Corrugated board is specified three ways: the flute profile, which sets thickness and cushioning; the strength rating, which is either an Edge Crush Test value in pounds per inch or a Mullen burst value in psi; and the wall construction, which is single, double or triple. This page collects the published figures for all three, names the standard each one comes from, and is explicit about where the industry has no standard at all.
We build custom boxes to these specifications every day, so we wrote the reference we kept failing to find. Where a number is a published standard, we say so and name it. Where a number is a rule of thumb repeated between suppliers, we say that too, because the difference matters when you are writing a purchase specification.
Flute profiles
The flute is the wavy layer of paper sandwiched between the liners. Its size and pitch determine how thick the board is, how much it cushions and how well it holds a printed image. Flute grades are identified by letter, and the letters are historical rather than sequential, which is why B flute is thinner than A flute and E flute is thinner than both.
| Flute | Flutes per linear foot | Approximate thickness | Typical use |
|---|---|---|---|
| A | 33 | 3/16 in to 1/4 in (4.8 mm to 6.4 mm) | Maximum cushioning, fragile and lightweight goods |
| B | 47 | About 1/8 in (3.2 mm) | Canned goods, products needing puncture resistance |
| C | 38 | About 5/32 in (4.0 mm) | The general purpose shipping box flute |
| E | 90 | About 1/16 in (1.6 mm) | Mailer boxes, retail packaging, high quality print |
| F | 125 | About 1/32 in (0.8 mm) | Small retail cartons, cosmetics, confectionery |
Why we publish thickness as a range
There is no standard that fixes corrugated board thickness by flute letter. The Fibre Box Association states that flute profiles vary, sometimes significantly, because corrugator rolls are manufactured to suit different production speeds, paperboard characteristics and cost targets. Two mills can both sell you C flute and deliver measurably different caliper.
A flute is where the disagreement is widest. Published figures range from 3/16 in to 1/4 in, and a measured caliper for A flute 200# board comes out at 0.201 in, which sits between the two. Anyone quoting a single authoritative A flute thickness is quoting their own supplier, not a standard. If your product needs a specific caliper, put the caliper in the specification rather than the flute letter.
Double and triple wall
Double wall combines two flute profiles between three liners, and is named for the flutes it contains. BC is the common heavy duty combination, measured at 0.258 in. AC is thicker still, measured at 0.338 in. EB pairs a fine flute with a medium one so you get a printable outer surface over a cushioning inner layer, and typically lands between 4 mm and 4.5 mm. Triple wall adds a third flute layer and is used for industrial loads that would otherwise need a wooden crate.
How corrugated strength is measured
Two tests dominate, and they measure genuinely different things. Confusing them is the most common error in packaging specifications.
Edge Crush Test (ECT)
ECT measures stacking strength. A short column of board is compressed on its edge, parallel to the flutes, until it crushes. The result is reported in pounds per lineal inch of loading edge. The governing standard is TAPPI/ANSI T 811, titled “Edgewise compressive strength of corrugated fiberboard (short column test)”, which covers single, double and triple wall board. The international equivalent is ISO 3037, which reports in kilonewtons per metre instead.
ECT is the right test to care about if your boxes will be palletised, stacked in a warehouse or stored on a shelf under other cartons. It predicts how much weight the box can carry from above.
Mullen burst test
The Mullen test measures puncture resistance. A rubber diaphragm is pushed through the face of the board until it bursts, and the pressure at failure is recorded. The governing standard is TAPPI/ANSI T 810, “Bursting strength of corrugated and solid fiberboard”, which covers the range from 690 kPa (100 psi) to 4,825 kPa (700 psi).
One point of vocabulary worth getting right. Mullen values are properly expressed in pounds per square inch. The trade shorthand “200#” is not a weight, it means a minimum bursting strength of 200 psi. Mullen is the right test to care about for rough handling, single parcel shipping and products with hard corners that could push through the wall.
