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Switch between imperial and metric takeoff units; current dimensions convert in place.
Enter the outside length from form to form.
Enter the outside width using the same unit family as length.
Use the planned edge cover so the grid count is based on the usable inner rectangle.
Spacing is measured across the slab width; actual spacing is rounded tighter than the target when needed.
Spacing is measured along the slab length; use the same value for a square grid.
Choose the size from the drawing, engineer note, or supplier quote.
Enter the bar length your supplier sells, such as 20 ft or 6 m.
{{ formatNumber(wastePercentClamped, 0) }}%
Use 5-15% for many simple layouts; verify lap and splice rules with the project design.
Choose whether the optional rate is per full stock bar, lineal unit, or weight unit.
Set 0 when prices are unknown; the takeoff still returns bars, length, and weight.
$ {{ priceRateSuffix }}
Takeoff itemEstimateDetailCopy
Enter valid slab, cover, spacing, and bar inputs to show the grid takeoff.
{{ row.item }} {{ row.estimate }} {{ row.detail }}
Order itemQuantityBasisCopy
Enter valid dimensions to show the steel order.
{{ row.item }} {{ row.quantity }} {{ row.basis }}
CheckStatusDetailCopy
Enter valid rebar inputs to show placement review notes.
{{ row.check }} {{ row.status }} {{ row.detail }} 

        

      

    

  

                

                

                    

                        

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Rebar takeoff turns a slab reinforcement layout into countable steel. The useful estimate starts with the outside concrete dimensions, subtracts the planned edge cover, counts bars in each direction from center-to-center spacing, then converts the grid into cut length, stock bars, and weight.


    
A rectangular slab grid is easy to misread because the bar direction and the spacing direction are perpendicular. Bars that run lengthwise are counted across the slab width. Bars that run widthwise are counted along the slab length. That distinction matters because a small spacing change can add a full bar run, not just a small fraction of steel.


    

      Rebar takeoff flow from outside slab dimensions to cover offset, bar runs, cut length, stock bars, and review checks.
      
The takeoff counts the usable grid first, then adds waste and rounds the result to whole stock lengths.

    


    
The final number is not only a bar count. Stock lengths create purchase rounding, waste allowance creates extra cut length, and bar size changes weight. A layout that needs 387.2 ft of #4 steel may still require 400 ft of purchased stock if the supplier sells 20 ft bars.


    
A rebar quantity estimate is not a structural design. Bar size, spacing, cover, lap splice length, supports, chairs, crack control, loads, exposure, and local code requirements still need the project drawings, specifications, or a qualified reviewer. The estimate is most useful as a measured material takeoff and order check before the reinforcement is bought or placed.

  






  

    
Technical Details:


    
Rebar grid math is governed by the clear rectangle available inside the slab edges. Concrete cover is the offset between the concrete face and the steel, so it reduces both outside dimensions before any bar count is made. If the cover is too large for the slab, there is no usable grid rectangle to count.


    
Center-to-center spacing sets the maximum target gap between adjacent bar centerlines. Because a partial bar run cannot be installed, the count rounds upward and then adds the two edge runs. Actual spacing is then recalculated from the rounded count, which means actual spacing should be equal to or tighter than the target spacing.


    
Formula Core:

    
The equations below show the rectangular two-way grid used for the takeoff. Lengthwise bars run along the usable length and are counted across the usable width. Widthwise bars run along the usable width and are counted across the usable length.


