Understanding Shipping Box Strength

Understanding Shipping Box Strength

Posted By on Dec 20th 2023

When you ship your products, you must ensure that they remain safe. No matter how much you trust your shipping partner, you never know what might happen on the road, and you need to trust that your shipping box will protect your products so that they will arrive to your customers intact.

On the other hand, you also want to ensure your packaging is economical. Stronger boxes are heavier and, therefore, more expensive to ship, and they use more materials, which costs more and is less environmentally friendly. You should find that sweet spot between protection and practicality when choosing your boxes.

As packaging experts, we know the ins and outs of shipping box strength and how to choose the perfect box for your products. Here’s our guide.

How is cardboard strength measured?

The strength of the shipping box sheet is described with a number and letter (e.g., 23B or 23ECT B). The number notes the Edge Crush Test (ECT) rating, which describes how much weight a single sheet can withstand if standing on its edge. The letter describes the tightness of the waves of the corrugate, otherwise known as the fluting.

What is an edge crush test (ECT)?

Corrugated strength is most commonly described using ECT. It tells you how much weight a corrugated sheet can withstand when standing up on its edge in a lab setting. For example, a 23A corrugated sheet can withstand 23 pounds of pressure, as can 23B, 23C, and 23E.

Another common way to note corrugated box strength is with The Mullen Test. This test measures how much pressure (in PSI) a corrugated sheet can withstand when horizontally suspended in the air. While this method is relevant for specific applications, ECT works well for most small carrier shipping boxes. ECT is also the only strength measurement applicable to boxes made with recycled content.

ECT determines whether a box is strong enough for its contents. Heavier products will need boxes with a higher ECT. However, you must consider more than just the ECT. The size and type of box you choose will also play a part. For example, a 32C box can carry more weight than a 32E box because it’s thicker.

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.

What are flutes?

Flutes are the s-shaped, wavy paper between the linerboard of a corrugated sheet. It gives corrugated boxes their strength and resistance to bending and pressure. Flutes vary by their height and how tightly or loosely they are waved.

types of corrugated flutes

Below are standard flute sizes from thickest to thinnest.

  • A Flute: 36 flutes per foot and 1/4" thick. While it works great for shock absorption, strength, and cushioning, it exceeds the needs of 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. It’s designed to resist crushing and is good for folding, making it an excellent selection for TL and LM boxes.
  • E Flute: 94 flutes per foot and 1/16” thick. The thinnest of the shipping-appropriate flutes, it makes an excellent printing surface and has a nice overall presentation.
  • F Flute: 125 flutes per foot and 1/32” thick. It’s inappropriate for shipping or eCommerce, but it can be interesting for certain primary packaging applications. Not offered by EcoEnclose.

For added strength, corrugated sheets can also be double- or triple-wall. eCommerce shipping boxes are generally made with single-wall B, C, or E flutes, all striking an appropriate balance of strength and lightness.

Why shipping box strength is important

Cardboard strength ratings are essential to understand the type of box you need fully. Once you know the specs of your shipping box strength for your products, you can easily order the boxes you need.

In terms of optics, choosing the proper shipping box strength will not only protect your items but also show your customers that you care. A high-quality box can often denote high-quality products and services to your customers. However, respecting your customers by keeping shipping costs low is essential. Thicker, heavier boxes might portray high quality, but they’ll cost more to ship. The shipping box strength you choose must protect your products, display quality, and minimize costs; it’s a delicate balance that can sometimes be difficult to find.

If you’re having trouble determining which shipping box strength measurements you need, contact us or order free samples to see which packaging suits your eCommerce business.

Learn more: The Ultimate Guide to Custom Shipping Boxes

Which corrugated material should I use?

When deciding which shipping box durability is best for your business and products, there are a few things to consider. It goes beyond ETC and fluting. You’ll need to consider box style, cost efficiency, and more to determine which custom shipping box best fits your brand.

