Rigid mailer thickness is stated in points (thousandths of an inch, so 28 point is 0.028 inch) or as GSM, which is weight per square meter, not thickness. The common standard is 28 point: Uline uses 0.028-inch board for every size of its Stayflats mailers, from 6 x 8 to 22 x 27 inches, and sizes them for contents no more than 1/4 inch thick. Stiffness rises with the cube of thickness, so a 32 point board of the same material is about 1.5 times as stiff as 28 point, and 40 point about 2.9 times. Close fit, backing board and board grade matter as much as the number, and anything thicker than 1/4 inch or breakable belongs in a box.
Points, inches and GSM
| Rating on a listing | What it measures | How to compare |
|---|---|---|
| Points (pt) | Thickness in thousandths of an inch | 28 pt = 0.028 in = about 0.71 mm |
| Inches or mm | Thickness directly | Uline lists its Stayflats as .028" |
| GSM (g/m²) | Weight per area | Not convertible to thickness without the board’s density |
| ECT (edge crush test) | Strength of corrugated board, in lb per inch | Only comparable with other corrugated ratings |
GSM appears on many marketplace listings. The Juvale stay-flat mailer , for example, is listed as 450 GSM with no thickness given. Two boards of the same GSM can differ in thickness and stiffness, so when thickness matters, ask the seller for caliper.
ECT belongs to corrugated board. UPS’s box strength chart, for example, pairs 32 ECT single-wall board with packages up to 30 lb. An ECT figure and a point figure describe different materials and cannot be compared.
Why a few points make a big difference
For a flat panel of a given material, bending stiffness (flexural rigidity) is proportional to the cube of its thickness: D = E h³ / 12(1 − ν²), where h is thickness. Holding the material constant, that gives:
| Board | Thickness | Stiffness relative to 28 pt |
|---|---|---|
| 24 pt | 0.024 in | 0.63x |
| 28 pt | 0.028 in | 1.0x |
| 32 pt | 0.032 in | 1.49x |
| 36 pt | 0.036 in | 2.13x |
| 40 pt | 0.040 in | 2.92x |
Two cautions. First, the comparison only holds within one board type; clay-coated newsback, solid bleached board and chipboard have different stiffness at the same thickness. Second, paperboard is stiffer in one direction than the other. Paperboard makers publish bending stiffness separately for the machine direction and cross direction because stiffness depends on how the fibers line up during manufacture, so a mailer may resist bending along its length better than across it.
Mailer listings rarely give a measured bending stiffness. Thickness within one board type is the best comparison most sellers have.
What the standard 28 pt mailer is sized for
Uline’s white self-seal Stayflats are a useful reference because the specs are published per size. All are 0.028 in clay-coated newsback; the kraft versions are also 0.028 in. Usable dimensions assume contents no more than 1/4 inch thick:
| Uline size | Outside (in) | Usable (in) | Fits |
|---|---|---|---|
| #1 | 6 x 8 | 5 1/2 x 7 3/4 | 5 x 7 photo or card |
| #2 | 9 x 11 1/2 | 8 1/2 x 11 1/4 | 8 x 10 print with a sleeve; 8.5 x 11 is a tight fit |
| #13 | 22 x 27 | 21 1/2 x 26 3/4 | Poster-size prints up to about 21 x 26 |
Two things follow. A 28 pt mailer is the normal choice even for very large sizes, so a bigger print does not automatically require thicker board. And usable size, not outside size, decides fit: a sleeve and backing board add both width and thickness.
When to add stiffness, and how
A wider panel flexes more under the same load, and a print that slides inside an oversized mailer can catch a corner. Options, roughly in order of cost:
- Fit the mailer to the protected item, so there is no room to slide.
- Add backing board on both faces for valuable or thin-paper prints. One board on one side still leaves the other face exposed.
- Choose thicker board of the same type where a seller publishes caliper, using the cube rule above to judge the gain.
- Move to a corrugated flat mailer or a box for framed or mounted work, stacks thicker than 1/4 inch, or anything with hard edges.
Uline markets its Stayflats as resisting bending and crushing, but the board sits directly against the contents with no cushioning space around them. Treat a rigid mailer as bend protection for flat paper, not a substitute for a box. For when to use a rigid mailer at all, see poly mailer vs rigid mailer .
How USPS prices a rigid mailer
Stiffness changes your postage category:
- Large envelope (flat) prices need flexibility. In the test in DMM 201.4.3, the piece is extended halfway off a table and pressed 1 inch from the edge. It is not flexible if it cannot bend at least 1 inch without damage. Flat-size pieces that fail are priced as parcels (DMM 101.2.5). Most rigid mailers fail by design.
- Small rigid mailers can be letters with a surcharge. A piece within letter size (no more than 11 1/2 x 6 1/8 x 1/4 inches and 3.5 ounces) is nonmachinable if it is “too rigid,” meaning it does not bend easily around an 11-inch-diameter turn under 40 pounds of belt tension (DMM 101.1.2). Nonmachinable letters pay a surcharge.
- Size limits still apply. Flats can be no more than 15 x 12 x 3/4 inches (DMM 101.2.1), so a 22 x 27 mailer is a parcel regardless.
Weigh and measure the sealed mailer with its contents before buying postage.
Sources
- Uline White Self-Seal Stayflats #2, 9 x 11 1/2 in : 0.028 in clay-coated newsback; usable 8 1/2 x 11 1/4 in for contents up to 1/4 in.
- Uline White Self-Seal Stayflats #1, 6 x 8 in : 0.028 in; usable 5 1/2 x 7 3/4 in.
- Uline White Self-Seal Stayflats #13, 22 x 27 in : 0.028 in; usable 21 1/2 x 26 3/4 in.
- Uline Kraft Self-Seal Stayflats #5 : kraft version also 0.028 in.
- Flexural rigidity (Wikipedia) : plate flexural rigidity D = E h³ / 12(1 − ν²), proportional to the cube of thickness.
- Holmen Iggesund general technical information : paperboard bending stiffness measured per ISO 5628 and reported separately for machine and cross direction because it depends on fiber orientation.
- UPS Packaging Guidelines (PDF) : box strength chart pairing 32 ECT single-wall board with packages up to 30 lb.
- USPS DMM 101 : letter-size limits and the “too rigid” nonmachinable criterion (101.1), flat-size limits and flexibility (101.2).
- USPS DMM 201 : the 1-inch flexibility test for flats (201.4.3).
FAQ
Is 28 point board thick enough for a rigid mailer?
It is the common standard. Uline uses 0.028-inch board for every size of its standard Stayflats, from 6 x 8 to 22 x 27 inches, sized for contents up to 1/4 inch thick. For large or valuable prints, add backing board on both faces or move to a corrugated mailer rather than relying on the envelope alone.
Does a higher point rating always mean a stiffer mailer?
Among boards of the same type, yes, and by more than the numbers suggest, because stiffness rises with the cube of thickness: 32 point is about 1.5 times as stiff as 28 point. Between different board grades the comparison breaks down, because the material and fiber direction also set stiffness.
What does 450 GSM mean on a rigid mailer listing?
Grams per square meter, a measure of weight, not thickness. It cannot be converted to points without knowing the board’s density, so ask the seller for the caliper in points, inches or millimeters if thickness matters.
Will a rigid mailer ship as a USPS large envelope?
Usually not. Flat-size mail must bend at least 1 inch in the USPS flexibility test, and mailers built not to bend fail it, so they are priced as parcels. A small rigid mailer within letter size can instead be charged the nonmachinable surcharge for being too rigid.