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Understanding Shipping Box Strength

Understanding Shipping Box Strength

Mar 23rd 2022

Shipping box sheet strength is described with a number and letter. The number is the Edge Crush Test rating and describes how much weight a single sheet can withstand if it were standing up on its edge. The letter is the fluting and describes the tightness of the waves of the corrugate.

EcoEnclose's Corrugated Sheet Strengths

  • 23B (Economy): 1/8" thick. Our lightest weight and flimsiest option. This is our economy option - if it works effectively for your business, use it as it is both cheaper and uses less source material than our other boards.
  • 32B (Standard): 1/8" thick, but with a higher ECT rating, giving it more strength and rigidity than our 23B board. It also provides more cushion and shock resistance than our 32E board.
  • 32E (Design): 1/16” thick, making it thinner, sleeker and more aesthetically pleasing than our 23B and 32B options. While it has the same ECT rating as 32B, its thinness does mean it is more subject to bowing than 32B and offers less cushioning.
  • 32C (Strength, Cushion): 11/64” thick, making it one of our two thickest options, providing even more cushioning.
  • 44C (High Strength): 11/64” thick, making it one of our two thickest options and our heavy-duty board option. We use this for most of our outbound shipments, which tend to be over 20-30 lbs each. This is our most expensive board type.

EcoEnclose Box Board

Matching Sheet Strength to Box Style

Presentation Boxes

Standard Boxes

Boxes for Books

23B Packages are 5 lbs or less, boxes are small (L+W+H < 30 in), low cost. Not offered. Packages are 5 lbs or less.
32B Packages are up to 25 lbs, provides structural sturdiness and some cushioning. Preferred over 32E for larger boxes (L+W > 22 inches) because board is thicker, more cushioned and rigid, and less prone to bowing. Packages are up to 25 lbs and small (W+L+H < 30 in).

Packages are up to 25 lbs, more sturdiness than 23B.


Packages are up to 25 lbs, sleek aesthetic. Preferred over 32C for small to mid-sized boxes (L + W < 22), thinness makes it more ecologically efficient to ship.

Not offered.

Not offered.


XL sized box, packages are up to 25 lbs, provides more cushioning than 32B. Not standard. Contact us to see if this is right for you.

Packages are up to 25 lbs. Appropriate for almost all box sizes.

Packages are up to 25 lbs, more cushioning and sturdiness than B flute.


XL sized box, packages are up to 40 lbs, provided more cushioning than 32B. Not standard. Contact us to see if this is right for you.

Packages are 30-40 lbs. Most expensive option.

Not offered.

What are Flutes?

Flutes are the s-shaped, wavy paper between the linerboard of a corrugated sheet. It is what makes these sheets different from paperboard, giving them strength, and resistance to bending and pressure. Flutes vary by their height and how tightly or loosely they are waved.

Below are common flutes from thickest to thinnest. Corrugated sheets can also be double or triple wall for added strength. eCommerce shipping boxes are generally made with single-wall B, C, or E flutes, all of which strike an appropriate balance of strong and lightweight.

Shipping box fluting diagram

A Flute: 36 flutes per foot and 1/4" thick. While it works great when shock absorption, strength, and cushioning are needed, it is much more than is needed for small carrier shipping. Not offered by EcoEnclose.

C Flute: 39-43 flutes per foot and 11/64” thick. This is the go-to flute for standard box styles - RSCs and FPFs.

B Flute: 42-50 flutes per foot and 1/8" thick. Made to resist crushing and good for folding, making it a nice selection for TL and LM boxes.

E Flute: 94 flutes per foot and 1/16” thick. The thinnest of the shipping-appropriate flutes. Makes a great printing surface and overall presentation.

F Flute: 125 flutes per foot and 1/32” thick. Not appropriate for shipping or eCommerce, but can be interesting for certain primary packaging applications. Not offered by EcoEnclose.

What Is Edge Crush Test (ECT)?

Shipping box edge crush test

Shipping box strength is driven by three factors:

  • Box style
  • Box dimensions
  • Corrugated sheet strength rating

Corrugated sheet strength rating is driven by it's construction:

  • Flutes
  • Linerboard
  • Adhesive between flutes to linerboard

ECT is the most common way to describe corrugated strength. It tells you how much weight a sheet of corrugated can withstand in a lab setting when standing up on its edge. For example, a 23ECT corrugated sheet can withstand 23 pounds of pressure. This is true for 32B, 32C, and 32E.

The Mullen Test was the preferred method for measuring box strength for the last century. This test measures how much pressure (in PSI) a corrugated sheet can withstand when laying flat, suspended in the air. While this test is relevant for certain applications, ECT works well for the vast majority of small carrier shipping boxes. ECT is also the only strength measurement applicable to boxes made with recycled content.

Matching ECT with your exact package weight is not an exact science because the box dimensions and box type are also very important. For example, an 8" x 8" x 8" RSC box made with 32C can withstand more weight than 32B or 32E because it is thicker.

ECT is useful in ensuring your boxes are strong enough for their contents. Check out this reference from UPS, which has delivered millions of packages just like yours. UPS does not address 23B, so we have added our guidelines based on our experience.

Maximum Weight of Box and Contents (lbs/kg) Size Limit of Box Length, Width, and Depth Added (inches/cm)* Minimum Bursting Test (lbs per sq. inch/kg per sq. cm) Minimum Edge Crush Test (ECT) (lbs per inch/kg per cm width)
5-10* 30* 125 * 23*
30 75 200 32
40 75 200 40
50 85 250 44
65 95 275 55
80 105 350 N/A

*EcoEnclose recommendations; not provided by UPS.

Learn More & Shop EcoEnclose Shipping Boxes

Sheet strength is one of many decisions once you’ve chosen boxes as your shipping solution. Our Ultimate Guide to Shipping Boxes will answer:

  • What box style should I choose?
  • What corrugated sheet strength will best meet my needs?
  • What size should my shipping boxes be?
  • Should I custom brand my cardboard boxes?
  • How can I ensure my shipping boxes are as eco-friendly as possible?
custom corrugated shipping boxes

Custom Shipping Boxes

Choose your style, size, strength, and branding to create an exceptional unboxing experience. 100% recycled, 95% post-consumer waste.

custom discount shipping boxes

Discount Branded Shipping Boxes

Choose from set styles, sizes, and strengths customized with your branding. Discounts help you start your custom packaging journey.

discounted 100% recycled shipping boxes

Discount Blank Shipping Boxes

Choose from set styles, sizes, and strengths with no branding. Discounts make it even easier for you to ship sustainably.