Rebar Calculator

How to use this calculator

1. Measure the slab and enter length and width. The calculator trims the side cover off both ends of every bar, so the bar lengths it returns are what you actually cut.
2. Pick a bar size and spacing. #4 at 18 inches on center is the common residential default. The section below shows what each combination gives you against the ACI benchmark.
3. Add the slab thickness if you want the steel check. Without it the calculator still counts bars, it just cannot tell you whether the grid is enough.
4. Set your stock length. This matters more than people expect. It decides how many pieces come out of a stick and whether you need lap splices at all.
5. Read sticks to buy rather than linear feet, because you pay for whole bars.

Once you have the steel, the slab calculator gives the concrete, and rebar, mesh and vapor barrier covers choosing between reinforcement types rather than counting one.

Rebar sizes and weight per foot

US bar is sized in eighths of an inch, so #4 is four eighths, half an inch. The weights below are the nominal figures the industry prices and ships against. They agree digit for digit across Caltrans Bridge Design Details Table 13.1.1, Illinois DOT standard 001001-02 and the WisDOT Bridge Manual, which is a good sign when a number is this widely copied online.

The bar marking column is the soft-metric designation rolled into the bar itself, which is the diameter in millimeters rounded off. US mills mark metric while ACI 318, the IRC and every DOT specify in inch-pound, so both numbers are live and the bar in your hand may be stamped with the one you were not expecting.

What you can actually buy matters as much as the table. Big box stores carry #3, #4 and #5. Contractor yards add #6. #7 and up is special order at retail, and a steel supplier will have the full range. If your drawing calls for #7 bar, price it before you plan around it.

How much rebar a slab actually needs

This is the part almost every rebar calculator leaves out, and it is the only part that tells you whether your grid is any good.

ACI 318 section 24.4.3.2 sets a minimum for shrinkage and temperature steel as a ratio of the gross concrete area: 0.0018 for Grade 60 deformed bar or welded wire, 0.0020 for Grade 40 or 50, with an absolute floor of 0.0014. For a slab, gross area per foot of width is the thickness times 12, so a 4 inch slab needs 0.0018 × 4 × 12 = 0.0864 square inches of steel per foot.

Work that against the bar areas above and the common spacings stop being folklore:

So the two combinations you see everywhere, #3 at 15 inches and #4 at 18 inches, are not arbitrary. They are the cheapest grids that clear the benchmark for a 4 inch slab. And #3 at 18 inches, which also gets recommended a lot, does not.

There is a second limit, and on a thin slab it is usually the one that bites. ACI caps shrinkage and temperature steel spacing at the lesser of five times the slab thickness and 18 inches. On a 4 inch slab that is 18 inches, on a 3 inch slab it is 15. Flexural steel in a one-way slab is tighter again, the lesser of three times the thickness and 18 inches. This is why #4 at 18 inches turns up so often: the 18 inch cap, not the area calculation, is what sets it.

The honest caveat, and it is a large one. ACI 318 section 13.2.4 only reaches a slab on ground that transmits structural loads to the ground. An ordinary house, garage or patio slab sits outside ACI 318 entirely, and the governing document is ACI 360R, which is a guide rather than a code. IRC section R506 requires no reinforcement in a residential slab on grade at all; it only says where steel goes if you use it. So 0.0018 is a defensible, conservative benchmark for how much steel a slab wants, not a requirement you are failing to meet.

Most residential slabs are reinforced by convention or because the designer called for it. That is worth knowing before you let anyone tell you a number is code. Slab thickness itself is a different question, covered in how thick a concrete slab should be.

Where the steel goes, and why the middle is wrong

You will be told to put the steel in the middle of the slab. That is the floor of what is acceptable, not the target, and the reason is about how slabs crack.

ACI 302.1R puts it plainly: temperature and shrinkage reinforcement in a slab on ground "should be positioned in the upper third of the slab thickness," and quoting the Wire Reinforcement Institute, "placed 2 in. below the slab surface or within the upper one-third of slab thickness, whichever is closer to the surface." The reason it gives is that cracks "originate at the surface of the slab and are wider at the surface, narrowing with depth." A crack is a V, widest where you can see it. Steel does its work where the crack is wide, so steel near the bottom of the slab is steel doing very little.

That is also the argument against light mesh, below.

