Every corrugated box you've ever used has a small circular stamp printed on one of its flaps. That stamp — called the Box Maker's Certificate or Box Certificate — contains critical information about the box's strength rating. Yet most people have no idea what those numbers mean or how to use them. Whether you're shipping consumer products, stacking pallets in a warehouse, or choosing packaging for fragile electronics, understanding ECT and Mullen ratings can save you money, prevent damage, and help you select the right box every time.
What Is ECT (Edge Crush Test)?
The Edge Crush Test measures a corrugated board's ability to withstand top-to-bottom compressive force. In simple terms, it tells you how much weight the box can support when stacked. The test involves placing a small section of the corrugated board on its edge between two parallel plates and applying increasing pressure until the board crushes.
ECT values are expressed in pounds per linear inch of load-bearing edge. A box rated at 32 ECT, for example, means that each linear inch of the box's edge can withstand 32 pounds of compressive force before collapsing.
Common ECT Ratings
Lightweight items up to 20 lbs. Mailer boxes, retail packaging, small consumer goods.
Standard lightweight, up to 30 lbs. Apparel, books, dry goods, subscription boxes.
Medium duty, up to 40 lbs. Small appliances, household items, retail merchandise.
Standard workhorse, up to 65 lbs. The most common rating — used for the majority of shipping boxes.
Heavy duty, up to 80 lbs. Electronics, automotive parts, canned goods, dense products.
Extra heavy duty, up to 95 lbs. Industrial components, heavy machinery parts.
Maximum single-wall strength, up to 120 lbs. Large industrial items, heavy equipment, warehouse storage.
ECT-rated boxes became the industry standard in the 1990s because they're more cost-effective. By focusing on edge compression rather than overall burst strength, manufacturers can produce lighter, cheaper boxes that still perform well in stacking scenarios — which is how most boxes are actually stressed during shipping and warehousing.
What Is Mullen (Burst Test)?
The Mullen Test — also known as the Burst Test — measures a corrugated board's resistance to puncture or rupture. A rubber diaphragm is pressed against the surface of the board with increasing hydraulic pressure until it bursts through. The result is measured in pounds per square inch (PSI).
A 200# Mullen rated box means the board can withstand 200 PSI of pressure before the surface ruptures. This is critical when the box must resist puncture from sharp edges, rough handling, or objects pressing against the sides.
Common Mullen (Burst) Ratings
Maximum contents up to 20 lbs. Lightweight consumer goods, small retail packages.
Maximum contents up to 35 lbs. Books, clothing, medium-weight consumer items.
Maximum contents up to 50 lbs. General purpose — hardware, small appliances, food products.
Maximum contents up to 65 lbs. The standard Mullen rating — equivalent in use to 32 ECT. Most common for shipping.
Maximum contents up to 95 lbs. Heavy-duty applications — dense products, canned goods, industrial items.
Maximum contents up to 120 lbs. Maximum single-wall Mullen strength. Heavy industrial, machinery, bulk storage.
How to Read the Box Certificate Stamp
Every corrugated box manufactured in the U.S. carries a round stamp on one of its bottom flaps, known as the Box Maker's Certificate (BMC). This stamp is your box's “birth certificate” and contains all the vital stats you need to evaluate its capability.
What the Stamp Tells You
- 1.Box style — Typically RSC (Regular Slotted Container), which accounts for about 80% of all shipping boxes.
- 2.Board construction — Single wall, double wall, or triple wall. This tells you how many layers of fluting exist.
- 3.Burst strength OR ECT — The stamp will show either a Mullen burst rating (e.g., “Bursting Test 200 lbs per sq. inch”) or an ECT rating (e.g., “ECT 32 lbs per in.”). Never both.
- 4.Size limit — The maximum combined length + width + height the box should be manufactured at for this rating.
- 5.Weight limit — The maximum gross weight (contents + box) the box is rated to carry.
- 6.Manufacturer — The name of the box maker and the facility where it was produced.
