Audited 27 Jul 2026·Last updated 28 Oct 2026·6 citations·Tier 1·0 uses

Rebar Calculator

Estimate rebar for a rectangular slab grid: bar lines each way, laps, total feet with waste, 20 ft stock bars to buy, weight by bar size (#3 to #8) and cost.

Rebar Calculator

Bar size
Leave at 0 to use the standard weight for the bar size (ASTM A615). Enter a value only if your supplier tag gives a different weight.
Total rebar length before waste (ft)
868
Total bar length for the whole grid in feet, laps included, before any waste allowance.
Lengthwise bar lines
14
Crosswise bar lines
21
Takeoff length per lengthwise line
32
Takeoff length per crosswise line
20
Total rebar length to order (ft)
911.4
Stock bars to buy
46
Estimated total weight (lb)
608.8152
Estimated material cost
683.55

Background.

Ordering rebar for a slab starts with a drawing that says something like "#4 at 18 in. o.c. each way" and ends with a number the supplier understands: so many 20-foot sticks of #4, weighing so many pounds. Getting from one to the other is a takeoff, and it is easy to get wrong in small ways that add up, such as forgetting the extra bar at the far edge, the overlap where two bars join, or the offcuts.

The takeoff here handles a single rectangular grid of bars running both ways. It counts the bar lines in each direction from the slab dimensions and the spacing, works out whether each line can be made from one stock bar or needs two or more joined with a lap, totals the length, adds a waste allowance, and turns the result into stock bars, pounds of steel and a material cost. For a 30 by 20 foot slab with #4 bars at 18 inches each way, that comes to 868 feet of bar before waste, 911.4 feet to order, 46 twenty-foot bars and about 609 pounds of steel.

Spacing is the input that moves the answer most. The same slab at 12-inch spacing takes 1,292 feet before waste, about half as much again, and at 24 inches only 672 feet. Bar size barely affects the length, but it changes the weight sharply: a #5 bar weighs about 56% more per foot than a #4.

What the takeoff will not do is decide whether the slab needs rebar, which size, or at what spacing. Those are design choices that belong to the drawings, the building code or an engineer, and so does the lap length. The count also assumes a plain rectangle with one layer of bars; openings, thickened edges, a second mat, dowels and bent bars all need adding separately.

What is rebar calculator?

A rebar takeoff is the list of reinforcing steel a job needs, measured as lengths of bar of each size and then converted into the stock bars a supplier sells. For a slab reinforced with a grid, the bars run in two directions at a set spacing, called on centre. Bar sizes in the United States are numbered by nominal diameter in eighths of an inch, so #4 is 4/8 or 1/2 inch, and #8 is 1 inch; the soft-metric names in ASTM A615M are #10 to #25 for the same bars. Where a line of reinforcement is longer than the stock bar, two bars overlap and are tied together, which is a lap splice; the overlap is extra steel that has to be counted. The weight per foot of each size is set by its nominal diameter and the density of steel, and ASTM A615 lists 0.668 lb/ft for #4.

How to use this calculator.

  1. Enter the slab length and width in feet. Lengthwise bars run along the length and are spaced across the width; crosswise bars do the reverse.
  2. Enter the bar spacing in inches as shown on the drawings, for example 18 for 18 in. o.c. The same spacing is used in both directions.
  3. Choose the bar size from #3 to #8. Its standard weight per foot is used unless you type a different figure into the override box, for example from a supplier's tag or for a metric bar; leave the override at 0 otherwise.
  4. Enter the stock bar length you will buy (20 ft is a common stock length) and the lap length from the drawings or engineer's details. A lap of 0 suits a slab small enough for single bars.
  5. Enter a waste allowance and the price per foot. Check your supplier's price for the size you chose, since changing the bar size does not change the price automatically.

The formula.

L = Nl×Tl + Nc×Tc
L · total bar length before waste, ftNl · number of lengthwise lines (across the width)Tl · steel per lengthwise line, laps included, ftNc · number of crosswise lines (along the length)Tc · steel per crosswise line, laps included, ft

The count of bar lines follows the fence-post rule: lines = floor(dimension in inches ÷ spacing) + 1. Lengthwise lines are counted across the width and crosswise lines across the length. The +1 puts a bar at the starting edge as well as at the end of each full space; when the dimension is not a whole number of spaces, the leftover gap is left at the far edge.

