The Total Grid Rebar Length calculator computes the
total length of rebar needed for one or more mats of rebar in a concrete slab.
INSTRUCTIONS: Choose units and enter the following:
- (L) Length of Slab
- (W) Width of Slab
- (oC) On-center spacing of rebar
- (i) Inset distance from edge of slab to rebar grid
- (m) Number of Mats.
- (rL) Length of the pre-cut rebar
- (rS) Size of Rebar
- (lF) Lapping Factor. 40 (default) and 60 are common.
Length of Rebar (LR): The calculator returns the length of rebar in feet. However, this can be automatically converted into other length units (e.g. meters or yards) via the pull-down menu.
The Math / Science
Rebar is short for "Reinforcement Bars". Rebar is often used inside concrete for added strength, including use in common slabs. This formula provides a length of rebar calculation useful in both calculating the weight of rebar and the cost of rebar. The Total Grid Rebar Length calculator computes the total length of rebar in a rectangular grid. It employs rebar lapping for dimensions (length or width) in excess of the user specified rebar lengths (rL: see Lapping below). A tolerance for determining the need for an extra rebar is set to less than one inch for a spacing fraction above the tolerance. Lapping is a function of the lapping factor and the diameter of the rebar size.
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 (onCenter - see diagram). Once the algorithm calculates the length of rebar, the length is used with the user specified rebar size for the rebar lapping length (see below).
The user specified parameters are:
- (L) Length - length of the slab
- (W) width - width of the slab
- (rS) size - this is the size of the rebar. The standard sizes include: 3-11,14,18
- (oC) OnCenter - the on-center spacing between rebar, which defaults to 18"
- (i) inset - an offset distance from the side of the slab to the first row the rebar grid, which has a default of 3"
- (m) mats - this is the number of mats (layers of grids) of rebar.
- (OC) Output choice - this lets the user choose between the weight of the rebar in the grid or the length of rebar in the grid.
- (rL) Rebar Length - this is the length of the rebar sticks used in constructing the rebar grid.
- (LF) Lapping Factor - this the multiple of the rebar diameter used to define the overlap length at rebar joints (extension).
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".
Lapping
Rebar lapping is required when the dimensions of your slab or wall exceed 
the length of a single piece (stick) of rebar. In this case, it is required to lap and tie the rebar to create the added length. There are a few considerations. First, the length of the rebar lap is specified as a factor (LF) times the diameter of the rebar which is different for each size of rebar. Lapping factors of 40 and 60 are common, but the engineering lapping factor specification should always be applied. See the following YouTube videos to better understand rebar lap:
Second, the typical lengths of pre-cut rebar 20', 40' and 60' in the United States.
Rebar and Slab Calculators:
- Rebar and Concrete in a Slab: The calculator returns all of the following:
- Total Length of rebar in feet
- Total Weight of rebar in pounds
- Total Volume of concrete in cubic yards. Note: this accounts for the concrete displaced by rebar.
- Total Weight of Slab in pounds. Note: this includes the weight of rebar and concrete.
- 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