Cover is the other constraint, and it comes from ACI 318 Table 20.5.1.3.1:

• 3 inches when concrete is cast against and permanently in contact with ground, which is a footing poured against earth.
• 2 inches for #6 and larger, 1.5 inches for #5 and smaller, when formed but exposed to weather or in contact with ground.
• 0.75 inches for slabs, joists and walls not exposed to weather, for #11 and smaller.

The IRC matches these exactly in section R403.1.3.5.3. For a slab on grade the practical upshot is that a 4 inch slab with steel 2 inches down and 2 inches of side cover satisfies both the position guidance and the cover requirement at once, which is why the calculator defaults to 2 inches of side cover.

Whatever you decide, the steel has to be held there. Chairs or supports at 2 to 4 ft centers, not hooked up with a rake as the concrete goes in.

Rebar or wire mesh

Welded wire reinforcement is specified by ASTM A1064, which superseded the old A82, A185, A496 and A497 designations, and the Wire Reinforcement Institute is explicit that those older references "are no longer considered appropriate specification references for new construction." The style is written as spacing then wire size, so 6x6-W1.4xW1.4 is wires 6 inches apart each way, and the W number is the cross-sectional area in hundredths of a square inch. W1.4 is 0.014 square inches.

That last fact is the one that settles the comparison. Steel per foot of width is the wire area times 12 divided by the spacing:

The light roll mesh sold for residential slabs, 6x6-W1.4, provides about a third of the steel the ACI benchmark asks for in a 4 inch slab. It is not a substitute for #3 at 15 inches or #4 at 18 inches. It is roughly a quarter of the steel in the latter.

Then there is the placement problem. Light mesh gets walked flat during the pour and ends up near the bottom, and the industry's own authority on slabs is blunt about what that means. Wayne W. Walker, chair of ACI Committee 360, the committee that writes the slab-on-ground guide, has written with Jerry Holland in Concrete Construction that slab curling "produces a significant tension stress in the top" and that it is best to "design the bottom of the slab as unreinforced." The chair of the relevant ACI committee saying to treat bottom steel as if it is not there is about as clear as guidance gets.

The fix, per the American Society of Concrete Contractors, is sheet mesh in W4.0 wire or heavier at 12 inch or wider spacing, so it is stiff enough to survive being stood on, supported on chairs like bar. Which condemns the cheap roll mesh twice over. And a caution worth keeping in proportion: ASCC also states that "neither wire reinforcement nor reinforcing bars prevent cracking." Reinforcement controls crack width and holds cracks tight after they form. Nothing in a slab stops them.

Lap splices, and why 40 bar diameters is not a code number

Any run longer than a stick of bar needs a splice, and the overlap is real steel you have to buy. This calculator adds it, which some do not.

What ACI 318 actually requires is a calculation. A Class A tension lap is one development length, a Class B lap is 1.3 development lengths, and Class B is required when the steel provided is less than twice what is required, or when more than half the bars are spliced at the same place, which on a slab grid is usually the case. Development length itself comes from an equation with factors for bar size, concrete strength, cover, spacing, epoxy coating and lightweight concrete. There is a 12 inch absolute minimum applied after the calculation, and #14 and #18 bars cannot be lap spliced in tension at all.

What you will read online is "40 bar diameters", sometimes 30. We went looking for that in ACI 318 and in the CRSI material and it is not in either. It appears only on contractor blogs and product pages. Our best guess at where it came from is coincidence: a #4 Class B splice in 4,000 psi concrete works out at 20 inches, which happens to be exactly 40 times half an inch.

So the calculator defaults to the greater of 12 inches and 40 bar diameters, and labels it in the result as a field allowance rather than a code value. If you have a splice length from a drawing or an engineer, type it into the lap field and use that instead. The only defensible simple rule is the greater of the calculated length and 12 inches.

Note also that the IRC only requires reinforcing in footings and stem walls in the higher seismic design categories, D0 through D2, and points at a table for the lap length rather than publishing a multiplier of its own.

Offcuts, stock length and why a waste percentage is the wrong tool

Every other rebar calculator ends with "add 10 percent for waste." We looked for an authoritative source for that figure across CRSI, ACI, WRI, RSMeans and DOT specifications and there is none. Every result was a contractor blog or an estimating service's marketing page. Nobody publishes a rebar waste allowance.