Each line then gets a takeoff length. A span no longer than the stock bar takes one piece. A longer span takes ceil((span − lap) ÷ (stock length − lap)) pieces, because each extra piece adds only the stock length less the lap, and the takeoff is span + (pieces − 1) × lap. A 30 ft span from 20 ft bars with a 2 ft lap needs ceil(28 ÷ 18) = 2 pieces and one lap; a 20 ft span from 20 ft bars needs one piece and no lap; a 39 ft span needs ceil(37 ÷ 18) = 3 pieces and two laps, 43 ft of steel. The total before waste is L = Nl × Tl + Nc × Tc, where N is the number of lines and T the takeoff per line in each direction. The length to order is L × (1 + waste ÷ 100). Stock bars to buy = ceil(order length ÷ stock length). Weight = order length × weight per foot, and cost = order length × price per foot.

Several simplifications are built in. The lap must be shorter than the stock bar when a line needs more than one piece, otherwise the calculator refuses the input. Bars are measured to the full slab dimension, with nothing taken off for concrete cover at the edges. The stock-bar count divides total length by bar length, which assumes offcuts are used somewhere else; a real cut list can need more bars. And everything is a single layer of a rectangular grid.

The standard weights come from the bar's nominal area and the density of steel, about 0.2836 lb per cubic inch. A #4 bar's nominal 1/2-inch diameter gives a cross-section of about 0.196 square inches, so one foot holds 0.196 × 12 ≈ 2.36 cubic inches of steel and weighs 0.668 lb. Weight rises with the square of the diameter, which is why the step from #4 to #5 adds 56% rather than 25%.

A worked example.

Example

Take the 30 by 20 foot slab with #4 bars at 18 inches each way, 20 ft stock bars, a 2 ft lap, 5% waste and a price of 0.75 per foot. Lengthwise lines are counted across the 20 ft width: 20 × 12 = 240 inches, 240 ÷ 18 = 13.33, so 13 full spaces and 14 lines. Crosswise lines are counted across the 30 ft length: 360 ÷ 18 = 20 spaces exactly, so 21 lines. Each lengthwise line spans 30 ft, more than one 20 ft bar, so it takes two pieces and one 2 ft lap: 30 + 2 = 32 ft. Each crosswise line spans 20 ft and fits one bar: 20 ft. The total before waste is 14 × 32 + 21 × 20 = 448 + 420 = 868 ft. Adding 5% gives 868 × 1.05 = 911.4 ft to order, and 911.4 ÷ 20 = 45.57, rounded up to 46 stock bars. At 0.668 lb/ft the steel weighs 911.4 × 0.668 = 608.8152 lb, about 276 kg, and at 0.75 per foot the material costs 683.55. A cut list tells a slightly different story. Each lengthwise line uses one full 20 ft bar and a 12 ft piece cut from a second bar, leaving an 8 ft offcut. Unless those fourteen 8 ft offcuts find another use, the job takes 28 bars lengthwise and 21 crosswise, 49 in all, so a 5% allowance is too thin for this layout.

spacing In18
stock Bar Length Ft20
slab Width Ft20
waste Percent5
lap Length Ft2
slab Length Ft30
price Per Ft0.75
bar Size#4
bar Weight Lb Per Ft0

Frequently asked questions.

