Length and Weight of Rebar in Concrete Slab

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Equation / Last modified by KurtHeckman on 2017/09/28 18:11
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KurtHeckman.Length and Weight of Rebar in Concrete Slab

The Length and Weight of Rebar in Concrete Slab computes the total length and weight of reinforcement bars (rebar) needed for a concrete slab based on the dimensions of the slab and the spacing and gauge of the rebar.  See the Rebar Calculator for more concrete and rebar functions and features.  VIDEO INSTRUCTION
Note: 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.

slabtotalgridrebarweight-illustration.png  Concrete Slab with Rebar Grid 


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 at 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"
  • gauge - this is the gauge of the rebar.  The standard gauges include: 3-11,14,18
  • mats - this is the number of rebar grids (mats). 
  • Output choice - this lets the user choose between the weight of the rebar in the grid or the length of rebar in the grid.
  • Rebar Length - (RL) this is the length of the uncut rebar used on the project.  This length is used to determine if lapping (see below) is needed.
  • Lapping Factor - (LF) this is the factor that is used with the diameter of the rebar to compute the length of lapping.

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".


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.

This is useful for:

  • The determination of bar length in concrete slab
  • The weight of rebar in concrete slab


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.  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