Rebar Calculator

Span, spacing, splice — in that order

The worked slab: 20 ft minus 3 in of clearance each side leaves a 19.5 ft clear span — 234 in. At 12 in on-center that is floor(234 ÷ 12) + 1 = floor(19.5) + 1 = 20 bars each direction, every bar 19.5 ft long: 40 bars, 780 linear feet. At #4's ASTM nominal 0.668 lb/ft the grid weighs 521.04 lb (236.34 kg). Give the tool your stock length and lap requirement and it adds the splice overlap before the weight. Computed at build time by the same engine wired to the form.

The weight table is the one sourced figure on this page, and it is catalogued — with its source, its verification date, and its status beside every other coverage constant on the site — in the guide Coverage figures and where they come from. This page does not repeat that catalogue. It explains what the calculator does with the numbers you give it, stage by stage, which input moves which figure, how to read the result panel, and where the model stops.

What the calculator computes, stage by stage

Every estimator on this site follows the five-stage pipeline documented on the methodology page: geometry, coverage, waste, package rounding, user-priced cost. The rebar tool bends that pipeline to fit its material. Steel is bought by the piece and the foot, not by the bag, so the coverage stage is a count rather than a division; the waste stage is replaced by a splice allowance you specify; the only round-up is a stock-piece count; and the cost stage is absent. The chain as the engine runs it:

1. Geometry — the clear spans. Length and width go in as decimal feet and are multiplied by 12 to work in inches; the edge clearance is subtracted from each dimension twice, once per side. Both spans must come out positive — a clearance that consumes the slab is refused with an error rather than counted — and the slab length, width, and stock length are accepted up to 1,000,000 ft. For the worked slab both spans are 234 in, or 19.5 ft.
2. Coverage — the count. Bars across a span are floor(span ÷ spacing) + 1: one bar at the start of the span plus one per full spacing interval. The engine's own function gives 20 for the worked 234 in span at 12 in; 21 for a 240 in span, where the intervals divide exactly and the end bar lands on the last boundary; and 1 for a 6 in span shorter than the spacing, because every span gets its first bar. Direction matters: bars that run the length are spaced across the width, so the bars-along-length figure is the count across the clear width, and each of those bars is the clear length long. The grid footage is the sum of both directions — count × bar length for each.
3. Splices — in place of waste. This stage runs only when you give a stock bar length. Each bar longer than a piece of stock needs ceil(bar ÷ stock) pieces and one splice per joint, and every splice adds the overlap you typed, converted from inches to feet. Total linear feet is the grid footage plus the overlap. No waste percentage is applied anywhere on this tool; there is no field for one.
4. Package rounding — stock pieces. The one round-up on the page is ceil(total linear feet ÷ stock length): the footage to buy, as whole pieces of your stock. It is a pooled figure, and the examples below show what that means.
5. Weight — the sourced multiplier. Choosing a bar size multiplies the total linear feet, overlap included, by that size's nominal pounds per foot from the ASTM A615 table in the site's materials data. Size enters nowhere else: it never changes a count. In metric mode the feet are multiplied by 0.3048 to report meters of bar and the pounds by 0.45359237 to report kilograms — both exact definitions.
6. User-priced cost — none. This form has no price field, and the site holds no steel price. Multiply the stock-piece count or the weight by whatever your supplier quotes you, depending on how they sell.

Why each input matters

Slab length and width set the two clear spans and go in as decimal feet; this form has no separate inches box, so a tape reading in feet and inches is converted first — the feet & inches calculator does that for any reading. The engine models one rectangle. An L-shaped slab is two rectangles: run the tool once for each and add the results, treating each as its own grid. The width-of-one trick that works on the area-based tools does not apply here, because a grid depends on both dimensions separately, not on their product.

Spacing comes from your plan. Both counts move inversely with it and in whole-bar steps, so a change that does not push span ÷ spacing across a whole number changes nothing, and the inch that does tips a bar on or off. The engine has one spacing field and applies it in both directions; a plan with different spacing each way needs two runs — take the bars-along-length figure from the run at the first spacing and the bars-along-width figure from the run at the second. The 12, 16, and 18 in figures on the field are placeholders, not suggestions.

Edge clearance is small but it touches every bar: each span shortens by twice the clearance, so every bar is shorter and, at a boundary, the count drops by one. The worked slab at 3 in has 20 bars each way on 19.5 ft spans; set the field to 0 and the spans are 20 ft, an exact multiple of the spacing, so the count steps to 21 each way and the footage to 840 lf. The default of 3 in is a labeled convention held by the form, not a sourced figure; the field accepts any value, including zero. Your plan's edge setback wins.

Bar size affects the weight line and nothing else. Leave it on "No weight line" and the result is footage only. The table in the next section shows the worked grid at every size the menu offers.

Stock bar length and overlap per splice switch on the splice stage together: a stock length without an overlap is refused, because the engine will not invent a lap. Both are yours — the stock length is what your supplier cuts, and the lap is design-governed, varying with bar size, concrete strength, and code, as the FAQ below says. Longer stock means fewer splices and less overlap; the examples show the same grid from two stock lengths.