    

      
        
          
            Linner
            =
            L-2C
          
          
            Winner
            =
            W-2C
          
          
            Nlengthwise
            =
            ⌈WinnerSlengthwise⌉+1
          
          
            Nwidthwise
            =
            ⌈LinnerSwidthwise⌉+1
          
          
            Lcut
            =
            (NlengthwiseLinner+NwidthwiseWinner)(1+R100)
          
          
            Bstock
            =
            ⌈LcutLstock⌉
          
        
      
    


    

      

        
        
          

            Symbol
            Meaning
            Result affected
          

        
        
          

            L and W
            Outside slab length and width after unit conversion.
            Slab footprint
          

          

            C
            Concrete cover subtracted from both ends of each slab dimension.
            Usable grid rectangle
          

          

            S
            Target center-to-center spacing for each bar direction.
            Lengthwise bars, Widthwise bars, and Actual spacing
          

          

            R
            Waste allowance percent, limited to 0% through 40%.
            Waste allowance and Total cut length
          

          

            Lstock
            Purchased straight bar length used for order rounding.
            Stock bars to buy, Purchase length, and Stock rounding
          

        
      

    


    
Bar size controls diameter, steel area, and standard weight per length. The supported preset set covers #3 through #11 U.S. bars with soft metric labels No.10 through No.36. Weight is calculated from the selected standard pounds per foot and converted to kilograms per meter when the metric display is active.


    

      

        
        
          

            Bar size
            Nominal diameter
            Area
            Weight
          

        
        
          

            #3 / No.10
            0.375 in
            0.110 in2
            0.376 lb/ft
          

          

            #4 / No.13
            0.500 in
            0.200 in2
            0.668 lb/ft
          

          

            #5 / No.16
            0.625 in
            0.310 in2
            1.043 lb/ft
          

          

            #6 / No.19
            0.750 in
            0.440 in2
            1.502 lb/ft
          

          

            #7 / No.22
            0.875 in
            0.600 in2
            2.044 lb/ft
          

          

            #8 / No.25
            1.000 in
            0.790 in2
            2.670 lb/ft
          

          

            #9 / No.29
            1.128 in
            1.000 in2
            3.400 lb/ft
          

          

            #10 / No.32
            1.270 in
            1.270 in2
            4.303 lb/ft
          

          

            #11 / No.36
            1.410 in
            1.560 in2
            5.313 lb/ft
          

        
      

    


    
The placement review statuses are estimating cues, not code approvals. They flag inputs that commonly deserve another look before ordering steel or placing a grid.


    

      

        
        
          

            Review item
            Rule used
            Status meaning
          

        
        
          

            Spacing band
            Maximum target spacing is at most 8 in, at most 18 in, or greater than 18 in.
            Tight grid, Common planning range, or Wide grid.
          

          

            Actual spacing
            Rounded bar counts are used to recompute spacing in both directions.
            Rounded tighter when the count keeps spacing from exceeding the target.
          

          

            Edge cover
            Cover is less than 1.5 in, from 1.5 in through 4 in, or above 4 in.
            Review, Captured, or Large offset.
          

          

            Waste allowance
            Allowance is below 5%, from 5% through 15%, or above 15%.
            Low buffer, Typical buffer, or High buffer.
          

          

            Stock rounding
            Offcut buffer is more than 20% of purchased length or not.
            High offcut or Order-ready.
          

          

            Design check
            Always shown when inputs are valid.
            Confirms that the result is a material takeoff, not a structural design.
          

        
      

    

  






  

    
Everyday Use & Decision Guide:


    
Set Measurement system before entering dimensions. Imperial mode uses feet for slab dimensions, inches for cover and spacing, and pounds for weight. Metric mode uses meters for slab dimensions, millimeters for cover and spacing, and kilograms for weight. Switching systems converts current values, so recheck the units after changing modes.


    
Use the outside form-to-form dimensions for Slab length and Slab width. Enter Concrete cover as the edge offset to the nearest bar centerline. The result row named Usable grid rectangle should match the rectangle where the bar centers actually sit.


