The Rebar for an Irregular Shaped Slab calculator estimates the total length and weight 
of reinforcement bars (rebar) needed for an irregular shaped concrete slab. NOTE: this calculator was updated on 16 Feb 2022 to accommodate the inset of rebar placement from the edge of the slab. The net result is a more accurate and less conservative estimate.
INSTRUCTIONS: Choose units and enter the following:
- (A) Approximate Area of the irregular shaped slab (e.g. 72 ft2)
- (rP) Estimate of Perimeter of the slab (e.g. 178 ft)
- (i) Inset of Rebar from Edge of Slab
- (D) Depth of Slab
- (S) Size of Rebar. Choose from the pull-down list size 2 to size 20.
- (oC) On-center spacing of the rebar.
- (M) Number of Rebar Mats
Irregular Slab Rebar and Concrete (IR): The calculator returns:
- approximate length of rebar in slab in feet.
- approximate weight of rebar in pounds, and the
- approximate volume of concrete in cubic yards.
However, these can be automatically converted to compatible units via the pull-down menu.
Rebar and Slab Calculators:
- Rebar and Concrete in a Slab: The calculator returns the length and weight of rebar, the volume of concrete and the total weight of the slab.
- Length of Rebar in Grid
- Weight of Rebar in Grid
- Rebar in a Wall
- Rebar in an Irregular Shaped Slab
- Rebar in a Circular Slab
- Rebar Lapping Length
- Rebar Cost Estimate
- Weight of Length of Rebar
- Concrete Displaced by Rebar
- Compute the Total Weight of a Slab with Rebar
- Spanish Version: Calculadora de Barras de Refuerzo
Reducing Rebar Waste
A major challenge with rebar in an irregular 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.
The Math / Science
The rebar weight algorithm calculates the length and weight of reinforcement steel bars in an irregular shaped concrete slab. The algorithm uses the Rough Area, Perimeter and the onCenter spacing to estimate the total length of rebar in one mat. The total length of rebar is the length in one mat times the number of mats.
Once the algorithm calculates the length of rebar, the length is used with the user specified rebar size, and density constants from the vCalc library to calculate the total weight of the reinforcement steel bars. The area and depth are used to compute the volume of concrete. The formula the length of rebar in one mat of an irregular shaped slab is:
L = 4*oC*(A/(2*oC*oC)) + rP/2
where:
- L = length of rebar in an irregular slab
- oC = on center spacing of rebar
- A = area of slab
- rP = perimeter of slab
To accommodate an inset from the edge of the slab for the placemen to the rebar, the area (A) and perimeter (rP) have to be reduced BEFORE the formula above. The method used is to treat the slab as a circular slab and reduce the area and perimeter accordingly as follows:
- //Reduce area by inset
- rR = sqrt(A/PI) - i
- A = PI*rR*rR
- //Reduce perimeter by inset
- rR = rP/(2*PI) - i
- rP = 2*PI*rR
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.
Usage
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.
Lapping
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
- Total Slab Weight - This includes the weight of the rebar and the concrete.
- Brick or Block Wall - Number of brick or block needed for a wall.
- Foundation - Poured - Amount of concrete needed for a poured foundation.
- Foundation - Block - Number of blocks needed for a foundation.