The units toggle changes what the fields expect and how the headline is presented, not what is computed. In metric mode meters are divided by 0.3048 and centimeters by 2.54 — both exact — before the engine sees them. The result leads with meters of bar and, with a size chosen, kilograms; the per-direction, grid, and splice lines stay in feet, the engine's working unit.

The figures this page relies on

• Nominal weight per foot, #3–#8 — the ASTM A615/A615M nominal figures: properties of the standardized bar sizes, identical across producers and grades, and handled by the site the way a manufacturer figure is. Recorded in the site's materials data with their source and verification date beside them, and pinned by a data test. Not adjustable, because the sizes are standardized — pick the size, not the weight.
• Nominal diameter — a definition: size n is n eighths of an inch, so #4 is 0.5 in. Shown in the table for reference; the engine does not use it.
• Edge clearance default, 3 in — a labeled convention held by the form, not a sourced figure and not a code citation. Adjustable, including to zero.
• Spacing placeholders, 12/16/18 in — common plan callouts shown as placeholders only. The field takes any spacing; nothing is suggested.
• Inches per foot, 12 — a definition, exact; used to turn the feet you type into the inches the count works in and the lap inches back into feet.
• Meters per foot, 0.3048; centimeters per inch, 2.54; kilograms per pound, 0.45359237 — exact definitions, used only to convert metric entries into the engine's feet and inches and to present meters and kilograms back.
• Coverage divisor, waste default, and price — none. This page holds no product yield, applies no waste percentage, and carries no price.

Worked examples

Every figure in this table was produced at build time by the same engine that runs the form above. The inputs are chosen to show different behaviours of the chain — a rectangular slab, a zero clearance, a wider spacing, two stock lengths, a long driveway, and a metric entry converted exactly before the engine ran, which is what the metric toggle does. Stock lengths and laps are illustrative inputs, typed as a plan would supply them; none is a recommendation.

The rectangular slab. A 20 × 12 ft slab at 16 in on-center with 3 in of clearance has clear spans of 19.5 and 11.5 ft. The bars that run the length are spaced across the shorter clear width, so there are only 9 of them, each 19.5 ft; the bars that run the width are spaced across the longer clear length, so there are 15, each 11.5 ft. The two directions contribute 175.5 and 172.5 lf for a grid of 348 lf — 232.46 lb as #4. Swap the length and width in the form and the two lines trade places while the totals stay the same, which is a quick check that you have read the result's directions the right way round.

The zero-clearance grid. The worked slab with the clearance field set to 0 has 20 ft spans, which the 12 in spacing divides exactly, so each direction gets 21 bars rather than 20: 42 bars and 840 lf against 40 and 780. The extra bar each way is the "+1" doing its job at an exact boundary, and the longer spans add to every bar besides. It also shows plainly what the clearance field is for: the difference between the two runs is entirely the 3 in setback on four sides.

The shed pad. A 12 × 10 ft pad at 18 in on-center — the widest of the placeholder spacings — has 11.5 and 9.5 ft spans, neither of which divides evenly by the spacing. The counts are 7 bars across the width and 8 across the length, 15 in all, for 156.5 lf. Weighed as #3 at 0.376 lb/ft that is 58.84 lb (26.69 kg). The leftover fraction of an interval — the part of each span that is not a whole number of spacings — is normal: the count is whole intervals plus the end bar, and where that odd fraction falls on the slab is a layout choice the tool does not make.

The worked slab from two stock lengths. With 10 ft stock and a 12 in lap typed from the plan, each 19.5 ft bar needs 2 pieces and 1 splice; 40 bars give 40 splices, each adding 1 ft, so the overlap is 40 lf and the total 820 lf — 82 pieces of stock and 547.76 lb as #4. From 20 ft stock every bar fits in one piece: 0 splices, 780 lf, 39 pieces, 521.04 lb. The weight difference between the two runs, 26.72 lb, is the overlap steel, and the stock length is the input that decides whether it exists at all.

The driveway. A 40 × 24 ft slab at 12 in on-center has 39.5 and 23.5 ft spans: 24 bars run the length and 40 run the width, 1,888 lf of grid. Both bar lengths exceed the 20 ft stock, so every one of the 64 bars carries a splice — 64 splices at the 18 in lap typed from the plan add 96 lf, for 1,984 lf in all, 100 pieces of stock, and 2,069.31 lb as #5 (938.62 kg). On a grid this size the overlap is a visible share of the total, which is why the lap figure belongs to the plan and not to this page.

The metric grid. In metric mode, 8 × 5 m at 30 cm on-center with 7.5 cm of clearance reaches the engine as 26.2467 × 16.4042 ft at 11.811 in with 2.9528 in of clearance. The spans are 25.75 and 15.91 ft — 7.85 and 4.85 m — and the counts 17 and 27, for 867.45 lf of grid. From 6 m stock with a 30 cm lap, only the longer bars need a splice: 17 splices add 16.73 lf, the total is 884.19 lf, and the headline reads 269.5 m of bar with 45 stock pieces and 267.91 kg (590.64 lb) as #4. The count is the same one the imperial path produces from the converted dimensions; only the entry and the headline changed.

