01How corrugated board is built
The Fibre Box Association describes two parts, the liner and the medium, both made of a heavy paper called containerboard. The medium is formed into flutes and glued between flat linerboard facings; the flutes hold the liners apart for bending stiffness, and the space between them cushions the contents.
The freight classification (NMFC) defines the builds by facings and mediums:
| Board | Build | In the UPS parcel chart |
|---|---|---|
| Single face | One medium glued to one liner | Not listed |
| Single wall | One medium between two liners | Recommended up to 80 lb gross weight |
| Double wall | Three liners, two mediums | Recommended from 81 to 150 lb |
| Triple wall | Four liners, three mediums | Not listed; ask the box maker |
02Flute sizes: A, B, C, E and F
Flute letters follow roughly the order the profiles were developed, not their size; size is counted in flutes per foot. The Fibre Box Association says larger flutes give more cushioning and vertical compression strength, smaller flutes a better surface for graphics, and that exact profiles vary by corrugator, so a "B flute" from two plants may differ. More in the flutes glossary entry.
| Flute | Flutes per foot (about) | What the Fibre Box Association says |
|---|---|---|
| A | 33 | The original flute profile |
| B | 47 | Developed for self-supporting canned goods |
| C | 38 to 39 | Developed as an all-purpose flute |
| E | 90 | The next profile after C; a small flute with a better print surface |
| F | 125 | Developed for small, folding carton type boxes |
Ask the box maker which flutes its corrugator runs before you design around one, and get the board caliper in writing with the quote.
03ECT vs burst: what "32 ECT" and "200 lb test" mean
- Edge crush test (ECT): a strip of board stood on its edge, flutes vertical, and crushed. The result is compression resistance in pounds per inch of width: "32 ECT" means 32 pounds per inch.
- Bursting strength (Mullen test): how much pressure punches through the board, in pounds per square inch. "200 lb test" means a 200 psi burst. Burst-rated boxes must also meet a minimum combined weight of the facings (liners).
They measure different things, so 32 ECT is not a conversion of 200 lb burst. NMFC Item 222 accepts either as alternatives: burst plus liner weight (its Table A) or edge crush (Table B), and its packaging items pair them, as in "200 pounds or edge crush test of not less than 32 pounds." More in ECT and burst strength.
The UPS single-package box strength chart, by gross weight of the packed box:
| Gross weight | Burst (psi) | ECT (lb per inch) | Walls |
|---|---|---|---|
| Up to 30 lb | 200 | 32 | Single |
| 31 to 50 lb | 250 | 44 | Single |
| 51 to 65 lb | 275 | 55 | Single |
| 81 to 100 lb | 350 | 61 | Double |
| 101 to 120 lb | 400 | 71 | Double |
| 121 to 140 lb | 500 | 82 | Double |
04The box maker’s certificate
The round stamp, usually on the bottom flap, is the box manufacturer’s certificate. Under NMFC Item 222-1, a box made to the Item 222 specs must carry it, about 3 inches across, naming the box maker and its city and guaranteeing the box conforms. NMFTA lists the lines on a burst-rated certificate as burst test, liner weight, size limit and gross weight limit. A box that does not meet Item 222 may carry only a voluntary straight-line stamp.
- The shipper is responsible for checking the box against the contents, according to NMFTA. UPS tells shippers never to exceed the gross weight on the certificate.
- Boxes made abroad for freight imported into the US need not bear a certificate if the shipper certifies on the bill of lading that they comply. If you import packed product, ask your factory what its boxes test at.
05RSC and other corrugated box styles
The regular slotted container (RSC) is the plain shipping box: one piece of board, scored and slotted, with flaps that close each end. The NMFC defines it and the other common styles:
| Style | What it is |
|---|---|
| RSC (regular slotted container) | All flaps the same length; the outer flaps meet in the center |
| FOL (full overlap slotted container) | All flaps the width of the box; closed, the outer flaps are within an inch of full overlap |
| CSSC (center special slotted container) | Inner and outer flaps of different lengths, and both pairs meet in the center |
| HSC (half slotted container) | An RSC with one set of flaps and a separate cover less than two-thirds its depth |
| Telescope (FTD, PT) | A separate top that fits over the body and covers more than two-thirds of its depth |
For printed e-commerce boxes with a folding lid, see mailer boxes and the box styles glossary entry.
