Rebar Calculator
ConstructionCalculate total linear feet and weight of rebar needed for a rectangular concrete slab grid. Enter slab size, spacing, and bar size for an instant estimate.
Total Rebar Weight
424.2
Total Linear Feet
635
What is a Rebar?
A Rebar Calculator determines the total linear feet and weight of reinforcing steel bar needed for a rectangular grid inside a concrete slab. Rebar reinforces concrete against cracking and improves tensile strength, and getting an accurate material estimate before a pour is essential for both budgeting and ordering the correct quantity from a supplier.
The calculator works out how many bars fit across the slab's width and length based on your grid spacing, sums their total length, then converts that footage to weight using standard US rebar size weight-per-foot values. If you're also estimating the concrete volume for the same slab, pair this with the Concrete Calculator for a complete material takeoff.
How to use this Rebar calculator
- Enter the Slab Length in feet.
- Enter the Slab Width in feet.
- Enter the Grid Spacing in inches — the on-center distance between parallel bars, commonly 12 to 18 inches for residential slabs.
- Select the Rebar Size — #3 through #8 — based on your project's engineering specification.
- Review the Total Rebar Weight result, shown as the highlighted primary output.
- Check the Total Linear Feet output to plan how many standard-length bars (20 ft or 60 ft) you'll need to order and cut.
Formula & Methodology
The calculator counts bars in a standard two-way grid and sums their total length: bars_along_width = floor((slab_width × 12) ÷ grid_spacing) + 1 bars_along_length = floor((slab_length × 12) ÷ grid_spacing) + 1 total_linear_feet = (bars_along_width × slab_length) + (bars_along_length × slab_width) total_weight = total_linear_feet × weight_per_foot Whereslab_lengthandslab_widthare in feet,grid_spacingis in inches, andweight_per_footis the selected bar size's standard weight (#3 0.376, #4 0.668, #5 1.043, #6 1.502, #7 2.044, #8 2.670 lb/ft). Worked example: A 20 ft × 15 ft slab with 12-inch grid spacing, using #4 rebar. - bars_along_width = floor((15 × 12) ÷ 12) + 1 = 16 bars - bars_along_length = floor((20 × 12) ÷ 12) + 1 = 21 bars - total_linear_feet = (16 × 20) + (21 × 15) = 320 + 315 = 635 ft - total_weight = 635 × 0.668 = 424.2 lb
Frequently Asked Questions
The amount depends on your slab's length, width, and the grid spacing between bars — tighter spacing means more bars and more total linear feet. The Rebar Calculator finds how many bars run in each direction based on your spacing, sums their total length, then converts that to weight using the selected bar size. Enter your slab dimensions and spacing to get an instant total.
Rebar sizes in the US are numbered based on their diameter in eighths of an inch — a #3 bar is 3/8 inch diameter, a #4 bar is 1/2 inch, and a #5 bar is 5/8 inch. Larger bar sizes weigh more per foot and provide greater tensile strength, so #4 and #5 bars are common in residential slabs while larger sizes are used in heavier structural applications.
Grid spacing is typically specified by an engineer or local building code based on the slab's thickness and expected load, commonly ranging from 12 to 18 inches on center for residential slabs. Tighter spacing (smaller numbers) increases the total rebar needed but improves crack control and load distribution.
On center' spacing measures the distance from the center of one bar to the center of the next, which is the standard way rebar grids are specified on construction drawings. The calculator's Grid Spacing input represents this on-center distance in inches.
Multiply the total linear feet of rebar needed by the weight-per-foot of your chosen bar size (for example, 0.668 lb/ft for #4 bar). This calculator handles that automatically once you enter slab dimensions, spacing, and bar size, returning both total linear feet and total weight.
A grid needs one more bar than the number of spacing intervals to cover the full width or length — for example, three 12-inch spacing intervals across a 36-inch span require 4 bars, not 3, to have a bar at both the start and end. This is why the formula uses floor(span ÷ spacing) + 1.
This calculator provides total weight and linear footage, which are the inputs needed to request a quote from a supplier — rebar is typically priced per linear foot or per ton, and pricing varies by region and market conditions. Use the weight and footage totals from this tool when requesting supplier pricing.
Yes, the calculator assumes a standard two-way grid where bars run in both the length and width directions of the slab, which is the typical configuration for residential and light commercial concrete slabs. It sums the total length of bars running each direction to produce the combined linear footage.
Rebar is placed within the concrete formwork before the pour to reinforce the slab against cracking and improve tensile strength, since concrete alone is strong in compression but weak in tension. Once you know your rebar requirements from this calculator, use the [Concrete Calculator](/concrete-calculator/) to estimate the volume of concrete needed for the same slab.
Residential slabs commonly use #3 or #4 rebar on 12 to 18-inch spacing, though local building codes and soil conditions can change this recommendation. Always check with a structural engineer or your local building department for the correct specification before pouring.
The calculation is mathematically precise for a standard rectangular grid based on the dimensions and spacing entered, using the same method structural estimators use for material takeoffs. Real-world projects may require slight overage for lap splices where bars overlap, so add 5-10% to the total for waste and splicing when ordering.
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