Understanding Shipping Box Strength

Understanding Shipping Box Strength

Mar 23rd 2022

What Sheet Strength Should I Choose For My Custom Shipping Boxes?

Once you’ve chosen corrugated shipping boxes as your shipping solution, you have a few more decisions:

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

In this post, we help you decide what Corrugated Sheet Strength to choose for your shipping boxes.

At a Glance: Finding The Right Sheet Strength For Your Boxes, Business, And Products

There are a lot of different sheet types out there, and they are typically described with a number and then a letter, i.e.: 32B, 26B, 44C, etc. The number is the Edge Crush Test rating and describes how much weight a single sheet can (in a lab setting) withstand if it were standing up on its edge. The letter is the fluting and describes the tightness of the s curves or waves of the corrugate. 

When selecting your sheet strength, consider both the ECT and the fluting! Learn more about the technical characteristics of fluting and ECT below. Looking for some quick guidance? Stay here for a summary of recommendations.

The Five Sheet Types EcoEnclose Offers for 100% Recycled Custom Corrugated Shipping Boxes

  • 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 EcoEnclose Board Types to Your Box Style And Needs

Presentation Boxes

Standard Boxes

Boxes for Books

Tab Locking | Literature Mailer | Tray Regular Slotted Container | Five Panel Folder One Panel Folder
23B Use When: Your packages are 5 lbs or less, your boxes are reasonably small (L+W+H are less than 30 in), and you want to keep your costs as low as possible. Not offered. Use When: Your packages are 5 lbs or less.
32B Use When: Your packages are up to 25 lbs, structural sturdiness and some cushioning is important.

Preferred over 32E for larger, wider boxes (where L+W are over 22 inches) because the board is thicker which offers more cushioning, is more rigid, and is less prone to bowing in the middle.

Use When: Your packages are up to 25 lbs and relatively small, with W+L+H < 30 in.

Use When: Your packages are up to 25 lbs and you want more sturdiness than 23B would give you.


Use When: Your packages are up to 25 lbs and a sleek aesthetic is critical.

Preferred over 32C for small to mid-sized boxes (where L + W < 22) because it creates a more stylish box, and because its thinness makes it more ecologically efficient to ship.

Not offered.

Not offered.


Not standard.

If you are creating a very large box, your packages are up to 25 lbs, and/or want more cushioning than 32B would give you, contact us to see if this is right for you. 

Use When: Your packages are up to 25 lbs. Appropriate for almost all box sizes.

Use When: Your packages are up to 25 lbs and you want more cushioning and sturdiness than B flute would give you.


Not standard.

If you are creating a very large box, your packages are up to 40 lbs, and/or want more cushioning than 32B would give you, contact us to see if this is right for you. 

Use When: Your packages are consistently over 30 lbs but do not exceed 40 lbs. Do not use it if this level of strength isn’t needed for your business as 44C is more expensive than all other options.

Not offered.

Fluting in Action: Matching Board Types to Example Shipments

Small Accessories: If you are shipping small, lightweight accessories in a 6” x 4” x 2” box, we recommend 32E. If you prefer the slightly lower cost of 23B, this strength would also work well, but it won’t have the visual appeal of a 32E flute box.

Wine Bottles: If you are shipping multiple, heavy wine bottles in a large RSC box, we recommend 44C flute, which is strong and sturdy enough for these heavy glass bottles. You’d still need eco-friendly cushioning and protection, but 44C would withstand the weight of these products nicely.

Backpack: If you are shipping a large backpack in a Literature Mailer, we recommend 32B flute. This would provide more thickness and sturdiness than both 32E and 23B. This would minimize the risk of bowing or crushing in the middle of the large, wide box.

Water Bottle: If you are shipping a water bottle in an FPF box, we recommend 32B, which is a great economical option for a lightweight FPF.

Technical Information: What are flutes exactly?

Shipping box flutes for strength

The flute of the corrugated is the s-shaped, wavy paper between the linerboard. The flute is what makes corrugated sheets unique and different from paperboard, giving them strength, and resistance to bending and pressure. Flutes vary in two ways – their height and how tightly or loosely they are waved or fluted.

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

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

C Flute: 11/64” thick and 39-43 flutes per foot. This is the go-to flute for standard boxes - RSCs and FPFs. This is a good time to mention that fluting does NOT go in order! A is the thickest, then comes C. And while B is thinner than both A and C, it is not as thin as E or F flute.

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

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

Corrugated sheets can also be double wall or even triple wall for added thickness and strength. eCommerce shipping boxes are generally made with single-wall flutes and are typically B, C, or E flutes, all of which strike an appropriate balance of creating strong boxes that are relatively lightweight.

Technical Information: What does Edge Crush Test mean?

Shipping box edge crush test

The strength of a specific box is driven by three factors:

  • The box style
  • The box dimensions
  • The strength rating of the corrugated sheets used to make the box

This strength rating of a corrugated sheet can be measured in different ways and is driven by several aspects of how the corrugated is constructed, including:

  • It’s fluting
  • The material and thickness of the linerboard
  • The adhesive used to glue the flutes to the linerboard

ECT (or Edge Crush Test) is the most common way to describe corrugated strength today. ECT tells you how much weight a sheet of corrugated can withstand when is standing up (on its edge). For example, a 23ECT corrugated sheet can withstand 23 pounds of pressure stacked on its edge, while a 32ECT can withstand 32 pounds. This is true for 32B, 32C, and 32E.

You may sometimes read about the Mullen Test, which was the preferred method for measuring box and corrugated 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 the Mullet Test may be relevant for certain applications, ECT strength works well for the vast majority of small carrier shipping boxes. ECT is also the only strength measurement that can apply to boxes made with recycled content.

Unfortunately, it is not an exact science of matching the ECT rating with your exact package weight. The box dimensions and box type do matter quite a bit. For example, an 8x8x8 inch RSC box made with 32C can withstand more weight than 32B or 32E, because it is thicker.

Despite the importance of considering your ECT (the number) and your fluting (the letter), ECT ratings are very useful in ensuring your boxes will be strong enough for their contents. Check out this reference from UPS, which has delivered millions of packages just like yours. Note that UPS does not address the 23B material, but based on our own experience, we have added our guidelines to the chart below.

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.

Shop EcoEnclose Shipping Boxes & Learn More

Check out our  Definitive Guide to Shipping Boxes - We break down what you need to know about this shipping solution - when to use them, when not to use them, what strength and size you need, how much does it cost, and how you can consider mother nature throughout your decision process.

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