Single Wall vs. Double Wall vs. Triple Wall
The “wall” of a corrugated box refers to the layered construction of the board. Each wall consists of a fluted (wavy) medium sandwiched between flat linerboard sheets. More walls mean more strength — but also more weight and cost.
Single Wall (Double Face)
One layer of fluting between two linerboards. This is by far the most common type, making up roughly 80% of all corrugated boxes produced. It's versatile, lightweight, and cost-effective for the vast majority of shipping applications.
Double Wall
Two layers of fluting separated by three linerboards. Used when you need significantly more stacking strength or puncture resistance than single wall can provide. Common for heavy products, export shipments, and items requiring extra protection.
Triple Wall
Three layers of fluting with four linerboards. This is industrial-grade packaging used in place of wooden crates. It can support palletized loads, heavy machinery, and bulk industrial products. Some triple wall boxes are rated for contents over 300 pounds.
Flute Profiles: A Through F
The fluting — the wavy, corrugated medium between the linerboards — comes in different sizes called “flute profiles.” Each profile offers a different balance of cushioning, stacking strength, and printability. The profile is identified by a letter, and the choice dramatically impacts how the box performs.
| Flute | Thickness | Flutes/ft | Best For |
|---|---|---|---|
| A Flute | 3/16" (4.8mm) | 33 ± 3 | Maximum cushioning. Fragile items, glass, delicate products. Best stacking strength per thickness. |
| B Flute | 1/8" (3.2mm) | 47 ± 3 | Flat crush resistance. Canned goods, die-cut displays, point-of-purchase packaging. Superior print surface. |
| C Flute | 5/32" (4.0mm) | 39 ± 3 | The all-rounder. Most common flute profile globally. Balances cushioning, stacking strength, and economy. |
| E Flute | 1/16" (1.6mm) | 90 ± 4 | Thin, excellent print surface. Retail-ready packaging, pizza boxes, cosmetics, small electronics boxes. |
| F Flute | 1/32" (0.8mm) | 125 ± 4 | Ultra-thin micro-flute. Replaces folding cartons. Jewelry, cosmetics, premium retail packaging. |
Double wall boxes combine two different flute profiles to optimize performance. The most common combinations are BC flute (B flute + C flute) for general heavy-duty use, and AC flute (A flute + C flute) for maximum cushioning with good stacking strength. You'll sometimes see EB flute for applications that need a smooth outer print surface with reasonable cushioning.
When to Use ECT vs. Mullen Rated Boxes
Choosing between ECT and Mullen rated boxes depends entirely on how the box will be stressed during its life cycle. Here's a practical breakdown:
Choose ECT When:
- ●Boxes will be stacked on pallets in a warehouse
- ●Contents have smooth, flat surfaces (won't poke through)
- ●You're shipping via parcel carriers (UPS, FedEx) with palletized loads
- ●Cost efficiency is important (ECT boxes are typically 10–15% cheaper)
- ●You want the lightest box that still meets stacking requirements
Choose Mullen When:
- ●Contents have sharp edges, corners, or irregular shapes
- ●Boxes will be handled individually (not stacked)
- ●You're shipping LTL (Less Than Truckload) where boxes face rough handling
- ●Products are dense and heavy relative to box size
- ●Carrier or government regulations require Mullen certification
It's worth noting that approximately 80% of corrugated boxes sold in the United States today are ECT rated. The shift from Mullen happened because ECT testing more accurately reflects real-world stacking conditions, and ECT boxes can be produced with less material while maintaining equivalent performance.
Box Strength Calculator: Rule of Thumb
Need to quickly estimate whether a box can handle your product? Use McKee's Formula, the industry standard for estimating box compression strength (BCT) from ECT values:
McKee's Formula (Simplified)
BCT = 5.87 x ECT x (Box Perimeter)^0.5 x (Board Caliper)^0.5
Where BCT is the Box Compression Test value in pounds, ECT is the edge crush value in lbs/inch, the box perimeter is 2 x (Length + Width) in inches, and the board caliper is the thickness of the board in inches.