06Kraft, recycled and white liners
Liners made mostly from virgin fiber are kraftliner; liners made mostly from recycled fiber are testliner, which Europe grades from 1 (strongest) to 4. Semi-chemical fluting (a pulp grade used for the medium) is generally stronger than recycled fluting, according to the paper maker Holmen. Ask for the liner and medium grades, or the ECT result, rather than a description. Our kraft paper page covers how kraft is made and how it prints.
On brown kraft, ink sits over the brown, so colors look more muted and warmer than on white. For brighter color, ask which white outer liners the box maker stocks, and proof on the actual board.
07Printing on corrugated
| Method | How it works | Fits |
|---|---|---|
| Flexo direct print (post-print) | Combined board printed directly with flexographic plates | The most common process, and the lowest print quality of the five |
| Flexo pre-print | The top liner is printed before the board is combined on the corrugator | Needs a very high volume to pay off |
| Litho-lamination | Liner sheets printed offset, then laminated to the board | The best print quality; fairly long runs, though much shorter than pre-print |
| Litho-label | An offset-printed label glued to one face of the board | Low volumes |
| Digital inkjet | Inkjet, with no plates | Economical on short runs; changeover in minutes |
Flexo and litho need plates, a setup cost spread over the run; digital does not. Ask for quotes by method at your real quantity. See printing methods.
08Recycling, recycled content and FSC claims
An AF&PA study put US recycling access for corrugated cardboard at 93% of the population in 2021.
- "Recyclable": under the FTC Green Guides (16 CFR 260.12), an unqualified claim is fine when recycling facilities are available to a substantial majority, at least 60%, of the consumers or communities where the item is sold, and when the whole package, minor incidental parts aside, is recyclable. If any component significantly limits recycling, the claim is deceptive.
- "Recycled content": unqualified only if the entire package, minor incidental components aside, is made from recycled material (16 CFR 260.13). A partly recycled box should state the amount, such as "contains 50% recycled fiber."
- FSC: chain-of-custody certification lets a company put the FSC label on finished products, so the box maker needs its own certificate; retailers that do not manufacture generally use FSC’s promotional license.
Rules checked October 22, 2026. This is general information, not legal advice.
09Testing the packed box
ECT and burst grade the board, not the box. Box compression testing (ASTM D642) loads the finished container, empty or filled, on its faces, edges or corners, which matters when cartons stack. ISTA test procedures test the packed product: the 3 series is general simulation, 3A covers packages shipped individually through parcel networks, and the 6 series holds member procedures such as Amazon.com SIOC.
UPS says its strength chart is only a guideline and that packaged products should be tested to ISTA 3A. See ISTA testing and quality control inspection.
10How to spec it with a supplier
Send every box supplier the same lines, or the quotes won’t line up. The Vault lists packaging factories with contacts; the public list is at packaging manufacturers.
| No. | Field | What to write |
|---|---|---|
| 01 | Style | RSC, half slotted, full overlap, telescope or die-cut; attach a dieline for anything but a plain RSC |
| 02 | Dimensions | Internal length x width x depth (length and width are the open face), and whether you mean inside or outside size |
| 03 | Board | Single or double wall, flute (or flute pair for double wall), and caliper agreed on the sample |
| 04 | Strength | ECT or burst grade for your packed weight, and whether you need the box maker’s certificate printed |
| 05 | Liners | Kraft or recycled, brown or white outer face, inside face |
| 06 | Method (flexo, litho-lam, digital), colors with Pantone references, which panels, inside print if any |
Questions to ask the supplier
- Which ECT or burst grade do you propose for a box packed to my gross weight, and why?
- What is your minimum per design and per print method, and what are the plate or setup charges?
- If I am paying for FSC board, what is your chain-of-custody certificate code?
11FAQ
Q01What does 32 ECT mean?
The board passed an edge crush test of at least 32 pounds per inch of width. It is the lowest single wall grade in the UPS parcel chart, for packed boxes up to 30 lb.
Q02Is 32 ECT the same as 200 lb test?
No. 200 lb test is a burst (Mullen) rating in pounds per square inch; 32 ECT is an edge crush rating. Freight rules accept either at the same tier, so they are alternatives, not conversions.
Q03What is an RSC box?
A regular slotted container: one piece of board with flaps of equal length on each end, where the outer flaps meet in the center. PCA calls it the most common box style.
Q04When do I need double wall boxes?
When the packed box is heavy. UPS’s chart moves to double wall from 81 lb gross weight; test your packed product too.
Q05What is E flute used for?
E flute is a small profile of about 90 flutes per foot. Smaller flutes print better and larger flutes cushion more, per the Fibre Box Association, so pick E when print matters more than cushioning.