rebar - circular slab

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Equation / Last modified by AndrewBudd on 2018/01/19 21:05
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KurtHeckman.rebar - circular slab

The Rebar in a Circular Shaped Slab calculator estimates the total length or weight Rebar Circular Slab.png of reinforcement bars (rebar) needed for a circular shaped concrete slab.  

INSTRUCTIONS: Choose units and enter the following:

  • (r) This is the radius (half of the diameter) of the circular slab (e.g. 12 ft).
  • (i) This is the inset from the edge of the circular slab (e.g. 3 inches)
  • (S) This is the size of the rebar. Choose from the pull-down list size 2 to size 18.
  • (oC) This is the on-center spacing of the rebar.
  • (M) This is the number of rebar mats
  • (WL) Output choice:  Choose rebar weight or length

Weight or Length of Rebar in a Cirular Slab: The calculator returns the weight rebar in pounds, and the length of rebar in feet.  However, this can be automatically converted to compatible units (e.g. meters or kilograms) via the pull-down menu.

Related Calculators:See the Rebar Calculator for more concrete and rebar functions and features. 

  • To compute the Cubic Yards or Volume of concrete in a circular slab, CLICK HERE.
  • To compute the Length of Rebar needed for a slab, CLICK HERE.
  • To compute the Weight of Rebar in a slab, CLICK HERE.
  • To compute the Surface Area of a slab, CLICK HERE.
  • To compute the Surface Area of Concrete Forms, CLICK HERE.
  • To compute the Volume of Concrete in a slab, CLICK HERE.
  • To compute the Weight of Concrete in a slab or wall, CLICK HERE.
  • To compute the Total Weight of a Slab with Rebar, [Could not locate page KurtHeckman.slab – total weight estimate].

Reducing Rebar Waste

A major challenge with rebar in a circular slab is minimizing wasted rebar.  The general guidance is this.  When possible, start with the longest cross-section of of your slab and work outwards from there. These will be the longest stretches of rebar in your slab.  When you cut off the ends, line them up in a row from shortest to longest.  When you get to shorter runs, first see if there is a cut-off section of rebar from your row of cut-offs.  This is only a little extra work and will save in both materials and labor removing cut-offs at the end of the project.   

General Information

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 


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