EstimatorKitFree rebar calculator
Every bar in the mat, cut to the cover
Give a slab, a footing or a wall its two dimensions, a bar size and the spacing on the drawing, and this returns the mat as a schedule: how many bars run each way, what length each one is cut to once the clear cover is taken off both ends, how many sticks those pieces come out of, where a bar has to be spliced and by how much, and what the whole lot weighs. Bar counts come from the clear span between the outer bar centers, so 18 inches on a 12 ft width returns nine bars at an actual 17 9/16. Copy gives a plain-text bar list; Download gives a CSV schedule.
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- #3 to #8
- Laps and splices counted
- CSV bar schedule
A two-way mat: bars each way, every one of them stopping a cover short of the form.
Feet unless the field says otherwise, and a written measure reads correctly: 20 in, 1' 8" and 0.6 m are the same footing width.
ACI 318-19 Table 20.5.1.3.1: formed concrete exposed to weather or in contact with ground — 1.5 in for #5 and smaller, 2 in for #6 and larger.
One length, every size, at a fraction of the shelf price per foot — and 20 ft of steel to get home.
The usual site practice: the mat only has to hold its geometry until the concrete arrives.
Price the steel with your own figures
A yard quotes by the pound or the hundredweight; a shelf tag is per stick, so divide it by the stick length to get the per-foot figure. Whichever you fill is the one used, and it is the steel you buy that is priced, not the steel that ends up in the pour.
#4 for this slab or pad
21 sticks · 281 lb
21 × 20 ft — 2.81 cwt, 0.14 tons
- Steel in the concrete
- 283 lf · 189 lb
- Steel you pay for
- 420 lf · 281 lb
- Left on the ground
- 137 lf (49%)
- Lap length
- 20"
| Mark | Cut | Buy | Working |
|---|---|---|---|
| A · Bars running the length | 9 × 15.8 ft | 9 × 20 ft | spaced across the width at 17 9/16" centers; one piece per stick, 4.25 ft off each |
| B · Bars running the width | 12 × 11.8 ft | 12 × 20 ft | spaced down the length at 17 1/8" centers; one piece per stick, 8.25 ft off each |
| Crossings | 108 | 9 × 12 |
|---|---|---|
| Ties | 73 | perimeter, then every other — the outside row always, because that is what stops the mat spreading |
| Tie wire | 1 roll | 73 ft at 12 in a tie; 16 gauge black annealed tie wire measures 0.0625 in across, so a foot weighs 0.0104 lb and a 3.5 lb coil runs about 336 ft |
| Chairs | 20 | at 4 ft centers — bar supports under a slab mat are commonly set at 3 to 4 ft each way |
49% of this order never makes it into the concrete — 137 lf, or 91.7 lb of the 281 lb you carry. That happens whenever the piece length is a little over half the stock length, which is most slabs between 10 and 19 ft. A fabricator prices cut-and-bent bar on the weight of the finished bar plus a shop charge and keeps the drop, so above roughly this much waste the schedule is worth phoning through instead of buying off the rack.
| Bar | Diameter | Area | lb/ft | 20 ft stick | Lap at 40d |
|---|---|---|---|---|---|
| #3 stamped 10 | 0.375 in | 0.11 in² | 0.376 | 7.5 lb | 15" |
| #4 stamped 13 | 0.5 in | 0.2 in² | 0.668 | 13.4 lb | 20" |
| #5 stamped 16 | 0.625 in | 0.31 in² | 1.043 | 20.9 lb | 25" |
| #6 stamped 19 | 0.75 in | 0.44 in² | 1.502 | 30 lb | 30" |
| #7 stamped 22 | 0.875 in | 0.6 in² | 2.044 | 40.9 lb | 35" |
| #8 stamped 25 | 1 in | 0.79 in² | 2.67 | 53.4 lb | 40" |
Cover, lap, spacing cap and bend diameter are transcribed here from ACI 318-19 as reference values to measure a layout against. Choosing the bar and the spacing is a design decision, and it stays with the drawing, with whichever code edition your jurisdiction has adopted, and with the engineer who stamped it.
How to work out rebar for a slab, footing or wall
Three inputs come from the drawing. Everything after that is cutting and buying.
Pick the element and give it two dimensions
A slab or pad gets a two-way mat, a strip footing gets continuous bars down the trench with cross bars and corner bars, a wall gets verticals and horizontals in one or two curtains. Then set the clear cover, which on this page means the edge distance — the gap between the concrete face and the end of a bar. That single number decides both how long every piece is cut and how many of them fit, which is why it sits above the spacing rather than below it.
Enter the bar size and spacing from the drawing, not from here
Which bar and how close together is a structural decision that belongs to the drawing, the adopted code and the engineer who stamped it. What this page does with them is arithmetic: it treats the spacing as the maximum it is written as, fits whole bars into the clear span between the outer bar centers, and hands back the spacing that actually results. Ask for 18 inches on a 12 ft width and you get nine bars at 17 9/16, because 18 does not divide the span and the drawing said not to exceed it.