The Mullen to ECT equivalency table
Carriers allow a box to qualify by either test. The tables below come from National Motor Freight Classification Item 222, which applies to motor carriers, and Rule 41 of the Uniform Freight Classification, which applies to rail. They set the minimum board strength, the maximum weight of the box plus contents, and the maximum outside dimension, measured as length plus width plus depth added together.
| Min. burst | Min. ECT | Max weight of box and contents | Max outside dimensions (L+W+D) |
|---|---|---|---|
| 125# | 23 | 20 lb | 40 in |
| 150# | 26 | 35 lb | 50 in |
| 175# | 29 | 50 lb | 60 in |
| 200# | 32 | 65 lb | 75 in |
| 250# | 40 | 80 lb | 85 in |
| 275# | 44 | 95 lb | 95 in |
| 350# | 55 | 120 lb | 105 in |
| Min. burst | Min. ECT | Max weight of box and contents | Max outside dimensions (L+W+D) |
|---|---|---|---|
| 200# | 42 | 80 lb | 85 in |
| 275# | 48 | 100 lb | 95 in |
| 350# | 51 | 120 lb | 105 in |
| 400# | 61 | 140 lb | 110 in |
| 500# | 71 | 160 lb | 115 in |
| 600# | 82 | 180 lb | 120 in |
| Min. burst | Min. ECT | Max weight of box and contents | Max outside dimensions (L+W+D) |
|---|---|---|---|
| 700# | 67 | 240 lb | 110 in |
| 900# | 80 | 260 lb | 115 in |
| 1100# | 90 | 280 lb | Not confirmed, see note |
| 1300# | 112 | 300 lb | Not confirmed, see note |
These rows are not a conversion formula
This is the part most published versions of this table leave out, and it causes real specification errors.
200# Mullen and 32 ECT sit on the same row because a carrier will accept either as qualification for the same duty class. They are alternate routes to the same permission. They are not physically equivalent, and one does not convert into the other. Two boards can both test at 32 ECT and behave very differently under puncture, because different corrugator formulas reach the same edge crush value by different means. If you need both stacking strength and puncture resistance, specify both numbers rather than assuming one implies the other.
Why you will see 40 lb and 65 lb quoted for the same board
Search for the maximum load of a 32 ECT box and you will find two different answers, both from credible sources. The carrier classification tables say 65 lb. Several packaging suppliers and test equipment manufacturers publish 40 lb.
Both are correct, because they answer different questions. 65 lb is the regulatory ceiling, the heaviest that board is permitted to carry under the classification. 40 lb is a conservative working recommendation that leaves headroom for handling, humidity and stacking time, all of which reduce real world performance below laboratory values.
If you are writing a compliance specification, use the classification figure. If you are deciding what to actually ship in the box, the lower number is the safer planning assumption. Quoting one while meaning the other is how boxes fail in transit while remaining technically compliant.
The box maker’s certificate
The round or rectangular stamp printed on the bottom flap of a shipping box is the box maker’s certificate. It is required by NMFC Item 222 for motor freight and UFC Rule 41 for rail, and it certifies that the board meets the classification the box is sold under.
- Round stamp. Standard boxes meeting the published requirements for their size and weight class.
- Rectangular stamp. Numbered or special packages certified under a specific item number.
- What it shows. Either the burst strength or the ECT value, the maximum gross weight, the size limit and the manufacturer’s identity.
Two things worth knowing. The certificate is a carrier requirement, not a government regulation, so no federal agency enforces it. And the size limit it references is the sum of length, width and depth in inches, not a cubic volume. We mention that because it is stated incorrectly in a surprising number of places, including at least one publication from a carrier association.
Board caliper: points, mils, microns and GSM
Folding cartons and rigid boxes are specified by caliper rather than flute, and the units cross between imperial and metric constantly.
One point equals 0.001 inch, which is the same as one mil, 25.4 microns, or 0.0254 millimetres. The conversion is exact and applies to every board grade.
| Points | Inches | Microns | Millimetres |
|---|---|---|---|
| 12 pt | 0.012 | 305 | 0.30 |
| 14 pt | 0.014 | 356 | 0.36 |
| 16 pt | 0.016 | 406 | 0.41 |
| 18 pt | 0.018 | 457 | 0.46 |
| 20 pt | 0.020 | 508 | 0.51 |
| 24 pt | 0.024 | 610 | 0.61 |
| 28 pt | 0.028 | 711 | 0.71 |
| 36 pt | 0.036 | 914 | 0.91 |
There is no universal conversion from points to GSM
This trips up more artwork and quoting conversations than any other unit question, so it is worth being blunt about.
Points measure thickness. GSM measures mass per square metre. The relationship between them is density, and density changes with the fibre furnish, the coating, how hard the board was calendered and whether it is virgin or recycled. Take three boards at 350 gsm, one kraft, one coated SBS and one recycled, and you will measure three different calipers.
So a chart that claims to convert points to GSM across all grades is describing one supplier’s stock, not a physical law. If your specification depends on thickness, specify points or microns. If it depends on weight, specify GSM. Specifying one and assuming the other is how a carton arrives visibly flimsier than the sample that was approved.
