ASTM A955 bar weighed by alloy family, the coating factor that keeps the lap at the black bar length, and the PREN a mill certificate will give you.
← Back to the calculatorA #4 is 0.500 in and 0.668 lb/ft in either material, which is why the drawing does not change when the alloy does. The word stainless does the selling and hides the arithmetic. ASTM A955 keeps the black bar's nominal sizes and prints two densities — 494.78 lb/ft³ for the 300 series and 489.59 lb/ft³ for duplex — so the weight per foot does change, by about a percent. What does not change is the splice: ACI 318-19 gives bare and zinc-coated bar a coating factor of 1.0, so a stainless #8 laps 50 in, exactly where a black #8 laps.
That makes the buying decision narrower than the pages that rank for this query suggest. The alloy is bought for the exposure, and it is priced per pound, so the only takeoff work is (1) which density family, (2) how many feet, and (3) whether the certificate's chemistry passes the PREN you asked for. All three are on this page.
ASTM A955 produces stainless reinforcement in the same imperial sizes as conventional black bar — #3, #4, #5 and up, with soft metric labels such as 10M and 13M that are rounded for convenience rather than converted. The nominal dimensions are identical, so a #4 is 0.500 in whichever alloy it is rolled from.
The weight is not identical, and the reason is in a footnote rather than in the table. A955 Table 1 computes nominal weight on two densities: 489.59 lb/ft³ (7,833.4 kg/m³) for the duplex alloys and 494.78 lb/ft³ (7,916.5 kg/m³) for the 300 series. The black bar table this site uses is built on 490 lb/ft³. Divide, and the 300 series is 1.0098 times the black weight while duplex is 0.9992 — about a percent one way, and level the other.
| Bar | Black, lb/ft | 300 series | Duplex | 100 ft, 300 | 100 ft, duplex |
|---|---|---|---|---|---|
| #3 | 0.376 | 0.380 | 0.376 | 38.0 lb | 37.6 lb |
| #4 | 0.668 | 0.675 | 0.667 | 67.5 lb | 66.7 lb |
| #5 | 1.043 | 1.053 | 1.042 | 105.3 lb | 104.2 lb |
| #6 | 1.502 | 1.517 | 1.501 | 151.7 lb | 150.1 lb |
| #7 | 2.044 | 2.064 | 2.042 | 206.4 lb | 204.2 lb |
| #8 | 2.670 | 2.696 | 2.668 | 269.6 lb | 266.8 lb |
| #9 | 3.400 | 3.433 | 3.397 | 343.3 lb | 339.7 lb |
| #10 | 4.303 | 4.345 | 4.299 | 434.5 lb | 429.9 lb |
| #11 | 5.313 | 5.365 | 5.309 | 536.5 lb | 530.9 lb |
Put that on an order and it stops being a rounding error at the scale of a project: 1,000 ft of #8 is 2,670 lb in black, 2,696 lb in 300 series and 2,668 lb in duplex. A 10-ton order in 300 series weighs about 195 lb more than the same bar count in black. Nothing dramatic, but it is a number the freight and the invoice both use, and it is not on any of the ten pages that rank for this query — lb/ft appears zero times across them, measured 2026-10-28.
Why the two densities, and not one. The duplex family is a ferritic-austenitic mix with a different alloy balance, and A955 chose to publish a nominal weight for each family rather than one figure for both. It also warns that density varies with alloy content, so individual heats can sit a few percent away from the nominal. The purchase weight is the nominal one; the mill certificate carries the chemistry that explains the difference.
