Rebar Calculator

Measurement system:

Switch between imperial and metric takeoff units; current dimensions convert in place.

Slab length:

Enter the outside length from form to form.

Slab width:

Enter the outside width using the same unit family as length.

Concrete cover:

Use the planned edge cover so the grid count is based on the usable inner rectangle.

Lengthwise bar spacing:

Spacing is measured across the slab width; actual spacing is rounded tighter than the target when needed.

Widthwise bar spacing:

Spacing is measured along the slab length; use the same value for a square grid.

Bar size:

Choose the size from the drawing, engineer note, or supplier quote.

Stock bar length:

Enter the bar length your supplier sells, such as 20 ft or 6 m.

Waste allowance: {{ formatNumber(wastePercentClamped, 0) }}%

Use 5-15% for many simple layouts; verify lap and splice rules with the project design.

Check rebar inputs

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Cost basis:

Choose whether the optional rate is per full stock bar, lineal unit, or weight unit.

Material rate:

Set 0 when prices are unknown; the takeoff still returns bars, length, and weight.

$ {{ priceRateSuffix }}

Takeoff item	Estimate	Detail	Copy
Enter valid slab, cover, spacing, and bar inputs to show the grid takeoff.
{{ row.item }}	{{ row.estimate }}	{{ row.detail }}

Order item	Quantity	Basis	Copy
Enter valid dimensions to show the steel order.
{{ row.item }}	{{ row.quantity }}	{{ row.basis }}

Check	Status	Detail	Copy
Enter valid rebar inputs to show placement review notes.
{{ row.check }}	{{ row.status }}	{{ row.detail }}

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Reinforcing a slab is partly a structural decision and partly a quantity problem. The structural decision sets the bar size, spacing, cover, laps, supports, and any special detailing. The quantity problem turns those design choices into a material takeoff that a supplier, estimator, or contractor can check before ordering steel.

Rectangular slab takeoffs can be deceptively easy to miscount. Bars are not counted from the outside form edge. They sit inside the concrete cover, so the usable grid rectangle is smaller than the slab footprint. A 50 mm cover removes 100 mm from each slab dimension, and a 2 in cover removes 4 in from each dimension. That smaller rectangle is where bar centerlines are counted.

Rebar takeoff terms and common estimating mistakes
Term	What it means in a slab takeoff	Mistake to avoid
Concrete cover	The distance from the concrete surface to the nearest reinforcing steel.	Counting the grid from the form edge instead of from the cover line.
Center-to-center spacing	The intended distance between neighboring bar centerlines in the same direction.	Assuming the target spacing stays exact after whole bars are counted.
Lengthwise bars	Bars that run along the slab length, counted across the usable width.	Counting them along the length because of the word lengthwise.
Stock length	The straight bar length sold or delivered by the supplier.	Buying only the calculated cut length without rounding to full bars.

Direction language causes many slab-grid errors. A lengthwise bar is a bar that runs along the long dimension, but the number of those runs depends on how many spaces fit across the width. A widthwise bar runs across the slab, but its count depends on the usable length. Once both directions are counted, the total lineal steel is the sum of every run in both directions.

Whole-bar rounding is the other important step. A spacing target such as 450 mm or 18 in is treated as a maximum planning distance between bar centerlines. Because the count must be a whole number, the final actual spacing usually becomes equal to or slightly tighter than the target. A purchase order then rounds the waste-adjusted cut length up again to full stock bars, which creates offcut buffer.

A takeoff can estimate material, weight, and cost basis, but it cannot approve the reinforcement design. Slab thickness, loads, subgrade, exposure, crack-control requirements, joints, bar grade, lap splice length, chairs, coatings, and local code rules still need the project drawings, specifications, and qualified review.

How to Use This Tool:

Work from the geometry to the order quantity. The most reliable path is to confirm the usable grid rectangle first, then check counts, stock-bar rounding, and review notes.

Choose Measurement system. Unit selectors for slab dimensions, cover, spacing, stock length, and displayed weight follow that choice, so review converted values after switching systems.
Enter Slab length, Slab width, and Concrete cover. The Usable grid rectangle row should match the rectangle where bar centerlines can sit after cover is removed from all four edges.
Set Lengthwise bar spacing and Widthwise bar spacing. Use the Lengthwise bars, Widthwise bars, and Actual spacing rows to catch direction swaps or unexpectedly tight grids.
Select Bar size from the #3 through #11 preset list and enter the supplier's Stock bar length. Steel Order uses the standard weight for the chosen size and rounds cut length up to full stock bars.
Adjust Waste allowance for cut loss, laps, offcuts, delivery variation, and field changes. The value is clamped from 0% to 40% and is applied before stock-bar rounding.
Open Advanced only when a material-only cost estimate is useful. Cost basis can apply the entered Material rate per stock bar, per primary length unit, or per primary weight unit.
If Check rebar inputs appears, fix the named value before reading the tables. Results are blocked when a required dimension, spacing, or stock length is zero, when cover consumes the slab, or when the spacing would produce more than 10,000 bar runs.
Compare Grid Takeoff, Steel Order, Placement Review, and Rebar Length Bars. A purchase count is ready for estimating only when the cover, actual spacing, waste, offcut, and design-check notes make sense for the project.