How much does the spacing change the amount of rebar?
A great deal, because spacing sets the number of lines in both directions. For the 30 by 20 foot example, 24-inch spacing needs 672 ft before waste, 18-inch spacing 868 ft and 12-inch spacing 1,292 ft, about half as much again as 18 inches. Spacing is a design decision, so take it from the drawings rather than choosing it to save steel.
Why is there an extra bar line, and why is one gap smaller?
A grid needs a bar at both edges, so the number of lines is one more than the number of spaces, as with fence posts. When the dimension is not a whole number of spaces, the count stops at the last full space: across 240 inches at 18 inches, the 14th bar sits at 234 inches, leaving 6 inches at the far edge. Placers often re-space the bars evenly instead.
Does the takeoff allow for concrete cover at the slab edges?
No. Each bar is measured to the full slab dimension, so the length is slightly generous. In practice bars stop short of the edge form by the cover the drawings require. If they called for 3 inches at each end, every line in the example would be 6 inches shorter, 17.5 ft less over the whole grid, an over-measure of about 2% of the 868 ft.
How long should the lap splice be?
As long as the drawings or the engineer specify. Lap length depends on bar size, steel grade, concrete strength, spacing, cover and coating, and the Concrete Reinforcing Steel Institute treats it as a design value rather than a rule of thumb. Its effect on the takeoff is easy to see: doubling the lap in the example from 2 to 4 ft adds 28 ft, two extra feet on each lengthwise line.
Why does my cut list need more stock bars than the calculator shows?
Because the stock-bar figure divides total length by bar length, as if every offcut could be used. In the example, each 32 ft lengthwise line leaves an 8 ft offcut, and if those go in the skip the job needs 49 bars, not 46. Cutting by hand from a list, or asking the supplier for longer bars, shows the real count.
What if my slab is just under twice the stock bar length?
The calculator allows for the lap when it counts pieces: a line longer than one bar takes ceil((span − lap) ÷ (stock length − lap)) pieces. A 39 ft span from 20 ft bars with a 2 ft lap takes ceil(37 ÷ 18) = 3 pieces and two laps, 43 ft of steel, because two 20 ft bars overlapped by 2 ft reach only 38 ft. Whatever the count, the offcut assumption still applies, so check a long line against a cut list.
When should I type in my own bar weight?
When the supplier's tag or mill certificate gives a different weight, or when you are using metric bar. A 12 mm bar has a cross-section of about 113 square millimetres and weighs about 0.888 kg per metre, which is about 0.597 lb per foot; entering that figure, with the slab in feet, gives the weight correctly. Leave the box at 0 to use the standard weight for the size.
Does a bigger bar size change the length or only the weight?
Only the weight. The length and the number of stock bars depend on the slab, spacing, laps and waste, not on diameter. Changing the example from #4 to #5 raises the weight from 608.8 lb to 950.6 lb, but the cost stays at 683.55 until you enter the higher price per foot that #5 bar carries.
What does the takeoff leave out?
Everything that is not straight bar in the grid: chairs or bolsters that hold the mat at the right height in the slab, tie wire, dowels into existing concrete, hooks and bends, and extra bars around openings or at thickened edges. Bars laid directly on the ground end up at the bottom of the concrete, where they do little, so supports belong on the order.
Can I use it for a slab with two layers of reinforcement, or for a footing?
For two identical mats, top and bottom, double the length, bars, weight and cost; for different mats, run the calculator once for each. A footing or wall can sometimes be treated as a long, narrow grid, but footings often have bars in one direction only, plus hooks, stirrups and dowels, so take quantities from the bar schedule on the drawings.

References& sources.

  1. [1]Concrete Reinforcing Steel Institute (n.d.). "Rebar Properties." CRSI.
  2. [2]Concrete Reinforcing Steel Institute (n.d.). "Reinforced Concrete Terminology." CRSI.
  3. [3]Concrete Reinforcing Steel Institute (n.d.). "Bar Identification." CRSI.
  4. [4]Concrete Reinforcing Steel Institute (n.d.). "Lap Splices." CRSI.
  5. [5]Concrete Reinforcing Steel Institute (n.d.). "Bar Tags." CRSI.
  6. [6]Concrete Reinforcing Steel Institute (n.d.). "Reinforcing Bars: Frequently Asked Questions." CRSI.

How this page was produced

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Daniel, Quanta Calculator Researcher
Primary sources
6 cited below
Method
L = Nl×Tl + Nc×Tc
Published
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Built with AI assistance and verified by automated tests against the cited sources — every worked example on this page is computed by the same code that runs the calculator. How we build and check calculators.

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