Order from the stick count, never from the total footage
The schedule lists each bar mark as a cut length and a piece count, then the sticks those pieces come out of — one per stick where the piece is more than half the stock, three per stick where it is short, and spliced with laps where nothing reaches. The offcut line is the difference and it is often a quarter of what you carry home. Copy gives a plain-text list to read at the trade counter; Download gives the same schedule as CSV for a spreadsheet or a fabricator.
Technical specifications
| Elements | Slab or pad as a two-way mat, strip footing with continuous bars, cross bars and corner bars, wall stem in one or two curtains with a dowel lap on the verticals |
|---|---|
| Bar sizes | #3 to #8 — 0.375 to 1.000 in diameter, 0.376 to 2.670 lb/ft, marked 10 through 25 by the mill |
| Bar layout | Count fitted into the clear span between outer bar centers, so the entered spacing is a maximum and the resulting spacing comes back to the nearest sixteenth |
| Cover presets | 3 in cast against ground, 1 1/2 in formed and exposed (2 in at #6 and above), 3/4 in dry interior — ACI 318-19 Table 20.5.1.3.1, editable |
| Lap splices | 40 bar diameters by default, adjustable from 12 to 80, with the 12 in ACI floor applied and segments counted when a bar outruns the longest stock |
| Stock lengths | 2, 3, 4, 8, 10 and 20 ft off a home-center rack; 20 ft at a rebar yard; 20, 30, 40 and 60 ft to order |
| Ties and supports | Crossings counted, perimeter always tied, field at every, every other or every third; 12 in of wire a tie and 96 ft of 16 gauge to the pound |
| Output | A plain-text bar list on Copy and a nine-column CSV schedule on Download, both generated in the browser from the fields above them |
Frequently asked questions
How much rebar does a 20 by 20 ft slab take?
Thirty bars and 600 linear feet, if you build it as a #4 mat at 18 inches with 1 1/2 inches of edge cover: 15 bars each way, every one of them cut to 19 ft 9 in, which is 30 sticks of 20 ft and 401 pounds of steel on the truck. The actual spacing lands at 16 7/8 rather than 18, because 14 spaces have to share the 19 ft 8 1/2 in between the outer bar centers and the figure on a drawing is a ceiling rather than a target. It is also an unusually tidy slab to buy for — a 19 ft 9 in piece leaves 3 inches of a 20 ft stick, where the same mat in a 12 ft bay leaves 8 ft 3 in of every one.
Why does the stick count come out higher than the footage divided by 20?
Because a mat is pieces and not a run of steel. Every bar has to reach from cover to cover in one continuous length, so a bar 11 ft 9 in long consumes a whole 20 ft stick and leaves 8 ft 3 in on the ground — you cannot join that offcut to the next bar without a lap splice nobody detailed and nobody wants in the middle of a mat. On a 16 by 12 ft slab at 18 inches the steel in the concrete is 283 linear feet, which divides into 15 sticks and would leave you six short: the real order is 21. Every calculator that reports a total footage and stops has handed you a number that cannot be bought.
How far from the edge of the concrete does rebar go?
Three inches where the concrete is cast straight against the ground, an inch and a half where it is formed and then exposed to weather or earth, and three quarters of an inch in a dry interior slab or wall — those are the ACI 318-19 values for #5 and smaller, and the middle one rises to two inches at #6 and above. Cover is a durability number rather than a placement convenience, because it is the thickness of alkaline concrete keeping the steel passive. Note that the cover at the edge and the cover at the bottom are two different measurements doing two different jobs: the edge cover fixes how many bars fit across the width, while the bottom cover is set by the height of the chairs the mat sits on.
How much do two bars overlap where they splice?
Forty bar diameters is the figure most residential drawings carry, which is 15 inches on a #3, 20 on a #4 and 25 on a #5, with a 12 inch floor underneath it that ACI 318 puts on any tension lap splice. That 40 is a rule of thumb standing in for a calculation: ACI derives a development length from concrete strength, bar grade, coating, cover and bar spacing, and a Class B splice — the usual one — is 1.3 times it, so a lap can land anywhere from under 20 diameters to over 60. Epoxy-coated bar needs more of it again. Wherever splices are unavoidable, stagger them rather than lining them up, since a row of laps at one station is a plane the section is weakest on.
Why is my #4 bar stamped with a 13?
Because US mills have rolled the soft-metric designation into the bar since the late 1990s, and 13 is the millimeter size that #4 became: #3 is 10, #4 is 13, #5 is 16, #6 is 19 and #8 is 25. The bar is physically unchanged — still half an inch — and the inch-pound number is what every drawing and every supplier still says out loud. While you are reading the marks, the rest of them are the mill's own letter, then the size, then a letter for the specification (S for A615 carbon steel, W for A706 low-alloy), then the grade: a single longitudinal line or the number 60 for Grade 60. The inch number itself is worth knowing for what it is — eighths of an inch of diameter, so #4 is four eighths — and worth knowing where it stops, since #9, #10 and #11 are 1.128, 1.270 and 1.410 inches rather than the ninths and tenths the pattern implies. Those three match the cross-sectional area of the old 1 inch, 1 1/8 inch and 1 1/4 inch square bars they replaced.
