Packaging Guide
ECT vs Burst Strength: Choosing Corrugated Box Strength
By Alice Wu, CEO & Founder · Editorial policy Published
Short answer
ECT (Edge Crush Test) measures how much edge pressure corrugated board takes before crushing, so it predicts stacking strength. Mullen burst measures the pressure needed to rupture the board face, so it predicts resistance to rough handling and punctures. For stacked shipping cartons, ECT is usually the more useful number.
The ect vs burst question comes down to what will hurt your box most: being stacked under weight, or being dropped, punctured and thrown around. ECT (Edge Crush Test) predicts stacking strength, while Mullen burst predicts how well the board resists rupture from rough handling. Both are valid ways to rate corrugated board, and most box makers can quote either one. For cartons that sit on pallets, in containers and in fulfillment center racks, ECT usually tells you more.
This guide explains what each test measures, how the two ratings roughly line up, how flute choice changes the picture, and how to pick a carton spec for FBA shipments from China.
What the Edge Crush Test Measures
The Edge Crush Test takes a small strip of corrugated board, stands it on its edge and presses down until it crushes. The result is reported in pounds per linear inch (lb/in). A board rated 32 ECT withstood 32 pounds of force per inch of edge before it failed.
Why does this matter? When cartons are stacked, the weight above presses down on the vertical walls of the box below. The walls behave like columns. Board that resists edge crushing makes stronger columns, so the box resists collapse under a stack.
ECT is also the input to the widely used McKee formula, which estimates a box’s compression strength from its ECT, board thickness and perimeter. You don’t need to run the formula yourself, but it explains why ECT is the number packaging engineers care about for stacking.
What the Mullen Burst Test Measures
The Mullen test clamps a piece of board over a rubber diaphragm and inflates it until the board bursts. The result is reported in pounds per square inch (lb/in²). In North America you will often see it written as 200#, 275# and so on, where the number is the burst strength.
Burst strength tracks closely with the strength of the outer liners. Heavier liners give a higher burst rating. That makes the Mullen number a good proxy for resistance to punctures, tears and bursting when a box is dropped or hit by another package.
It says much less about stacking. Two boards with the same burst rating can perform differently under a stack, because burst does not measure how well the fluted middle layer holds the walls upright.
ECT vs Burst at a Glance
| Factor | Edge Crush Test (ECT) | Mullen burst test |
|---|---|---|
| Unit | lb/in | lb/in² (written as 200#, 275#) |
| What is tested | Edge strip crushed vertically | Board face inflated until it ruptures |
| Main predictor of | Stacking and compression strength | Puncture and rough-handling resistance |
| Driven mostly by | Liner and medium combined, flute structure | Liner weight and quality |
| Best fit | Palletized cartons, container loads, warehouse storage | Loose parcels, heavy items in single-box shipments |
| Material use | Can reach a rating with lighter, better-engineered board | Often needs heavier liners for a higher rating |
Common ECT and Burst Equivalences
Box makers and buyers often convert between the two ratings using equivalence charts. The pairs below are the ones you will see most often in typical industry charts. They are approximate, not exact. The two tests measure different things, so a board that meets one rating does not automatically meet the other.
| Board type | Typical ECT rating | Typical burst equivalent |
|---|---|---|
| Single wall | 23 ECT | 125# |
| Single wall | 26 ECT | 150# |
| Single wall | 29 ECT | 175# |
| Single wall | 32 ECT | 200# |
| Single wall | 40 ECT | 250# |
| Single wall | 44 ECT | 275# |
| Double wall | 48 ECT | 275# |
| Double wall | 51 ECT | 350# |
The most common spec for general shipping cartons is 32 ECT, roughly equal to 200# single wall. When cartons are heavier or will be stacked high, buyers step up to 44 ECT single wall or a double wall board.
If your contract or customer specifies one test, ask for that test. Don’t assume the equivalent rating satisfies it.
Flute Types and Why They Matter
The flute is the wavy middle layer between the liners. Flute size changes thickness, cushioning, stacking strength and print quality.
B flute
B flute is relatively thin, at roughly 3 mm. It has good puncture resistance and a flatter surface for printing. It is common for retail shipping boxes and smaller cartons.
C flute
C flute is thicker, at roughly 4 mm. It gives more cushioning and better stacking strength than B flute. It is the most common single wall flute for general shipping cartons.
E flute
E flute is thin, at roughly 1.5 mm. It is used for printed mailer boxes and retail packaging where a smooth print surface and a slim profile matter. It is not meant for heavy shipping cartons.