More to the point, a percentage is the wrong shape of answer, because cut waste is deterministic rather than statistical. You know your bar length and you know your stock length, so you know exactly what is left over:

• A 10 ft bar out of a 20 ft stick: two pieces, nothing dropped. Zero waste.
• A 12 ft bar out of a 20 ft stick: one piece, 8 ft dropped. That is 40 percent waste, four times the rule of thumb.
• A 6.5 ft bar out of a 20 ft stick: three pieces, 6 inches dropped. About 2 percent.

No single percentage covers that range, which is why this calculator computes the sticks directly and reports the drop rather than inflating a total. The contingency dropdown is there if you want a field allowance on top, and it is labeled as one.

Stock length is a lever most people do not know they have. Retail bar is 20 ft. Suppliers carry 30, 40 and 60 ft, and fabricators stock 60. Take a 40 by 30 ft slab with #4 at 18 inches: in 20 ft sticks it needs 114 sticks and 67 lap splices, and in 60 ft sticks it needs 34 sticks and no splices at all, saving about 112 linear feet of steel that would otherwise have been overlap. One caution the other way, though: if your bar length does not divide into the longer stick, the drop gets worse rather than better. The trick is matching stock length to bar length, not just buying long.

Worth knowing that 20 ft is a convenience length with a premium on it. When the mills raised rebar prices in June 2025 the increase was $60 a ton generally, plus another $40 a ton specifically on 20 ft bars.

What rebar costs

Steel moves with the mill market, so every figure here is a snapshot with a date on it rather than a price. Retrieved 29 September 2026, from a regional building supply yard, Grade 60 in 20 ft sticks:

Two patterns in that table are worth acting on. Price per pound falls as bar size rises, so #3 is the most expensive steel you can buy by weight; if a #4 grid at wider spacing meets your benchmark, it is usually cheaper as well as stronger. And a big box store runs roughly 1.6 times a regional yard for the same bar, which on the same date was $1.06 to $1.19 per pound against $0.61 to $0.76. On a slab of any size that difference is worth a phone call.

The cost panel prices whole sticks, because that is what you pay for, and shows the by-the-foot figure separately so you can see what the offcuts cost you. Both are editable. Do not trust the defaults for long: they were a snapshot on one day at one supplier.

One warning about figures you will find elsewhere. There are two numbers circulating that look authoritative and are not. A widely quoted "$405 per ton for #4 Grade 60" is a producer price index value presented as dollars, and it is off by roughly half against the mill price and a factor of four against retail. A "$462 per metric ton" figure comes from an international export benchmark, not the US domestic market. Neither is usable for a US job.

If you are pricing the whole pour rather than the steel, the concrete cost calculator handles concrete, delivery and labor, and what a yard of concrete costs covers the material side.

The rest of the pour

Steel is one of four things to get right on a slab and the other three are geometry and mix. Size the concrete with the slab calculator, check depth against slab thickness, and read what psi you need, because the strength you specify interacts with cover and exposure. If you are reinforcing a footing rather than a slab, the footing calculator sizes it, and remember that concrete cast against earth wants 3 inches of cover rather than 2.

For the weight of the finished slab including the steel, the concrete weight calculator handles the concrete and this page gives you the steel to add. If you are batching the concrete yourself, the mix calculator works out cement, sand and gravel. And how to pour a slab covers the sequence, including getting the steel up on chairs before anyone starts placing.

Sources

• Caltrans Bridge Design Details, Chapter 13, for the bar size, area and weight table, cross-checked against Illinois DOT and WisDOT.
• ACI 302.1R, Chapter 3, for steel position in the upper third of the slab and the reason cracks are widest at the surface.
• CRSI standards and the Manual of Standard Practice, for lap splice classes and reinforcing practice.
• ACI 318-19, for cover (Table 20.5.1.3.1), the shrinkage and temperature ratio (24.4.3.2), spacing limits and the scope limit at 13.2.4.
• Wire Reinforcement Institute, Manual of Standard Practice WWR-500, for the W-number convention and ASTM A1064 superseding the older specifications.
• Concrete Construction, for Wayne W. Walker and Jerry A. Holland on curling and designing the bottom of a slab as unreinforced.
• American Society of Concrete Contractors position statements, for mesh stiffness and support, and that reinforcement does not prevent cracking.

Bar weights and the ACI ratios are published figures. The price table is a dated snapshot at one supplier. The lap default is a field convention, and the page says so where it appears.