This is the number that decides whether the alloy premium shows up in the bar list, and the answer is that it does not. ACI 318-19 Table 25.4.2.5 gives a coating factor ψe of 1.0 to uncoated bar and to zinc-coated bar, and 1.2 or 1.5 to epoxy-coated bar. Stainless reinforcement is bare metal with no film on it, so it takes 1.0 and the splice is the black bar splice.
| Bar | ld (in) | Class A lap | Class B lap | Class B, ft-in |
|---|---|---|---|---|
| #3 | 12 | 12 | 16 | 1 ft 4 in |
| #4 | 16 | 16 | 21 | 1 ft 9 in |
| #5 | 19 | 19 | 25 | 2 ft 1 in |
| #6 | 23 | 23 | 30 | 2 ft 6 in |
| #7 | 34 | 34 | 45 | 3 ft 9 in |
| #8 | 38 | 38 | 50 | 4 ft 2 in |
| #9 | 43 | 43 | 56 | 4 ft 8 in |
| #10 | 49 | 49 | 64 | 5 ft 4 in |
| #11 | 54 | 54 | 71 | 5 ft 11 in |
The comparison that makes the point is against the coated bars. On a #8 at 4,000 psi the black or stainless Class B lap is 50 in, an epoxy-coated bar with tight cover and spacing takes it to 60 in, and with loose cover to 75 in. A galvanized bar stays at 50 in for the same reason stainless does. So the three corrosion answers sort themselves by what they add to the bar list: zinc and stainless add nothing, epoxy adds 10 to 25 in per splice on a #8.
That is worth putting in the terms a buyer uses. Stainless costs more per pound, and on the bar list it costs only more per pound — no extra inches, no extra laps, no change to the cut list. The pages that sell it tend to bury this, and one of them states the opposite of the table: a mill's own stainless rebar page says a “customary overlap of about 4 feet of bar at both ends”. Four feet is 48 in, which sits between the #7 lap (45 in) and the #8 lap (50 in), and on a #4 it is 27 in more than the 21 in the table asks for.
The check to run before you cut. Take the drawing's bar size, look up the Class B lap above, and compare it with what the detail shows. If the detail says 40 in on a #8, the drawing is short by 10 in and the drawing still governs — this page shows where the number comes from, it does not replace the splice schedule. The full derivation, including ψt and the 1.7 cap, is on development length, and the offset rule is on splice length.
PREN is the one number in this family that a buyer can compute rather than look up, and it is the reason to ask for the chemistry page of the mill certificate instead of the cover sheet. The formula is a weighted sum of three elements:
PREN = Cr + 3.3 × Mo + 16 × N (each element as a weight percentage)
| Alloy | Cr % | Mo % | N % | PREN | Where it is used |
|---|---|---|---|---|---|
| 304 / 304L | 18.5 | 0 | 0.05 | 19.3 | Mild exposure, architectural |
| 316L | 17.0 | 2.5 | 0.05 | 26.1 | Deicing salt, coastal |
| 2304 (S32304) | 23.8 | 0.5 | 0.18 | 28.3 | Duplex, bridge decks |
| 2205 (S32205) | 22.5 | 3.25 | 0.17 | 35.9 | Marine splash, high chloride |
Work one row by hand to see how little there is to it. 2205 at 22.5 Cr, 3.25 Mo and 0.17 N is 22.5 + 10.73 + 2.72 = 35.9. The same arithmetic on 316L at 17.0 Cr, 2.5 Mo and 0.05 N gives 17.0 + 8.25 + 0.8 = 26.1. The gap between the two alloys that actually compete for a marine deck is therefore 9.8 points, and molybdenum is most of it: 3.25% against 2.5% is worth 2.5 PREN points on its own.
The one published comparison in the top ten says 2205's pitting resistance equivalent number is “approximately 50 percent higher than that of the other three alloys”. Divide and the claim depends on the divisor: 35.9 against 26.1 is 1.38, while 35.9 against the average of the other three (19.3, 26.1 and 28.3, so 24.6) is 1.46. Read it as an average, not as a comparison with 316L.
What PREN is not. It is a composition index, not a test result, and it carries no temperature and no crevice term. The test behind a pitting claim is ASTM G48, which measures a critical pitting temperature in ferric chloride on a polished specimen, and a real structure adds concrete pH above 12, a crack, a joint and a cover dimension to that. So PREN ranks alloys for a specification; it does not qualify an exposure. If the certificate reads 34 on a bar specified at 35, the question is a nonconformance, not a corrosion calculation.