    
    
      
  • Grid Takeoff is the main quantity view. It lists the slab footprint, usable grid rectangle, lengthwise and widthwise bar runs, base lineal rebar, waste allowance, total cut length, and steel density.
    • 
      
  • Steel Order turns the cut length into full stock bars, purchase length, purchase weight, cut weight, and optional material-only cost.
    • 
      
  • Placement Review is the sanity-check view for spacing, actual spacing, edge cover, waste allowance, stock rounding, cost status, and the design warning.
    • 
      
  • Rebar Length Bars shows how lengthwise bars, widthwise bars, waste, and stock rounding buffer contribute to the purchased length.
    • 
      
  • JSON keeps the exact inputs, computed outputs, row data, and assumptions together for a job note or estimate handoff.
    • 
    


    
The default 18 in spacing and 10% waste allowance are a planning starting point for a simple rectangular slab. Use the spacing values shown on the drawing, engineer note, or local specification when those exist. If the Spacing band status says Wide grid, verify crack-control and load requirements before treating the order as ready.


    
Use Stock bar length exactly as the supplier sells it. A small change from 20 ft to 6 m can alter Stock bars to buy, Purchase length, and Stock rounding. If the stock rounding buffer is large, compare another stock length or ask whether the supplier can cut to a better length before ordering extra steel.


    
The optional Material rate is for material comparison only. It can be entered per stock bar, per foot or meter, or per pound or kilogram depending on the selected cost basis. It does not include bending, tying, chairs, delivery, taxes, minimum-order charges, lap splice detailing, or field labor.


    
Before using the number, compare Total cut length, Stock bars to buy, Purchase weight, Actual spacing, and the Design check row. If any of those disagree with the drawing or supplier quote, fix the inputs before copying the table or placing the order.

  






  

    
Step-by-Step Guide:


    
Work from the slab geometry to the supplier order, then use the review rows to catch spacing and cover mistakes.


    
    
      
  1. Choose Measurement system. Confirm that the dimension, cover, spacing, stock length, and weight units match the job.
    2. 
      
  2. Enter Slab length and Slab width from the outside slab dimensions. The summary stays in the input-review state until positive dimensions are present.
    3. 
      
  3. Enter Concrete cover. Check Usable grid rectangle to make sure cover was subtracted from all four slab edges.
    4. 
      
  4. Set Lengthwise bar spacing and Widthwise bar spacing. Review Lengthwise bars and Widthwise bars to see the rounded run counts and actual spacing.
    5. 
      
  5. Choose Bar size. Confirm that Selected bar size, Purchase weight, and Cut weight reflect the size on the drawing.
    6. 
      
  6. Enter Stock bar length and set Waste allowance. Read Total cut length, Stock bars to buy, and Purchase length together because stock rounding happens after waste is added.
    7. 
      
  7. Open Advanced only when material price matters. Choose Cost basis, enter Material rate, and check whether Material cost changes from Not estimated to a dollar amount.
    8. 
      
  8. If the red Check rebar inputs alert appears, fix the named field. Common messages include a zero slab dimension, zero spacing, zero stock length, cover that is too large for the slab, or spacing that creates more than 10,000 bar runs.
    9. 
      
  9. Review Grid Takeoff, Steel Order, Placement Review, and Rebar Length Bars before using the quantity in a quote, purchase list, or job record.
    10. 
    

  






  

    
Interpreting Results:


    
Start with Total cut length and Stock bars to buy. Total cut length is the estimated steel length after waste allowance. Stock bars to buy is the rounded purchase quantity after that length is divided by the selected stock bar length.


    

      

        
        
          

            Output cue
            What it means
            Check before relying on it
          

        
        
          

            Usable grid rectangle
            Outside slab size after cover is subtracted from each edge.
            Confirm that cover is the planned offset to the nearest bar centerline.
          

          

            Actual spacing
            Spacing after bar counts round upward in each direction.
            Make sure tighter spacing is acceptable and practical to place.
          

          

            Base lineal rebar
            Two-way grid length before waste allowance.
            Recheck direction-specific spacing if the number looks high or low.
          

          

            Purchase weight
            Rounded purchase length multiplied by the selected bar weight.
            Verify the bar size and stock length against the supplier quote.
          

          

            Stock rounding
            The offcut buffer created by whole stock bars.
            Review stock length when the status says High offcut.
          