BC double wall
BC double wall combines a B flute and a C flute with three liners. It is much stiffer and stronger under a stack. Use it for heavy products, dense goods and cartons that will sit at the bottom of tall stacks for weeks at sea.
| Flute | Approx. thickness | Typical use |
|---|---|---|
| E | ~1.5 mm | Printed mailers, retail boxes |
| B | ~3 mm | Smaller shipping boxes, printed outer boxes |
| C | ~4 mm | Standard shipping cartons |
| BC double wall | ~6.5–7 mm | Heavy or high-stacked master cartons |
Thickness values vary by mill and machine, so treat them as rough guides.
Reading a Box Certificate Stamp
Many North American boxes carry a round stamp on the bottom flap called a box maker’s certificate. It usually shows:
- The box maker’s name and location
- Whether the rating is ECT or burst
- The rating itself, such as 32 ECT or 200 lb/in²
- For burst-rated boxes, often a maximum gross weight and a size limit
Chinese box factories don’t always print this stamp unless asked. If you need it for a customer or carrier requirement, request it before production and ask for a matching test report. A stamp is only as good as the board behind it.
A Note on Chinese Board Grades
Suppliers in China often describe corrugated board by liner paper grade, flute combination and grammage rather than by ECT or burst. You may see letter codes for liner grades, such as K for kraft liner, together with flute codes like “BC” and gram weights for each layer.
These descriptions are useful, but they are not a strength rating on their own. Paper quality varies between mills. If strength matters, ask for:
- The paper and gsm of each layer
- The flute type
- A measured ECT or burst value for the finished board
That gives you a number you can compare across suppliers.
How to Choose Strength for FBA Cartons
Start with carton weight
Heavier cartons need stronger board. Amazon sets limits on carton weight and dimensions, and cartons over certain weights need extra labeling. Check the current rules in our guide to Amazon FBA box size and weight limits before you set your packing quantity.
Think about the stack, not just the box
An FBA carton from China may sit on a pallet, ride in a container, wait in a port warehouse and then go into a fulfillment center. Humidity during sea freight can weaken board over time. If cartons will be stacked on pallets or floor-loaded high in a container, prioritize ECT.
Match board to the product
- Light, soft goods such as apparel: 32 ECT single wall in C flute is often enough.
- Dense or heavy goods close to the weight limit: consider 44 ECT single wall or a BC double wall.
- Fragile goods: board strength helps, but inserts, dividers and void fill matter more. See how to package fragile products for Amazon FBA.
Fill the carton properly
Underfilled cartons crush at the top because there is nothing supporting the lid. Overfilled cartons bulge and lose stacking strength at the walls. Size the carton to the contents and use fillers where needed.
Test before the full run
Pack a sample carton with real product and drop-test it at the corners, edges and faces. Stack a few full cartons and leave them for a day. It’s simple and catches most problems before they become damaged inventory.
Frequently Asked Questions
Is 32 ECT the same as 200# test board?
They are commonly treated as equivalent for single wall board, and many charts list them together. But they come from different tests, so a 32 ECT board is not guaranteed to pass a 200# burst test. If a spec names one test, ask for that one.
Is a higher burst rating always better for shipping cartons?
Not for stacked cartons. Burst mainly reflects liner strength and puncture resistance. A board with a high burst rating but a weak medium can still collapse under a stack, which is why ECT is usually preferred for palletized and containerized freight.
When should I switch from single wall to double wall?
Switch when cartons are heavy, dense or near your channel’s weight limit, or when they will sit at the bottom of tall stacks for long periods. Double wall costs more and adds volume, so test whether 44 ECT single wall is enough first.
Does humidity during sea freight weaken corrugated boxes?
Yes. Paper absorbs moisture, and damp board loses a significant part of its stacking strength. For long sea transits, choose a stronger board than the dry-condition minimum, keep cartons off wet floors and consider desiccants in the container.
Can I ask a Chinese factory for an ECT test report?
Yes. Ask for the measured ECT or burst value of the finished board, plus the paper gsm and flute for each layer. Compare suppliers on the measured values, not just the paper description.
How We Can Help
We produce shipping cartons, master cartons and printed boxes in our own factory in Longhua, Shenzhen, with a minimum order of 500 pieces per design. We can help you match board grade and flute to your product weight and shipping route. See our boxes and cartons service, or check pricing for how carton orders are quoted.