The industry body that promotes the material lists its own conditions, and they are narrower than the marketing: stainless reinforcement is chosen for structures with a long design life, for parts that are inaccessible for future maintenance, for chloride contamination with no relaxation in durability design — cover, concrete quality or waterproofing — and for reinforcement bridging joints or penetrating the concrete surface while also seeing chlorides, which is the dowel and the holding-down bolt rather than the slab mat. Read that list and the pattern is that stainless is bought where the chloride cannot be kept out and the bar cannot be reached.
What it does not buy is just as specific, and it is where most of the disappointment lives:
There is one detailing consequence that none of the ten pages raises, and it is a direction rather than a rule. In a chloride environment stainless is the noble member of a stainless-to-black couple, so the black bar becomes the anode and the attack concentrates there, scaled by how much stainless area it is connected to. A small stainless patch inside a large black mat is the wrong way round; a large stainless area against a small black one is the tolerable direction. No clause in ACI 318 covers it, so it is a decision for the engineer of record, and the clean answer is one material through a pour.
Put the two side by side on one bar and the comparison does not rank them. On a #8, 1,000 ft weighs 2,727 lb galvanized, 2,696 lb in 300-series stainless and 2,670 lb in black bar, and the Class B splice is 50 in either way — identical, because ACI 318-19 Table 25.4.2.5 gives zinc-coated and uncoated bar the same coating factor, 1.0. The difference that decides a job is not in the splice schedule at all: the zinc is 150 µm of metal with a fixed budget, and the stainless bar is the metal, with no layer to spend.
zinc lb/ft = oz/ft² × (π × d ÷ 12) ÷ 16
The zinc is sold by area and the alloy by volume, and that is the whole reason the two premiums do not scale the same way. A767 Class I asks for 3.50 oz/ft² at #4 and larger and 3.00 oz/ft² at #3, so the zinc per foot grows with the perimeter, which is proportional to the diameter. Steel weighs in proportion to the area of the section, which is proportional to the diameter squared. Divide one by the other and the zinc premium falls as 1/d. The alloy premium does not move at all, because 494.78 against 490 lb/ft³ is a ratio between two densities rather than a surface effect.
| Bar | Black, lb/ft | Zinc, lb/ft | Galvanized, lb/ft | vs black | Stainless 300, lb/ft | vs black |
|---|---|---|---|---|---|---|
| #3 | 0.376 | 0.0184 | 0.394 | +4.9% | 0.380 | +1.1% |
| #4 | 0.668 | 0.0286 | 0.697 | +4.3% | 0.675 | +1.0% |
| #5 | 1.043 | 0.0358 | 1.079 | +3.4% | 1.053 | +1.0% |
| #6 | 1.502 | 0.0430 | 1.545 | +2.9% | 1.517 | +1.0% |
| #7 | 2.044 | 0.0501 | 2.094 | +2.5% | 2.064 | +1.0% |
| #8 | 2.670 | 0.0573 | 2.727 | +2.1% | 2.696 | +1.0% |
| #9 | 3.400 | 0.0646 | 3.465 | +1.9% | 3.433 | +1.0% |
| #10 | 4.303 | 0.0727 | 4.376 | +1.7% | 4.345 | +1.0% |
| #11 | 5.313 | 0.0807 | 5.394 | +1.5% | 5.365 | +1.0% |
Swipe the table sideways on a phone. Zinc mass = A767 Class I oz/ft² × πd/12 ÷ 16; stainless from the A955 densities in section 01.
Read the last two columns together and the bar size changes the answer to the first question a buyer asks. On a #3 the zinc is a 4.9% weight penalty against 1.1% for the alloy, so galvanized carries 4.6 times the weight addition. On a #11 the same coating is 1.5% against the same 1.0%, and the gap has closed to 1.6 times. Neither penalty is large. What matters is that one of them moves by a factor of three across the size range and the other does not, so a rule of thumb learned on #4 bar is wrong on #11.
Hot-dip galvanizing puts a zinc layer on a carbon steel bar. Zinc is anodic to steel, so the layer works two ways at once: while it is intact it keeps chloride and oxygen off the steel, and where it is broken the zinc beside the break corrodes in preference to the steel, which is why a scratch does not turn into a rust site the same day. The layer is a fixed quantity of metal, so what it can do is bounded by how much is there and how fast the exposure spends it. Once the zinc is gone at a point, the steel under it behaves like black bar.
