Mill plate is allowed to be surprisingly far from flat, and the numbers that say how far are behind a paid standard. This chart publishes the flatness and bow allowances by thickness, width and length, converts them to your actual piece, and gives you a straight verdict: within spec and not worth rejecting, or rejectable with the clause to cite.
Last updated July 2026
ASTM B209 Commercial grade, 0.500 to 0.999" thick, over 36 to 48" wide.
The per-foot bow figure would give 0.320" over this length, which is capped at 1.5 times the full-plate flatness allowance. The reported bow allowance is the capped value.
R = c² / (8 x g). A short chord with a small gap can still mean a badly bowed plate, which is why the tool projects the local reading out to the full width before it gives you a verdict.
| Allowance, Full Plate | Bow Per Foot | Relative Cost | |
|---|---|---|---|
| Commercial (selected) | 0.200" | 0.0400"/ft | Baseline, no adder |
| Superior | 0.100" | 0.0200"/ft | Plus 10 to 20 percent |
| Special | 0.050" | 0.0100"/ft | Plus 25 to 50 percent, longer lead time |
Commercial is what you bought if the purchase order did not name a grade. Superior and Special have to be stated at order time, and Special usually has to be negotiated with the mill rather than the distributor.
| Product Form | Flatness, Full Plate | Basis |
|---|---|---|
| Rolled plate, Commercial | 0.200" | B209 grid, by thickness and width |
| Cast tool and jig plate | 0.015" | Product guarantee, entire surface, all sizes |
| Precision ground 6061 | 0.040" | 0.010"/ft of width |
| Blanchard ground | 0.040" | 0.0100"/ft at this thickness |
That is about 13 to 1 between rolled Commercial and cast tool and jig plate on this exact size. Grinding removes the bow that is there today, it does not remove the residual stress underneath it. A Blanchard ground plate with stress still in it will move again the first time you take a heavy cut out of one face.
FLATNESS: Plate shall be supplied to ASTM B209 Superior flatness grade. Maximum deviation from flat shall not exceed 0.100 in over the full plate, measured in free state on a surface plate at three point support, with no clamping or vacuum applied. Longitudinal bow shall not exceed 0.0200 in per foot of length. Material outside these limits is rejectable and returnable at supplier expense. Certification of flatness grade shall accompany the shipment.
A grade named in the body of the order is enforceable. A note on the drawing that the supplier never saw is not. If flatness actually matters for the part, buy cast tool and jig plate or ground plate instead of arguing about grades.
Flatness grid as of ASTM B209-14 (2025 reapproval) as reproduced by aluminum distributors. ASTM B209 is a paid standard, so verify these values against a current copy before you use them contractually. 6061, 7075, 7050 and 5083 rolled plate share the same B209 flatness grid, there is no alloy correction in the standard. In practice 7075 and 7050 arrive at the loose end of the band more often because of the harder quench, and they spring further when you cut them up.
Maximum deviation from flat over the full plate, in inches, measured in free state. Special grade is 25 percent of the Commercial value with a 0.015 inch floor, and has to be negotiated when the order is placed. Widths over 72 inches are outside the published grid and have to be agreed with the mill.