Interpreting Results:

Start with Total cut length and Stock bars to buy. Total cut length includes the waste allowance. Stock bars to buy is the rounded purchase quantity after full stock lengths are applied.

How to interpret rebar calculator outputs
Output	Useful reading	Verification cue
Usable grid rectangle	The bar-centerline rectangle after concrete cover is subtracted from length and width.	Confirm cover is the project cover to steel, not a rough edge gap.
Actual spacing	The recalculated spacing after whole bar runs are counted.	Actual spacing should not exceed the entered target, but it may be tighter.
Purchase weight	Rounded purchase length multiplied by the selected standard bar weight.	Check the bar size, grade, coating, and supplier data against the quote or drawing.
Stock rounding	The offcut buffer created by buying full straight bars.	High offcut may justify a different stock length or cut-plan review.
Material cost	Optional material-only cost from the selected rate basis.	It excludes bending, tying, chairs, delivery, taxes, minimums, labor, and waste beyond the entered allowance.

Common planning range is an estimating cue, not structural approval. It only means the entered target spacing falls in the calculator's light-to-medium slab planning band. The Design check row remains important because a material takeoff does not decide whether the reinforcement schedule is adequate.

A neat order quantity can still be wrong when units or direction labels are mixed. If the count jumps unexpectedly, verify the length unit, cover unit, spacing unit, and direction of each spacing value before changing the slab dimensions.

Technical Details:

A rectangular slab grid is counted from two inner spans, not from the outside footprint. Cover is subtracted twice from the outside length and twice from the outside width. If either inner span is zero or negative, there is no valid rectangle for the bar centerlines.

Target spacing is treated as a maximum planning distance. A ceiling operation counts enough spaces to keep the actual spacing from exceeding the target, then one more bar is added to place a run at both ends of the usable span. That count is deterministic, but it does not decide the structural spacing requirement.

Formula Core:

The formulas below use a common length unit after converting the entered units. Lengthwise bars run along the inner length and are counted across the inner width. Widthwise bars run along the inner width and are counted across the inner length.

\begin{array}{lcl} L_{inner} & = & L - 2 C \\ W_{inner} & = & W - 2 C \\ N_{lengthwise} & = & ⌈ \frac{W_{inner}}{S_{lengthwise}} ⌉ + 1 \\ N_{widthwise} & = & ⌈ \frac{L_{inner}}{S_{widthwise}} ⌉ + 1 \\ S_{actual lengthwise} & = & \frac{W_{inner}}{N_{lengthwise} - 1} \\ S_{actual widthwise} & = & \frac{L_{inner}}{N_{widthwise} - 1} \\ L_{cut} & = & (N_{lengthwise} L_{inner} + N_{widthwise} W_{inner}) (1 + \frac{R}{100}) \\ B_{stock} & = & ⌈ \frac{L_{cut}}{L_{stock}} ⌉ \end{array}

Rebar formula symbols and related outputs
Symbol	Meaning	Related output
L, W	Outside slab length and width after unit conversion.	Slab footprint
C	Concrete cover subtracted from both sides of each span.	Usable grid rectangle
S	Target spacing for the bar direction being counted.	Actual spacing
R	Waste allowance, limited to 0% through 40%.	Waste allowance, Total cut length
L_stock	Purchased straight stock bar length.	Stock bars to buy, Purchase length

With the default metric values, a 6 m by 3.6 m slab and 50 mm cover create a 5.9 m by 3.5 m usable grid. At 450 mm spacing both ways, lengthwise count is ceiling(3.5 / 0.45) + 1 = 9 runs, and widthwise count is ceiling(5.9 / 0.45) + 1 = 15 runs. The base lineal length is 9 x 5.9 m + 15 x 3.5 m = 105.6 m. With 10% waste, Total cut length becomes 116.16 m.

The stock order rounds the waste-adjusted cut length to full bars. With 6 m stock bars, 116.16 m rounds to 20 bars, giving 120 m of purchase length and 3.84 m of stock rounding buffer. Weight uses the selected standard bar weight per length, so changing from #4 to #5 changes purchase weight and cost but does not change the grid count.

