar in the grid or the length of rebar in the grid.

The calculation determines the need for an extra rebar using an internal tolerance for for a spacing fraction that exceeds the tolerance with a default of 1".  It also assumes that the uncut rebar is in 40' lengths and that lapping is needed for dimensions in excess of 40`.  Furthermore, the lapping factor, when lapping is needed, is 40.  For the same calculations with the ability to add a different "uncut" length and/or a different lapping factor CLICK HERE.


Reinforcement bars are often used in concrete including common slabs.  This formula provides a length and weigh calculation that is useful in understanding the additional load of the slab added by the rebar steel.  It is also useful for calculating the weight of rebar when considering transport.  Steel is dense and heavy.  Most vehicles would be considerably overloaded in weight of rebar long before their potential volume is full, which poses a significant safety issue.  In the U.S., most pickup trucks are rated at a half or three quarters ton load rating.  This rating indicates the safe weight of a load that can be carried.  The rebar weight formula can help determine how many trips are required to transport the load of reinforcement steel safely.


When the dimensions of your slab or wall exceed the length of a single piece of rebar, it is required to lap and tie the rebar to create the added length.  There are a few considerations.  First, the length of the lap is often specified as 40 times the diameter of the rebar.  In this case, 40 is lapping factor.  60 is also a common factor, but the engineering specifications should always be applied.  See these YouTube videos to better understand rebar lap:

Second, the typical length of pre-cut rebar is 40' and 60' in the United States.

See Also