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Rebar Calculator

Construction

Calculate 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.

Reviewed by the thecalcu.com team · Last updated July 12, 2026

1200
1200
424

Total Rebar Weight

424.2
Total Linear Feet
635

This calculator computes your Total Rebar Weight, Total Linear Feet from the values you enter.

Inputs
Slab LengthSlab WidthGrid SpacingRebar Size
Outputs
Total Rebar WeightTotal Linear Feet

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.

Why Use a Rebar Calculator?

Under-ordering rebar means a delayed pour while waiting for more material, while over-ordering wastes budget on excess steel. A contractor pouring a 20 ft × 15 ft driveway slab with 12-inch grid spacing needs an accurate linear footage and weight total to order the right amount of #4 bar from a supplier without guessing.

This calculator automates the grid math, counting bars in each direction, summing total length, and converting to weight, so you get a reliable estimate in seconds. Pair it with the Gravel Calculator when your project also requires a gravel base beneath the slab.

Who Should Use This Calculator?

  • Concrete contractors, estimate rebar quantities for slab, driveway, and patio pours before ordering material.
  • Structural estimators, quickly generate material takeoffs for rebar as part of a larger project bid.
  • DIY homeowners, plan a small slab pour like a shed foundation or walkway with an accurate rebar estimate.
  • Construction project managers, verify supplier quotes against an independent rebar quantity calculation.
  • Engineering students, check hand calculations for rebar grid layouts against a quick reference tool.

Anyone planning a reinforced concrete slab pour benefits from an accurate rebar estimate. Also see the Concrete Calculator and Metal Weight Calculator for related material calculations.

What Insights Does the Rebar Calculator Give You?

Total Rebar Weight (lb) is the primary output, the combined weight of all rebar needed for the slab grid, based on your selected bar size. Use this figure to request pricing from a supplier or budget the reinforcing steel line item of a project.

Total Linear Feet (ft) shows the combined length of all rebar in the grid, which is useful for confirming stock length requirements (standard rebar comes in 20 or 60-foot lengths) and calculating how many full-length bars need to be cut.

Together, these outputs give you the two figures most commonly needed to request a rebar quote and plan a material order.

How to use this Rebar calculator

  1. Enter the Slab Length in feet.
  2. Enter the Slab Width in feet.
  3. Enter the Grid Spacing in inches, the on-center distance between parallel bars, commonly 12 to 18 inches for residential slabs.
  4. Select the Rebar Size, #3 through #8, based on your project's engineering specification.
  5. Review the Total Rebar Weight result, shown as the highlighted primary output.
  6. 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.
Show formula & methodology ↓Show less ↑

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

Where slab_length and slab_width are in feet, grid_spacing is in inches, and weight_per_foot is 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

How much rebar do I need for a concrete slab?
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.
What is the difference between #3, #4, and #5 rebar?
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.
How is rebar spacing determined for a slab?
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.
What does 'on center' mean for rebar spacing?
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.
How do I calculate the weight of rebar for a project?
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.
Why does the calculator add 1 extra bar to the grid count?
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.
What is the difference between rebar weight and rebar cost?
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.
Can I use this calculator for a two-way rebar grid in both directions?
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.
How does rebar work with a concrete pour?
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.
What bar size is typically used for a residential driveway or patio 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.
How accurate is the total linear feet calculation?
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.
Also known as
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