| Thickness (in) | Grade | Width to 36" | Over 36 to 48" | Over 48 to 60" | Over 60 to 72" |
|---|---|---|---|---|---|
| 0.250 to 0.499 | Commercial | 0.200" | 0.300" | 0.400" | 0.500" |
| 0.250 to 0.499 | Superior | 0.100" | 0.150" | 0.200" | 0.250" |
| 0.500 to 0.999 | Commercial | 0.150" | 0.200" | 0.250" | 0.300" |
| 0.500 to 0.999 | Superior | 0.075" | 0.100" | 0.125" | 0.150" |
| 1.000 to 1.999 | Commercial | 0.100" | 0.150" | 0.200" | 0.250" |
| 1.000 to 1.999 | Superior | 0.050" | 0.075" | 0.100" | 0.125" |
| 2.000 to 2.999 | Commercial | 0.080" | 0.120" | 0.160" | 0.200" |
| 2.000 to 2.999 | Superior | 0.040" | 0.060" | 0.080" | 0.100" |
| 3.000 and over | Commercial | 0.060" | 0.100" | 0.140" | 0.180" |
| 3.000 and over | Superior | 0.030" | 0.050" | 0.070" | 0.090" |
| Thickness (in) | Commercial | Superior | Special | Commercial over 96", before cap |
|---|---|---|---|---|
| 0.250 to 0.499 | 0.050"/ft | 0.025"/ft | 0.0125"/ft | 0.400" |
| 0.500 to 0.999 | 0.040"/ft | 0.020"/ft | 0.0100"/ft | 0.320" |
| 1.000 to 1.999 | 0.030"/ft | 0.015"/ft | 0.0075"/ft | 0.240" |
| 2.000 and over | 0.020"/ft | 0.010"/ft | 0.0050"/ft | 0.160" |
Bow over the full length is capped at 1.5 times the full-plate flatness allowance, so a very long plate does not accumulate an unlimited allowance from the per-foot figure alone. The last column is the raw per-foot figure carried over 96 inches, before that cap is applied. On a 48 inch wide plate the cap bites first in the 0.500 to 0.999 and 1.000 to 1.999 bands, which land at 0.300 inch and 0.225 inch.
| Alloy and product form | Flatness basis | On a 48 x 96 x 0.500" plate | Practical note |
|---|---|---|---|
| 6061-T651 rolled plate, Commercial | B209 grid by thickness and width | 0.200" | Default when the order names no grade |
| 6061-T651 rolled plate, Superior | 50 percent of Commercial | 0.100" | Plus 10 to 20 percent, must be ordered |
| 7075-T651 rolled plate, Commercial | Same B209 grid as 6061 | 0.200" | Harder quench, arrives at the loose end more often |
| 7075-T7351 rolled plate, Commercial | Same B209 grid as 6061 | 0.200" | Overaged, more stable than T651 after cutting |
| 7050-T7451 rolled plate, Commercial | Same B209 grid as 6061 | 0.200" | Thick plate alloy, stress relieved, still springs on cut |
| 5083 / 5052 rolled plate, Commercial | Same B209 grid as 6061 | 0.200" | Non heat treatable, work hardens rather than quenches |
| 5000 series cast tool and jig plate | 0.015" over the entire plate | 0.015" | Product guarantee, not a B209 grade, about 20 to 1 flatter |
| Precision ground 6061 plate | 0.010" per foot of width | 0.040" | Ground after rolling, residual stress still present |
| Blanchard ground plate | 0.005 to 0.010" per foot | 0.020 to 0.040" | Tighter on thicker plate, will move again if stressed |
A 48 by 96 by 0.375 inch 6061 plate is allowed 0.30 inch of deviation from flat at Commercial grade. That is just under five sixteenths of an inch across the sheet, and it is legal. The same plate bought as 5000 series cast tool and jig plate is guaranteed to 0.015 inch, a twenty to one difference. If your fixture plate has to sit flat on a table without shimming, that gap is the whole argument.
These values are reproduced from distributor-published copies of the ASTM B209 and Aluminum Standards and Data flatness tables. ASTM B209 is a paid standard, so verify against a current copy of ASTM B209 or B209M before citing a number in a contract or a rejection.
F_full is the maximum permitted deviation from flat over the full plate, read from the grade table by thickness band and width band, in inches. It is a total deviation over the whole surface, the spread between the highest and lowest points, not a limit that applies to every local span.
Grade scaling: Superior = 0.50 x Commercial, Special = 0.25 x Commercial, floored at 0.015 in. Each grade step roughly halves the allowance, and the floor exists because no mill will commit to an unbounded fraction of a small number.
F_span = F_full x (span / width)², floored at 0.003 in. Scaling a full-plate allowance down to a shorter measurement span. Deviation from flat grows with the square of the unsupported span for a given curvature, so a 12 inch straightedge sees a small fraction of what a 48 inch sweep sees on the same bow.