Box style

For a presentation-style box (such as a Tab Locking Box or Literature Mailer), you will probably want to select a board that’s easy to fold, such as a B or E flute. While we make presentation boxes with thicker material, the folding and creasing may be a little more cumbersome. We typically suggest 32 ECT C or 44 ECT for RSC boxes, depending on the weight of the product shipped. We can make most custom shipping boxes from most boards, but we suggest ordering a sample first.

Protecting your product

If you’re shipping small, light items that are not fragile, you likely don’t need the heavy-duty box. However, you’ll likely need something a little sturdier if you're shipping fragile items. When determining which box will best support your product, consider the following.

  • Can your product be bent?
  • Is the product heavy, and could it potentially break through the box?
  • Will it need extra padding during transit?
  • Does it need protection from punctures or other potential hazards?

Answering these questions will help you narrow down your packaging offerings.


Some corrugated sheets, especially 32 ECT E, are thinner and sleeker, resulting in more successful printing. Conversely, the 44 ECT has fewer flutes that are twice as wide and create small ridges on the printing surface. Such ridges make it difficult to get an even print finish. Therefore, if the aesthetics of the box are essential to your brand, consider tighter fluting.

Cost efficiency

Consider the box cost, the inbound shipping (to your fulfillment location), and the outbound shipping (to your customer). Stronger board material usually has more cardboard material per square inch. This is going to cost more.

EcoEnclose's Corrugated Sheet Strengths

EcoEnclose offers five different corrugated sheet options: 23ECT B Flute, 32ECT B Flute, 32ECT C Flute, 32ECT E Flute, and 44ECT C Flute. We work with these five options because we have found these satisfy nearly all non-industrial shipping needs and are the most cost-effective options to meet said needs. As you consider your options, here’s a handy chart to determine the best sheet type for your box.

Sheet Box Style* Definition Use If: Avoid If:
23 ECT B Flute LM, TL, OPF, Tray Economy: 1/8" thick. Our lightest weight and least expensive board that uses minimal source material. More subject to crushing as it is not as strong or as stiff as other options. You ship items that collectively weigh less than 5lbs, want to minimize costs or want a basic print on the outside of the box. You ship heavy items (>15lbs), have a large box (greater than 32 inches of L + W + H), and want to print on the inside of the box.
32 ECT B Flute RSC, LM, TL, FPF, OPF Standard: 1/8" thick. Its higher ECT rating gives it more strength and rigidity than 23B and provides more cushion and shock resistance than 32E. You ship items that collectively weigh between 15lbs and 25lbs.
32 ECT C Flute Large LM, Large TL, RSC, FPF, OPF, Tray Strength & Cushion: 11/64” thick. One of our two thickest options, providing even more cushioning. You ship items that collectively weigh 15-25lbs or have a larger tab locking box or literature mailer greater than 30 inches in L+W+H. You ship very light or small materials. B flute can be 15-20% cheaper than C Flute.
32 ECT E Flute LM, TL Design: 1/16” thick. Thinner and more aesthetically pleasing than 23B and 32B. Same ECT rating as 32B, but is more subject to bowing and offers less cushioning. You ship items that collectively weigh 15-25 lbs. You want a very clean design, an excellent printing finish or to print on the inside of the box. Ideal for limited storage space because the material is thinner than B or C flute. You ship very light or small materials. Raw material on 23 ECT can cost 15-20% less than that of 32 ECT.
44 ECT C Flute RSC, FPF High Strength: 11/64” thick. Our thickest option is heavy-duty and our most expensive board. You ship items that collectively weigh up to 40lbs or you have a high-value product that needs the heavy-duty crush protection. You ship very light or small materials. Raw material on 44 ECT can cost 40-50% more than that of 32 ECT.

*EcoEnclose recommendations; not provided by UPS.

Our three most popular shipping box strengths are 32E, 32B, 23B. Notices the differences in thickness, and the faint lines in 23B.

EcoEnclose Box Board

Shop custom shipping boxes

Ordering custom shipping boxes is simple. Select your style, dimensions, strength, and branding - we'll handle the rest. For branded orders, we have a real, live human proof your artwork prior to production to ensure your vision is captured into a beautifully finished product.

Get started: order custom shipping boxes here