Can I weld rebar instead of tying it?
Not on ordinary A615 bar, which is the steel on every rack in the country and is not written to be welded. A615 has no limit on carbon content, so its carbon equivalent is unknown and a weld on it can harden and crack in the heat-affected zone; a tack weld is enough to do it. The weldable grade is A706 low-alloy, which caps the chemistry for exactly this reason and is what AWS D1.4 procedures assume. Tie wire is not a poor substitute here either — it is not structural in the first place. Its whole job is to hold the mat in the shape the drawing shows until the concrete goes off, and after that the bond between deformed bar and concrete does everything.
What actually cuts and bends rebar on site?
An angle grinder with a metal cutoff wheel does #3 through #5 without complaint, and a hydraulic bar cutter or a portaband does it more quietly. Bolt cutters manage #3 and start to disappoint at #4. Bending is cold bending around a pin, and ACI 318 sets a minimum inside bend diameter of six bar diameters for #3 through #8, rising to eight for the larger sizes — a tighter radius than that starts cracking the outside of the bend. Do not heat a bar to make it bend and do not straighten and re-bend one at the same point. If the schedule has more than a handful of bends in it, a fabricator will cut and bend the lot for a shop charge and keep the drop, which on a mat with a lot of waste can cost less than buying full sticks.
About bar sizes, cover and why footage is the wrong unit
Start with the number on the bar, because it is not arbitrary and it is not metric. A bar number is its diameter in eighths of an inch, so a #4 is four eighths — half an inch — and a #5 is five eighths. The pattern is exact from #3 to #8 and then stops: #9, #10 and #11 measure 1.128, 1.270 and 1.410 inches because they were sized to match the area of the old one inch, one and an eighth, and one and a quarter square bars, not to continue the eighths. Weight follows straight from the diameter — a square inch of steel a foot long weighs 3.4 pounds, so a #4 at 0.196 square inches is 0.668 pounds a foot and a 20 ft stick of it is 13.4 pounds. Which bar and how close together is not a decision this page makes or should make. That comes off the drawing, out of the code your jurisdiction has adopted, and from the engineer who put a stamp on it; what happens here is the arithmetic that follows once those are fixed. If you have not got that far yet and want the mat sized alongside the concrete, the sheeting and the joints, the concrete slab calculator carries the grid as one line of a whole-slab take-off, and this page is the one it hands off to.
The reason a bar count is not width divided by spacing is that bars stop short of the concrete face at both ends, and the outermost bar sits half a diameter inside that. So the spacing has a shorter span to fill than the element is wide, and the number written on a drawing is a maximum rather than a target: whole bars go into that span, and the spacing that comes out is smaller and almost never round. From there the same edge distance sets the cut length of every piece, and that is where footage stops being a useful unit. A mat is pieces. A bar 11 ft 9 in long takes a whole 20 ft stick, because the 8 ft 3 in left over cannot be joined into the next bar without a lap splice in the middle of a slab that nobody drew. Divide total footage by 20 on a 16 by 12 ft mat and you order 15 sticks against a real requirement of 21. The inverse case is a footing, where the run genuinely is continuous, the offcut genuinely does start the next bar, and the splices are the thing to count instead — which is why the schedule reports segments and lap lengths there and pieces per stick on the slab. Once the steel is settled, the concrete calculator sizes the pour it goes into, and the crushed stone calculator handles the compacted layer underneath.
Cover is the part of this that is worth understanding rather than just complying with. Fresh concrete is strongly alkaline, and that alkalinity holds a passive oxide film on the steel which stops it corroding at all — cover is simply how much of that protection there is between the bar and the weather. Carbonation eats into it slowly from the surface and chlorides from road salt or seawater punch through it directly, and once the film goes, rust occupies somewhere between two and six times the volume of the steel it grew from, which is more than enough to split the cover off from the inside. That is why three inches is specified against earth and three quarters of an inch is enough indoors, and why a bar pushed down into the mud with a boot ends up worse than no bar at all: it has nothing under it, so it starts corroding from the day the slab is poured and spalls the concrete off itself from below. The same logic governs the buried work on either side of it — the drainage and steel behind a segmental retaining wall, the joints in a masonry wall that the mortar calculator batches, and the backfill the topsoil calculator turns into yards once the trench is closed.
Your bar schedule never leaves the phone
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — measurements, quantities, the prices your supplier quoted you — is uploaded, logged or kept, which is also why the calculators carry on working on a site with no signal.
The CSV the Download button hands you is assembled in the tab and saved straight to the device — it is not generated on a server, so nobody sees the schedule before you send it to whoever is cutting the steel.