Placement review thresholds for rebar estimates
Review check	Boundary rule	Status shown
Spacing band	Maximum target spacing <= 8 in, > 8 in and <= 18 in, or > 18 in.	Tight grid, Common planning range, or Wide grid.
Edge cover	Cover < 1.5 in, >= 1.5 in and <= 4 in, or > 4 in.	Review, Captured, or Large offset.
Waste allowance	Waste < 5%, >= 5% and <= 15%, or > 15%.	Low buffer, Typical buffer, or High buffer.
Stock rounding	Offcut buffer > 20% of purchase length.	High offcut; otherwise Order-ready.

Validation blocks impossible or misleading inputs before results are shown. Length, width, spacing, and stock length must be positive; cover cannot consume the usable rectangle; bar size must be one of the supported presets; and extremely dense grids are rejected when the run count would exceed 10,000.

Accuracy and Privacy Notes:

This is a material takeoff for a simple rectangular two-way grid. It is not a structural design check, code-compliance review, fabrication drawing, bending schedule, or lap-splice calculation.

Standard bar weights are estimating presets. Use supplier mill data when the quote, specification, grade, coating, or local stock differs.
Waste allowance is a planning buffer. It does not replace a cut plan, lap schedule, chaired placement plan, or field-measured adjustment.
Material cost is material-only and excludes labor, bending, tying wire, chairs, delivery, taxes, minimum charges, and design review.
Calculations run in the browser from entered values. No slab dimensions or cost values need to be sent to a server for the estimate.

Advanced Tips:

Use separate spacing values when the drawing calls for different bar spacing by direction. A square grid is only appropriate when both directions use the same target spacing.
Check Actual spacing before ordering. A count rounded upward can make the installed spacing tighter than planned, especially on short spans.
Review High offcut before accepting the order quantity. A different stock length may reduce purchase buffer without reducing the cut-length estimate.
Keep the waste allowance consistent when comparing bar sizes or supplier stock lengths. Changing waste and bar size at the same time makes cost and weight comparisons harder to explain.
Use the length chart to see whether the order is dominated by one bar direction, waste, or stock rounding. That helps target the next cut-plan review.

Worked Examples:

Metric patio slab

A 6 m by 3.6 m slab with 50 mm cover, 450 mm spacing both ways, #4 bar, 6 m stock length, and 10% waste produces 9 lengthwise runs and 15 widthwise runs. The total cut length is 116.16 m, rounded to 20 stock bars. The result is useful for a first material quote, but the bar size and spacing still need to match the drawing.

Wide spacing warning

A small slab entered with 24 in target spacing can show a low bar count and a Wide grid placement note. That does not mean the order is invalid, but it tells the estimator to verify crack-control, load, and drawing requirements before treating the steel quantity as ready.

Cover consumes the grid

If a 3 ft wide strip is entered with 24 in cover, Check rebar inputs reports that cover is too large for the slab dimensions. Reduce the cover to the project value or correct the slab width before reading any bar count.

FAQ:

Why is actual spacing tighter than the target spacing?

Bar counts are rounded up to whole runs, then spacing is recalculated over the usable span. That keeps actual spacing from exceeding the target, but it can make the final spacing tighter.

Does this decide the correct bar size for my slab?

No. Bar size is an input that should come from drawings, specifications, or qualified design review. The calculator estimates quantities after that choice is made.

Why are lengthwise bars counted across the width?

Lengthwise describes the direction the bars run. The number of parallel lengthwise runs depends on how many spaces fit across the usable width.

What does the waste allowance include?

It adds a percentage to the raw lineal length for cut loss, laps, offcuts, field changes, and ordering margin. It is not a substitute for a detailed cut list or splice schedule.

Why are the result tables empty?

Fix the Check rebar inputs message first. Common causes are zero dimensions, zero spacing, zero stock length, cover that leaves no usable grid, or spacing so tight that the run count exceeds the tool limit.

Glossary:

Concrete cover: The distance from the concrete surface to the nearest reinforcing bar.
Usable grid rectangle: The slab rectangle remaining after cover is subtracted from each edge.
Center-to-center spacing: The distance from one bar centerline to the next in the same direction.
Stock bar length: The full straight length purchased from the supplier before cutting.
Offcut buffer: The extra purchase length left after total cut length is rounded up to full stock bars.
Material takeoff: An estimating list of quantities, lengths, weights, and ordering assumptions.

References:

A615/A615M Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, ASTM International.
The importance of concrete cover, American Concrete Institute.
Bar Identification, Concrete Reinforcing Steel Institute.
Splicing Bars, Concrete Reinforcing Steel Institute.