Quick Example:
For a 32 ECT single-wall C-flute box measuring 18" x 12" x 12":
Perimeter = 2 x (18 + 12) = 60 inches
Board caliper = 0.156 inches (C-flute)
BCT = 5.87 x 32 x (60)^0.5 x (0.156)^0.5
BCT = 5.87 x 32 x 7.75 x 0.395
BCT = approximately 576 lbs
Keep in mind that real-world conditions reduce this theoretical maximum. As a general safety guideline, apply a 3:1 safety factor for short-term storage (under 30 days) and a 5:1 safety factor for long-term warehousing. That means if your pallet load weighs 500 lbs on the bottom box, you'd want a box with at least 1,500 lbs BCT for short-term or 2,500 lbs BCT for long-term storage.
Environmental Factors That Affect Box Strength
The numbers printed on the box certificate assume ideal, dry conditions. In the real world, several factors can dramatically reduce a box's performance:
- ●Humidity: At 90% relative humidity, a corrugated box can lose up to 50% of its compressive strength. Even moderate humidity (60–70%) can reduce stacking strength by 20–30%. This is especially important in Atlanta's humid summer climate.
- ●Storage time: Boxes lose strength over time under continuous load. After 30 days of stacking, expect a 35–40% reduction. After 90 days, up to 50%.
- ●Pallet overhang: If boxes hang over the edge of a pallet by even an inch, stacking strength drops by 30% or more. Proper pallet pattern and box sizing is critical.
- ●Printing coverage: Heavy ink coverage on the box surface can reduce burst and compression strength by 10–20%, because the ink weakens the linerboard fibers.
- ●Hand holes and slots: Any cutout in the box wall reduces compression strength. A pair of hand holes can reduce BCT by 15–20%.
Practical Tips for Choosing the Right Box Strength
Weigh your heaviest product configuration
Include all inner packaging, void fill, inserts, and the product itself. Then add 10% for a safety margin.
Consider your stacking requirements
How many boxes high will they be stacked? Multiply the weight of one box by the number of boxes above the bottom box in your stack.
Factor in your environment
Will boxes be stored in a climate-controlled warehouse or an outdoor dock in July? Atlanta summers can push humidity above 80%, which significantly affects box performance.
Don't over-specify
Using 44 ECT boxes when 32 ECT will do wastes money and increases your shipping weight. Right-sizing your box strength is one of the easiest ways to cut packaging costs.
Test with real shipments
Before committing to a large order, send test shipments through your actual supply chain. Track damage rates and adjust your specifications accordingly.
ECT to Mullen Equivalency Guide
While ECT and Mullen measure different properties, there are generally accepted equivalencies for single-wall corrugated:
| ECT Rating | Mullen Equivalent | Max Weight (lbs) |
|---|---|---|
| 23 ECT | 125# Mullen | 20 lbs |
| 26 ECT | 150# Mullen | 35 lbs |
| 29 ECT | 175# Mullen | 50 lbs |
| 32 ECT | 200# Mullen | 65 lbs |
| 40 ECT | 275# Mullen | 80 lbs |
| 44 ECT | 275# Mullen | 95 lbs |
| 48 ECT | 350# Mullen | 120 lbs |
These equivalencies are approximate. An ECT-rated box of a given level will perform comparably to the corresponding Mullen-rated box for most shipping applications, but they are not interchangeable for regulatory or carrier compliance purposes. Always check your carrier's specific requirements.
Making the Right Choice for Your Business
Understanding box strength ratings isn't just an academic exercise — it directly impacts your bottom line. Over-specifying box strength means paying for material you don't need. Under-specifying means damaged products, returns, and unhappy customers. The sweet spot is finding the minimum strength that reliably protects your products through your specific supply chain.
At Box Atlanta, we help businesses throughout the greater Atlanta area find that sweet spot. Whether you need 23 ECT mailer boxes for e-commerce or double-wall 275# Mullen containers for industrial equipment, our team can recommend the right specification for your exact application — and supply it at competitive prices with new or quality used boxes.