B = bow_per_ft x (length / 12), capped at 1.5 x F_full. Longitudinal bow allowance over the full length, with bow_per_ft read from the bow table by thickness band and scaled by grade.
F_per_ft = F_full / (width / 12), and B_per_ft = bow_per_ft. The inches-per-foot readouts, which are the numbers most useful when you are talking to a supplier who quotes flatness that way.
R = c² / (8 x g). Equivalent curvature for cross-checking a straightedge reading, where c is the chord length and g is the gap at the center. It lets you check whether a local reading and a full-plate reading actually agree, or whether you are looking at a local wave rather than a continuous bow.
Projected full-width deviation = measured x (width / span)². The inverse of the span scaling. If the bow is a continuous arc, this is what a corner-to-corner sweep should read.
Verdict logic. A reading taken over a span shorter than the width is compared against F_span first. If it is inside F_span, the plate is clearly within spec. If it is over F_span but still inside F_full, the plate passes B209 on total spread and the supplier is technically right, but the local curvature projects to more than the allowance across the full width, so the honest next step is a corner-to-corner measurement rather than a rejection. Only a reading over F_full is reported as rejectable, with the overage in inches and as a percentage of the allowance.
Measurement adjustment. A reading taken by rocking the plate on a bench is halved before comparison, because total rock at a corner is roughly twice the deviation from flat when the plate pivots about its high center. Free-state surface plate readings and feeler-gauge gaps are used as entered.
A lot more than most buyers expect. At Commercial flatness grade, a 48 by 96 inch plate in the 0.250 to 0.499 inch thickness range is allowed roughly 0.30 inch of deviation from flat over the full plate, and longitudinal bow of about 0.050 inch per foot of length. Thicker plate is held tighter, around 0.10 inch total for 1 to 2 inch plate at that width. If you did not specify a flatness grade on the purchase order, Commercial is what you bought.
It depends on thickness and width, and it runs from roughly 0.06 inch on narrow thick plate to 0.50 inch on wide thin plate at Commercial grade. For the common case of 0.500 to 0.999 inch thick at 48 inch width, the Commercial allowance is about 0.20 inch over the full plate. B209 also defines Superior grade at roughly half that and Special grade at about a quarter, but you only get those if you name them on the order.
Probably, if you ordered Commercial grade, which is the default when no grade is stated. A reported case of 0.035 inch of bow over 7 inches scales to well inside the Commercial allowance for a 48 inch wide plate. The place to fight this is the purchase order, not the phone call. Specify Superior or Special flatness, or buy cast tool and jig plate, and expect to pay 10 to 50 percent more for it.
They are three flatness grades in ASTM B209, and each step roughly halves the allowed deviation. Commercial is the default and the loosest. Superior is about 50 percent of the Commercial allowance and typically adds 10 to 20 percent to the price. Special is about 25 percent of Commercial, has to be negotiated when you place the order, and can add 25 to 50 percent plus lead time. None of them are as flat as cast tool and jig plate.
Cast tool and jig plate is guaranteed flat within 0.015 inch over the entire plate surface, against roughly 0.30 inch for a 48 by 96 inch Commercial grade rolled plate of similar thickness. That is about a twenty to one difference. Cast plate gets there by being cast rather than rolled, thermally stress relieved, then machined on both faces, which is also why it holds a plus or minus 0.005 inch thickness tolerance.
In free state, on a surface plate, supported at three points, with nothing clamping it down. Sweep an indicator across the surface and record the highest and lowest readings. The flatness value is the total spread between them, not the reading at any one spot. Clamping the plate or laying it on a vacuum table pulls an artificial flatness into it, which is exactly how parts pass inspection and then potato chip after they come off the fixture.
Buying a full 48 by 96 sheet and cutting it down means you own whatever bow the mill sent. If you only need a few blanks, get an instant quote on 6061, 7075 and 7050 plate cut to your size, and inspect a piece you can actually put on a surface plate instead of a sheet you cannot lift.