150 µm is the whole of that budget on a Class I bar and 85 µm on a Class II bar, so the division is worth doing once. It converts a coating class into a number of years, and it also shows why the number is an argument rather than a specification:
| Zinc spent per year | Class I, 150 µm | Class II, 85 µm | Stainless |
|---|---|---|---|
| 1.0 µm/yr | 150 yr | 85 yr | no coating mass to divide |
| 1.5 µm/yr | 100 yr | 57 yr | |
| 2.0 µm/yr | 75 yr | 43 yr | |
| 3.0 µm/yr | 50 yr | 28 yr | |
| 5.0 µm/yr | 30 yr | 17 yr |
Swipe the table sideways on a phone. Each cell is the coating thickness divided by the rate in that row, nothing else.
The rate is the one number this page does not publish. How fast zinc is spent in concrete depends on chloride, moisture, pH and how much bare steel the layer is protecting, and no rate is quoted here because none was verified for this page. The table is a division, not a prediction: put the exposure rate you are willing to defend into the left column and the years follow.
Stainless has no equivalent table, and that is the point rather than an omission. There is no coating mass to divide because there is no coating: the chromium oxide film that does the work is nanometres thick and rebuilds itself where oxygen and pH allow, so a cut end or a handling scratch is not a place where a budget has been spent. What it will not do is work without oxygen, or above the chloride level the alloy was chosen for, and when it does fail it pits at a point instead of thinning evenly along the bar. So the comparison is not which one lasts longer. It is that one has a consumption rate you can argue about, and the other has a chloride threshold you have to specify.
The two specifications are checked by two different numbers, and that is the practical difference on a purchase order. A767 is a coating specification, so its certificate carries a coating mass in oz/ft² or g/m²: 3.50 oz/ft² is 1,070 g/m², and since 1 oz/ft² of zinc is 42.7 µm the thickness behind the class is 150 µm. A955 is a bar specification, so its certificate carries chemistry — chromium, molybdenum and nitrogen, which is what the PREN above is computed from. Ask a galvanizer for Cr and Mo and there is nothing to give; ask a stainless mill for a coating mass and the question does not apply.
The clause asymmetry runs the same way. A767 says flaking and cracking that come from fabrication after galvanizing are not grounds for rejection; A955 has no coating clause at all, because there is no coating to flake. And the repair budget that gets quoted in every coating argument — D3963's 2% and 5% of a foot — belongs to epoxy-coated bar only. Neither of these two products has a patch routine, one because damage is not a rejection item and the other because there is nothing on the surface to damage.
Every bar on a job gets cut, and the cut is the one place where the two products differ in a way that can be put in square inches. A #8 cut square exposes π/4 × 1.00² = 0.785 in², which is 0.00545 ft² of bare steel. The coated barrel of one foot of the same bar is π × 1.00 ÷ 12 = 0.262 ft², so the cut is 2.1% of one foot's coating area and 0.10% of the coating on a 20 ft bar. The area is small, and that is not the argument. The argument is what the area is: on a galvanized bar it is steel with no zinc on it, sitting at the end of the bar, which is where a bar usually arrives at a joint; on a stainless bar it is the same alloy as the rest of the bar.
Zinc is anodic to stainless steel, so if a galvanized bar is tied to a stainless bar in the same pour the zinc pays for the couple. The direction is fixed by the galvanic series; the rate is an exposure question, which is why it is written here as a direction and not as a number. The stainless-to-black couple in section 04 runs the same way with the roles moved. The practical rule is one material through a pour, and where two have to meet, keep them electrically separate rather than relying on distance.
The split that follows is about access, not about rank. A galvanized bar is a coating decision: it fits exposure that will be inspected, where coating damage from handling, cutting and bending can be repaired on site, and where the coating that was specified matches the exposure it is going into. A stainless bar is an alloy decision: it fits bar that will not be reached again — joint dowels, penetrations, restoration work, anything cast in and covered — where a coating cannot be inspected and a repair cannot be made. Neither is a stronger version of the other; one is a consumable layer on carbon steel, the other is a different steel. Galvanized rebar is worked through on its own page, and epoxy-coated bar is the third answer, the only one of the three that moves the splice.