How to read the result

The headline is the footage: N linear feet of bar, or in metric mode N m of bar with the feet in brackets. It already includes any splice overlap, so it is the figure to buy against. Beneath it, Bars along the length and Bars along the width each give a count and a bar length — read them as "this many bars, each this long", and remember that the bars along the length are the ones spaced across the width. Grid total repeats the bar count and the footage before overlap, so a dimension typo shows before it is padded. With a stock length entered, a lap splices line shows the number of splices and the overlap footage they add, and Stock bars to buy gives the pooled piece count. With a size chosen, the Weight line names the size and its pounds per foot and gives the weight in pounds and kilograms, metric first in metric mode. The copy button places a one-line plain-text summary — the bar count, the split by direction, and the total footage — on the clipboard.

Common misreadings

Spacing in feet. Spacing, clearance, and lap go in as inches; the slab and the stock length as feet. Type the worked slab's 12 in spacing as 1 — one foot, in the wrong unit — and the engine spaces a bar every inch: 235 bars each way, 470 in all, and 9,165 lf instead of 780 — 11.75 times the footage. It is the rebar form of the inches-versus-feet thickness error the concrete slab calculator warns about; a bar count in the hundreds for a small slab means the spacing field.

Stock length in inches. The slip in the other direction is quieter. Enter 20 ft stock as 240 and the engine sees 240 ft pieces: 0 splices and Stock bars to buy of 4 pieces for 40 bars — a piece count far below the bar count is the tell.

Lap in feet. Type a 12 in lap as 1 and each of the 40 splices on the 10 ft-stock grid adds 1 in instead of 12: the overlap falls from 40 lf to 3.33 lf and the total from 820 to 783.33 lf, with no warning, because a fraction of an inch is a valid lap entry. An overlap line that is tiny against the splice count means the lap field.

Metric figures in the imperial fields. The metric grid above is 8 × 5 m at 30 cm. Type those same numbers with the toggle still on Imperial and the engine sees an 8 × 5 ft slab at 30 in on-center with 7.5 in of clearance: 5 bars and 29.75 lf against the correct 44 bars and 884.19 lf. The error is not a simple factor, because the slab shrank and the spacing grew; a result implausibly small for the slab means the toggle.

Reading the directions backwards. Bars along the length are the bars that run the length — spaced across the width, so their count follows the width and their length follows the length. On a square slab the two lines match and the question never arises; on the rectangular example the counts are 9 and 15, and the longer bars are the fewer. Swapping the length and width fields swaps the lines, not the totals.

Treating the stock-piece count as a cutting list. It is the total footage divided by the stock length, rounded up once. On the spliced worked slab that is 82 pieces for 40 bars that each need more than one piece; on the metric grid it is 45 pieces for 44 bars, most of which fit in one piece with an offcut. The figure assumes offcuts are used elsewhere in the grid. A plan that cannot reuse them needs a per-bar count from the bar lengths on the result.

Where the model stops

The engine counts one flat rectangular mat of straight bars at a single spacing in both directions, each bar running its full clear span, and assumes that layout is what gets built. It does not count a second mat, dowels into an adjoining slab or footing, corner or diagonal bars, hooks or bends, supports, or tie wire; it does not deduct openings; and it does not model a footprint that is anything but a rectangle. The splice count is figured from each bar's clear length and the stock length alone — it does not re-check whether the lap itself pushes a bar into one more piece, and it places no splices — and the stock-piece figure is pooled footage rather than a cutting list. No waste percentage is applied and no price is held. Those are the model's simplifications; the methodology page lists the ones every estimator shares, and reinforcement layout is among them.

The boundary matters more than the simplifications. This tool estimates a material quantity only: whether this slab needs steel, of what size, at what spacing, with what lap and cover is design — from your plans, your engineer, or your code official — and nothing on this page determines structural adequacy, code compliance, load-bearing requirements, or engineering suitability. The dimensions and spacing you enter should already carry that review; the tool arithmetic-checks a layout that exists, as the note under the form says.

Related tools and guides

The slab this grid sits in is the concrete slab calculator's job — volume, waste, and quarter-yard ready-mix ordering. Bagged-mix comparisons live in the concrete bag calculator, footings under the slab edge in the concrete footing calculator, and piers in the concrete column calculator. The feet & inches calculator turns tape readings into the decimal feet this form takes, and the construction unit converter moves any figure between feet and meters or pounds and kilograms. For where the weight table comes from and what kind of fact it is, read coverage figures and where they come from; for the shared pipeline this tool bends, how material estimates are calculated; for the unit toggle, metric and imperial in the estimators; or browse all guides.

Frequently asked questions

Dimensions never leave your browser. Sources and the estimating pipeline are documented on the methodology page.