Does Amazon require 32 ECT?
We could not find any Amazon document that sets a minimum ECT value, and the claim that Amazon requires 32 ECT does not appear to come from Amazon.
The claim is repeated widely, almost always by packaging suppliers and always without a link to an Amazon source. What Amazon actually publishes is a performance protocol. ISTA 6-Amazon.com-SIOC qualifies packaging by putting it through drop, vibration, compression, impact, tip over and leak testing. Read it and you will find no ECT minimum, no burst minimum and no board grade requirement anywhere in it. It does not care how you reach the result, only that the package survives the test.
That distinction matters commercially. If you specify 32 ECT because a blog told you Amazon requires it, you may still fail the protocol, and you may equally be paying for board you did not need. The honest statement is that Amazon qualifies packaging by performance testing rather than by a published board specification. Amazon’s own seller requirement pages sit behind a Seller Central login, so if you need certainty for a compliance document, read them from your own account rather than trusting a supplier’s summary, including ours.
Corrugated specification FAQs
Is ECT or Mullen better?
Neither, they measure different failure modes. Choose ECT if the box will be stacked, palletised or warehoused, because it predicts stacking strength. Choose Mullen if the box ships as a single parcel through rough handling, or if the contents have hard corners that could puncture the wall. Products that face both risks should be specified against both.
What ECT do I need for a standard ecommerce parcel?
32 ECT single wall covers most ecommerce parcels up to the classification limit of 65 lb, and is what the majority of shipping boxes are built to. Heavier or larger loads move up to 44 ECT single wall or into double wall. If you are near a limit, specify against the weight and dimension you actually ship rather than the class you hope to fit.
Why do two suppliers quote different thicknesses for the same flute?
Because there is no standard fixing it. The Fibre Box Association states plainly that flute profiles vary between mills, since corrugator rolls are built around different production and cost priorities. If caliper is critical to your product, specify the caliper in inches or millimetres and let the mill choose the flute that delivers it.
What does the stamp on the bottom of a box mean?
It is the box maker’s certificate, required by NMFC Item 222 and UFC Rule 41. It states the board strength, the maximum gross weight and the size limit the box is certified for, plus the manufacturer’s identity. A round stamp indicates a standard box, a rectangular one indicates a numbered or special package.
How do I convert points to GSM?
You cannot, not reliably. Points measure thickness and GSM measures weight, and the bridge between them is density, which varies by grade, coating and calendering. Any universal conversion chart is describing one supplier’s stock. Specify whichever unit your requirement actually depends on.
Does a higher ECT always mean a stronger box?
It means a higher edge crush value, which is stacking strength only. It says nothing about puncture resistance, and nothing about how the box performs once the board has absorbed humidity or been stored under load for weeks. Laboratory values are a starting point, not a guarantee of field performance.
Sources
- Fibre Box Association, flute profiles and the variability caveat
- TAPPI/ANSI T 811 om-23, edgewise compressive strength of corrugated fiberboard
- TAPPI/ANSI T 810 om-22, bursting strength of corrugated and solid fiberboard
- ISO 3037, edgewise crush resistance, international equivalent to ECT
- National Motor Freight Classification Item 222, motor carrier requirements
- Uniform Freight Classification Rule 41, rail carrier requirements
- ISTA 6-Amazon.com-SIOC overview, published by the International Safe Transit Association
NMFC Item 222 and UFC Rule 41 are published behind a subscription. The tables above were confirmed against multiple independent reproductions that agree with each other, including one hosted by the Fibre Box Association. Item 222 has been amended over time, so for a contractual specification you should confirm against a current copy rather than any secondary source, this page included.
Using this to specify your own boxes
The practical version of everything above is short. Write down the weight and the outside dimensions you actually ship, pick the row that covers it with margin, decide whether stacking or puncture is your real risk, and specify the test that matches. Then specify caliper separately if thickness matters to the look or fit.
We make boxes to that specification with no minimum order, so you can order the quantity you need rather than the quantity that makes a mill’s setup worthwhile. Every order includes a free design proof before anything is printed, and free samples so you can test the board in your own packing line before committing. If you send us the product weight and dimensions we will tell you which board it needs and why, including when a cheaper board is sufficient.
A right sized custom box also usually costs less to ship than a stock box that is close enough, because carriers bill on dimensional weight. That saving tends to be larger than the difference in box price.