What neither one buys. Not a shorter lap — both take ψe 1.0, so the splice table in section 02 is unchanged. Not less cover — ACI 318's cover table is about fire and bond as well as corrosion. Not fewer bars — the count comes from the spacing on the drawing. The decision is the exposure, and the arithmetic is the two weight columns above.
Two published short lists disagree, and the disagreement is useful because it shows the choice is exposure-driven rather than ranked. The stainless rebar association names four commonly used alloys in order of preference: 2304, 2205, 316LN and XM-28. A mill's technical page names 2205, 316LN, 18Cr-3Ni-12Mn (S24000) and 304LN. The overlap is 2205 and 316LN; the difference is whether the lean duplex 2304 leads or the nitrogen-strengthened austenitics do.
Strength is a separate axis and it is written into A955 as three levels:
| A955 grade | Yield, min | Tensile, min | Elongation, min | Against black Grade 60 |
|---|---|---|---|---|
| Grade 40 [280] | 280 MPa / 40 ksi | 500 MPa / 70 ksi | 20% | Two thirds |
| Grade 60 [420] | 420 MPa / 60 ksi | 620 MPa / 90 ksi | 20% | Same yield |
| Grade 75 [520] | 520 MPa / 75 ksi | 690 MPa / 100 ksi | 20% | +25% |
The strength is where a stainless order can quietly change shape, and usually it does not. In a slab the bar count is set by the spacing the drawing calls for, and that spacing is set by temperature and shrinkage rather than by yield, so buying Grade 75 instead of Grade 60 removes no bars from the order. Where the extra 25% does work is where the bar is designed for a load rather than for crack control: column verticals, dowels across a joint, and ties. Ask which one the drawing is doing before paying for the higher grade.
Three inputs, and the three numbers that change the order. Pick the bar, pick the alloy family, enter the total linear feet.
The lap line is the point of the whole page: it is the same number whether the bar is black, galvanized or stainless, and the only way to make it longer is to coat the bar in epoxy. The weight line is where the alloy family shows up, and it is about a percent.
No. ACI 318-19 Table 25.4.2.5 gives a coating factor of 1.0 to uncoated and zinc-coated bar and 1.2 or 1.5 to epoxy-coated bar. Stainless bar is bare metal, so it takes 1.0 and the lap is the black bar lap: at 4,000 psi and Grade 60 a #4 is 21 in and a #8 is 50 in, Class B. The alloy premium buys corrosion resistance, not inches of lap.
Only if you buy the top grade. ASTM A955 offers three strength levels, Grade 40, 60 and 75, with minimum yield of 280, 420 and 520 MPa (40, 60 and 75 ksi) and minimum tensile of 500, 620 and 690 MPa (70, 90 and 100 ksi). Grade 60 stainless is the same 60 ksi as the black bar it replaces, and Grade 75 is 25% more. In a slab the bar count is set by spacing rather than by yield, so the extra strength usually does not remove bars from the order.
About 1% in the 300 series and nothing in duplex. ASTM A955 Table 1 computes nominal weight on two densities, 494.78 lb/ft3 (7,916.5 kg/m3) for the 300 series and 489.59 lb/ft3 (7,833.4 kg/m3) for duplex, against the 490 lb/ft3 behind the black bar table. A #8 is 2.670 lb/ft black, 2.696 lb/ft in 300 series and 2.668 lb/ft in duplex, so 1,000 ft of #8 gains 26 lb in 300 series and loses 2 lb in duplex.
It is a composition index for pitting resistance: PREN = Cr + 3.3 x Mo + 16 x N, with each element as a weight percentage. On typical compositions that puts 304 near 19, 316L near 26, 2304 near 28 and 2205 near 36. It takes about thirty seconds from the certificate, and it ranks alloys rather than qualifying an exposure, because the formula has no temperature term and no crevice term.