        
      

    


    
A positive status does not mean the reinforcement is structurally correct. Common planning range only means the target spacing is within the calculator's estimating band. Load, slab thickness, soil support, joint layout, bar support, corrosion exposure, and code requirements are outside the material count.


    
The strongest verification cue is agreement between the drawing and the result rows. Check that Lengthwise bars, Widthwise bars, Actual spacing, Selected bar size, and Stock bars to buy all describe the same real slab. If one row looks surprising, recheck the unit selectors before changing the design quantity.

  






  

    
Worked Examples:


    
Default rectangular slab

    
A 20 ft by 12 ft slab with 2 in cover, #4 bars, 18 in spacing in both directions, 20 ft stock bars, and 10% waste gives a Usable grid rectangle of about 19.7 ft by 11.7 ft. The grid rounds to 9 lengthwise runs and 15 widthwise runs, so the summary shows 24 runs. Total cut length is about 387.2 ft after waste, and Stock bars to buy rounds that to 20 full bars.


    
Metric slab with the same layout logic

    
A 6 m by 3.6 m slab with 50 mm cover and 450 mm spacing works out to the same run pattern: 9 lengthwise bars and 15 widthwise bars. Total cut length is about 116.2 m after 10% waste, and 6 m stock bars produce 20 bars, or 120 m of Purchase length. The result changes units, but the count still comes from the usable rectangle and rounded spacing.


    
Wide spacing that needs a design check

    
If the default slab keeps #4 bars but spacing is changed to 24 in in both directions, Spacing band changes to Wide grid. The calculator still returns bar counts and order length, but the warning means the spacing should be checked against the project documents before steel is ordered for crack control or load support.


    
Troubleshooting cover that removes the grid

    
A 4 ft by 3 ft small pad with 24 in cover cannot produce a usable rectangle because cover consumes the full width. The alert reports that concrete cover is too large for the slab dimensions, and the result tables stay empty. Reducing Concrete cover to the planned edge offset restores Usable grid rectangle and the downstream order rows.

  






  

    
FAQ:


    

      Does this calculate structural reinforcement?
      
No. It estimates a rectangular two-way rebar grid, stock bars, length, weight, optional material cost, and review cues. Structural design still depends on the project drawings, loads, slab thickness, supports, exposure, cover, laps, and code requirements.

    


    

      Why is actual spacing different from target spacing?
      
The bar count rounds upward with ceil(span / spacing) + 1. After that, actual spacing is recomputed across the usable span. This keeps spacing from exceeding the target, so the actual number can be slightly tighter.

    


    

      Which direction is lengthwise spacing measured across?
      
Lengthwise bar spacing is measured across the slab width because those bars run along the slab length. Widthwise bar spacing is measured along the slab length because those bars run across the slab width.

    


    

      Why does purchase length exceed total cut length?
      
Total cut length is the estimated steel after waste. Purchase length rounds that number up to full Stock bar length units, so the difference appears as the stock rounding buffer.

    


    

      What should I do when the result table is empty?
      
Read the Check rebar inputs alert and fix the named field. The tables remain empty when slab dimensions, spacing, or stock length are zero, when cover consumes the usable rectangle, or when spacing creates more than 10,000 bar runs.

    


    

      Are my measurements sent to a server?
      
The calculation runs in the browser. The page can place entered values in the URL for repeatable state, so avoid sharing a URL that contains private job details unless that is intentional.

    

  






  

    
Glossary:


    

      
Concrete cover

      
The distance from the concrete face to the nearest reinforcing steel, used here as the slab-edge offset before bar counts are made.

      
Center-to-center spacing

      
The distance from one bar centerline to the next bar centerline in the same direction.

      
Lengthwise bars

      
Bars that run along the slab length and are counted across the usable width.

      
Widthwise bars

      
Bars that run along the slab width and are counted across the usable length.

      
Stock bar length

      
The straight length sold or supplied, used to round the cut-length estimate into whole bars.

      
Lap splice

      
An overlap between reinforcing bars used to continue reinforcement, normally defined by project design rather than this takeoff.

    

  






  

    
References: