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Construction

Rebar Calculator for Concrete Slabs

By Nick Major · Founder, Calculator Campus Last updated July 1, 2026

Estimate rebar for a rectangular slab, wall, or footing. Enter the project dimensions, bar spacing, stock length, and bar size to get total linear feet, whole stock bars to buy, approximate weight, and a field lap-splice length.

Start with slab length, width, spacing, and bar size. Open advanced options for edge clearance, stock length, waste allowance, lap-splice factor, and wall / footing details.

Calculation mode

Use Slab grid for a flat rectangular mat, or switch to Wall / footing for horizontal courses and vertical bars.

Primary output 23 stock bars
Slab length in feet, or the horizontal wall / footing run in wall mode.
Slab width in feet. Wall / footing mode uses Wall height instead.
Vertical wall height or footing depth in feet for wall / footing mode.
On-center spacing in inches. In slab mode this applies to both directions; in wall mode it spaces vertical bars along the length.
Select the imperial bar size used for weight and lap-splice diameter.
On-center spacing in inches for horizontal courses across the wall height or footing depth.
Advanced options
Waste allowance Quantity takeoff only; engineered drawings control structural spacing and cover. Lap splice 20 in
This calculator computes with feet, inches, and U.S. rebar designations.
Planning context for presets; it does not change the grid count unless your drawings specify different spacing or cover.
Distance in inches from each slab edge to the first bar.
Length of each stock bar you plan to buy. The calculator rounds up to whole sticks.
Extra length for cuts, laps, and ordering buffer.
Field shorthand for lap length. Common takeoffs use 40–60 bar diameters; engineered splice schedules control permitted work.
Answer 23 stock bars
Lengthwise bars 11 bars
Widthwise bars 21 bars
Total rebar length 455.4 ft
Stock bars to buy 23 pieces
Approximate weight 304.21 lb
Lap-splice length 20 in
Show calculation details
Effective grid length 19.5 ft
Effective grid width 9.5 ft
Unit weight 0.668 lb/ft
Notes Quantity takeoff only; engineered drawings control structural spacing and cover.

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How to check the math

Grid length

Convert edge clearance to feet, then subtract it from both ends of the slab to find the usable grid length.

Grid Length = Slab Length - 2 × Edge Clearance
Grid width

Convert edge clearance to feet, then subtract it from both sides of the slab to find the usable grid width.

Grid Width = Slab Width - 2 × Edge Clearance
Bars along length

Divide grid width by bar spacing, then round up to a whole number. Add one bar to include the starting row.

Bars Along Length = Ceiling(Grid Width / Bar Spacing) + 1
Bars along width

Divide grid length by bar spacing, then round up to a whole number. Add one bar to include the starting column.

Bars Along Width = Ceiling(Grid Length / Bar Spacing) + 1
Total rebar length

Multiply each bar count by its run length, add both directions together, then scale up by the waste factor.

Total Rebar Length = (Bars Along Length × Grid Length + Bars Along Width × Grid Width) × (1 + Waste Percent / 100)
Rebar piece count

Divide total rebar length by the stock bar length, then round up to the nearest whole bar.

Rebar Piece Count = Ceiling(Total Rebar Length / Piece Length)
Approximate weight

Multiply total rebar length by the unit weight for the chosen bar size to get total weight in pounds.

Approx Weight = Total Rebar Length × Unit Weight
Lap splice length

Multiply bar diameter by the lap factor to estimate the required overlap length at each splice point.

Lap Splice Length = Lap Factor × Bar Diameter
Wall / footing bar counts

In wall mode, divide wall height by vertical bar spacing to count horizontal courses. Divide wall length by horizontal bar spacing to count vertical bars. Add one to each.

Horizontal Bar Count = Ceiling(Wall Height / Vertical Bar Spacing) + 1; Vertical Bar Count = Ceiling(Wall Length / Horizontal Bar Spacing) + 1
Approximate weight (blocklayer shorthand for #5)

Multiply total rebar length by 1.043 pounds per foot, the ASTM A615 nominal unit weight for a number 5 bar.

Approx Weight No. 5 = Total Rebar Length × 1.043
Methodology

How the answer is computed

Slab mode subtracts edge clearance from both sides of the slab, counts bars across each direction with ceil(effective dimension ÷ spacing) + 1, and multiplies each count by the perpendicular run length. Wall / footing mode counts horizontal courses across wall height and vertical bars across wall length with the same endpoint rule. The calculator then applies the waste allowance, rounds stock bars up to whole pieces, looks up nominal ASTM A615 weight by bar size, and computes lap splice length as lap factor × nominal bar diameter.

Worked examples

See the math step by step

20 × 12 ft garage slab with 16-inch bar spacing

Mario needs to order rebar for a 20-foot by 12-foot garage slab. He is going with 3-inch edge clearance, 16-inch bar spacing, 5 percent for waste, and 20-foot stock lengths.

Pulling 3 inches (0.25 feet) from each end, the length drops to 20 - 2 × 0.25 = 19.5 feet. The width does the same: 12 - 2 × 0.25 = 11.5 feet. With 16-inch spacing, 10 bars run the length and 16 bars cross the width. The raw steel totals 10 × 19.5 + 16 × 11.5 = 379 linear feet. Adding 5 percent for waste gives 379 × 1.05 ≈ 398 feet. Dividing by the 20-foot stock length gives 398 ÷ 20 ≈ 19.9, rounded up to 20 pieces.

When to use this calculator

Use this calculator when you already know the rebar size and spacing from a plan, inspector note, or standard detail and need a material takeoff for a slab, retaining wall stem, or strip footing. It is for ordering quantities, not structural design.

Weight and Cost Estimates

Beyond piece count, the tool estimates total steel weight based on the bar size you select. Weight matters for paving projects — a driveway pour can require several hundred pounds of rebar. Multiply weight by your supplier's price per unit and you get a cost estimate before committing to an order. Knowing your total weight upfront also lets you compare quotes from multiple suppliers.

Walls, Beams, and Columns

This tool is built around flat concrete slabs, but rebar planning for walls follows a similar logic. Bar spacing and wall height set the horizontal bar count, and wall length sets the vertical count. For simple retaining walls or basement walls, the grid method gives a reliable bar estimate. Beams and columns involve load-bearing requirements that structural drawings must address separately.

Assumptions

What we assume

  • Slab mode assumes one rectangular orthogonal mat with one bar size and uniform spacing in both directions.
  • Wall / footing mode treats the entered length as the horizontal run and the wall-height field as the vertical height or footing depth.
  • Edge clearance reduces slab grid length and width before bar counts are calculated.
  • Piece count rounds up because rebar is purchased as whole stock bars.
  • Weight uses nominal ASTM A615 unit weights; actual delivered weight can vary by mill tolerance.
  • Lap-splice length is a contractor shorthand for takeoff comparison, not a code-computed development length.
Limitations

What this skips

  • Does not size rebar for loads, soil, seismic design, or crack-control requirements.
  • Does not verify ACI cover, development length, hooks, bends, chairs, or two-mat reinforcement.
  • Does not create a bar-bending schedule or account for openings, curved slabs, columns, stirrups, or local supplier pricing.
Common mistakes

What people miss

  • You enter the total slab dimension instead of the net span, which adds bars that will not fit.
  • Mixing up on-center spacing with the clear gap between bars adds extra bars to the count.
  • Forgetting a waste factor means the order runs short when bars need cutting or lapping at joints.
  • You enter the slab thickness as the bar placement depth, ignoring the required concrete cover layer.
  • Adding the rebar counts for both grid directions into one field doubles the steel and skews the total.
References

References

  1. ASTM A615/A615M rebar specification

    ASTM International · accessed 2026-07-01

  2. ACI 318-19 Building Code Requirements for Structural Concrete

    American Concrete Institute · accessed 2026-07-01

  3. Omni Calculator rebar calculator

    Omni Calculator · accessed 2026-07-01

  4. BN Products vCalc rebar length and weight

    BN Products · accessed 2026-07-01

  5. Blocklayer rebar spacing layout and weight

    Blocklayer · accessed 2026-07-01

Frequently asked questions

How do I estimate the total cost of rebar for my project?
Use the stock-bar count from the calculator, then multiply by your local supplier price per bar. The tool intentionally leaves pricing out because rebar prices vary by region, bar size, coating, delivery, and order size.
How do I find the total length of rebar my project requires?
Add up the length of every bar in your grid to get the total rebar length. The yard charges for a full stick even when you need only part of the last one.
How does this calculator figure out rebar grid dimensions?
In slab mode, it subtracts edge clearance from the slab length and width, then counts bars in each direction from the on-center spacing. In wall / footing mode, it counts horizontal courses across height and vertical bars along length.
What method does this calculator use to find total rebar length?
The calculator sums the run of every bar in the grid to find a total length. Multiply that length by the weight per foot of your chosen bar size to get an estimated rebar weight. The grid math alone does not give you weight without that extra step.
How do I calculate bar spacing, total quantity, coverage area, and weight from my slab dimensions?
It returns bar count, coverage area, and estimated weight at once. For slabs with odd shapes, split the area into rectangles, run each section through the tool, and add the results together.

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