The austenitic grades are barely magnetic, which is why they are used near MRI rooms and in non-magnetic structures, and duplex 2205 is ferromagnetic in the annealed and hot rolled condition. So a cover meter or a rebar locator behaves differently on the two families, and a magnetic locator is not a reliable way to prove which alloy was supplied.
You can, but the couple has a direction. Stainless is the noble member, so in a chloride environment the black bar becomes the anode and corrodes faster than it would alone, and the effect grows with the area of stainless it is connected to. A small stainless patch inside a large black mat is the wrong way round. No clause in ACI 318 covers this; it is a detailing decision, and the usual answer is to keep the two electrically separate or to use one material throughout a pour.
It passivates rather than being inert. The chromium oxide film needs oxygen and a healthy pH, and it survives chlorides far better than black bar, but it is not immune: in a high chloride environment with thin cover, stainless pits, and duplex and 316L pit at different chloride levels. That is what PREN is ranking, and it is why the alloy choice is tied to the exposure rather than to the word stainless.
The same imperial sizes as black bar, #3 through #11 plus #14 and #18, with the soft metric labels 10M, 13M, 15M and so on rounded for convenience rather than converted. A #4 stainless bar is 0.500 in nominal diameter and 0.668 lb/ft black weight, so a drawing does not change when the alloy changes.
On the lap it is, by 25 in on a #8 bar. Epoxy coating takes the coating factor to 1.5, so a #8 Class B splice goes from 50 in to 75 in; stainless takes 1.0 and stays at 50 in. Ninety-six laps is 200 ft of extra bar — 534 lb at 2.670 lb/ft. On maintenance it is simpler too: a coated bar has to be patched to D3963's 2% and 5% limits after every cut and drag, and a stainless bar has nothing to patch. What it is not is cheaper: A955 bar is quoted per project, and no page ranking for this query publishes a dollar per pound.
It depends on whether the bar can be reached, not on a ranking. Both take a coating factor of 1.0, so a #8 Class B splice is 50 in either way, and neither moves cover or bar count. Two numbers do separate them. Weight: a #8 is 2.727 lb/ft galvanized against 2.696 lb/ft in 300-series stainless and 2.670 lb/ft in black bar, so 1,000 ft is 57 lb heavier than black galvanized and 31 lb heavier than stainless; on a #3 the zinc adds 4.9% against 1.1% for the alloy, because the zinc is bought by surface area and the alloy by volume. Durability: the zinc is 150 µm of metal with a fixed budget, spent at whatever rate the exposure sets, while the stainless bar is the metal with no layer to spend. Galvanized is the coating decision, inspectable and patchable on site; stainless is the alloy decision, for bar that will not be reached again.
Yes, at every size in the A767 Class I range. On a #8 it is 2.727 lb/ft galvanized against 2.696 lb/ft in 300-series stainless, on a #4 it is 0.697 against 0.675, and on a #3 it is 0.394 against 0.380. The reason is that the two premiums are calculated differently: the zinc is a mass per area, 3.50 oz/ft² at #4 and up and 3.00 at #3, multiplied by the surface of one foot of bar, πd/12, so it falls from 4.9% of the black weight on a #3 to 1.5% on a #11; the alloy is a density ratio, 494.78 against 490 lb/ft³, which is about 1% at every size. On an order, 1,000 ft of #8 is 2,727 lb galvanized against 2,696 lb stainless and 2,670 lb black.
Not on a fixed scale, because the two do not run out of the same thing. The zinc is a fixed 150 µm on a Class I bar and 85 µm on Class II, so the years are that thickness divided by whatever rate the exposure spends it at: at 2 µm a year a Class I coating is 75 years, at 3 µm a year it is 50, and once the zinc is gone at a point the steel under it behaves like black bar. Stainless has no coating mass to divide — the passive film is nanometres thick and rebuilds where oxygen and pH allow — so its limit is not a number of years but the chloride level the alloy was chosen for. In the same exposure stainless outlasts galvanized, because the galvanized bar has a clock and the stainless bar has a threshold. But no zinc consumption rate is published on this page, so a year figure for galvanized has to come from the exposure, not from the coating class.