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Edge Crush and Box Compression in Plain English
The edge crush test measures the strength of corrugated board on its edge. The box compression test measures how much weight a finished box can take from the top before collapsing. Use ECT to shortlist a board grade, then confirm BCT on your actual box size and print spec. Pair with snug inserts to prevent crush without overspec, weight or cost.
If you are reviewing cardboard postal boxes, start here, then explore our full range of Postal Boxes.
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ECT vs BCT: What They Actually Tell You
ECT stands for Edge Crush Test: It measures how much force the edge of a corrugated board strip can take before it crushes. Think side pressure on conveyors or boxes stacked tightly in cages.
If you are asking, what is edge crush test, this is it. It is a lab test done on the board itself, not the finished box.
BCT stands for Box Compression Test: It measures how much weight a fully made box can handle from the top before it collapses. This reflects the whole spec. Flute, liners, dimensions, glue, print coverage and even humidity all affect the result.
In simple terms: ECT tells you about the board, BCT tells you about the finished cardboard postal box.
If you need help choosing between flutes before looking at ratings, read Choose the Right Flute for Strength and Weight → /blog/choose-the-right-flute-for-strength-and-weight
Why Parcels Crush in the Real World
Boxes do not fail in labs, they fail in transit.
Conveyors squeeze from the sides, cages stack 10 or more layers high, vans wedge parcels into tight gaps, returns travel back through the same system again.
Flat, dense items like books concentrate load at the edges, and bottles create point loads that press panels inwards.
The goal is not maximum strength. It is just-enough strength plus good internal bracing.
Ratings, Units and Common Mix-Ups
What an ECT rating is and what it is not
An ECT rating is the result of the edge crush test method carried out in a lab. It applies to the board grade only.
Suppliers often quote values like 32 ECT. This means the board achieved 32 pounds per inch of edge crush strength under a US test standard.
An ECT rating helps you compare like for like board options. It does not guarantee a specific BCT for your box size. Dimensions and design still matter more.
If you are searching for an edge crush test calculator or edge crush test chart, be cautious. Conversions give estimates, but real BCT depends on your exact box.
Units: what you will see on spec sheets
Edge crush test unit of measure: Usually kN per metre in the UK and EU, or lb per inch in US style ratings. For example: 32 ECT is roughly 5.6 kN per metre. This is a guide only, not a precise conversion.
If you need an edge crush test unit of measure conversion, ask your supplier to confirm both values so you are comparing fairly.
Box compression test unit of measure: Usually kN or N of peak top load before collapse. Some older reports show kgf. Ask for SI units to keep comparisons simple.
Always confirm the test standard used in the edge crush test procedure. For example:
- ISO 3037 for ECT
- ISO 12048 or ASTM D642 for BCT
This avoids apples to oranges comparisons.
Safe Starting Points for Books, Bottles and Mixed Goods
Rules of thumb only. Always test before rollout.
| Category | Typical weight per parcel | Product risk | Starting board and cues | Insert or bracing | Step-up triggers |
|---|---|---|---|---|---|
| Books and media | 0.5–2.0 kg | Dense, edge loaded | B flute single wall around 29–32 ECT | Tight book wrap, corner stiffeners | Multi book packs over 2 kg or depot stacking → EB or higher ECT |
| Bottles, cosmetics, candles | 0.5–1.5 kg | Point loads, breakable | B flute around 32–38 ECT | Pulp tray or die cut card cradle | Twin packs or courier stacking → BC double wall |
| Mixed goods, gift sets | 1.0–3.0 kg | Voids and movement | B or C single wall 32–38 ECT depending on size | Partition cards to stop panel bowing | Larger formats or returns logistics → EB double wall |
These are starting ranges, not guarantees. Real world BCT depends on box size, print coverage, humidity, tape and palletisation.
For books specifically, see Book Wraps that Protect Corners and Spines.
How ECT Influences BCT Without the Heavy Maths
Higher ECT usually means higher potential BCT, if everything else stays the same.
But size changes everything.
- Bigger boxes bow more easily.
- Taller boxes buckle sooner.
- Heavily inked panels can slightly soften fibres if over printed.
- Double wall boxes generally hold more load than single wall.
- Tight glue joints also help.
Practical takeaway: Choose a sensible ECT tier. Then confirm BCT on your actual postal boxes cardboard spec with a lab test or controlled stack check.
How to Hit Strength Without Adding Bulk
When to move from single to double wall
Move from B flute to EB or BC when:
- Parcels exceed 2 kg.
- You are shipping multiple glass items.
- Warehouse stacking or long returns loops are involved.
Choose EB when height limits or print quality matter. Choose BC for maximum BCT and stacking strength.
When inserts beat heavier board
Moulded pulp or die cut card partitions stabilise products and stop panels bowing inwards. Often, the right insert can match the performance of a heavier flute at lower thickness and lower grams. Mono material paper based inserts also help keep packs recyclable and aligned with sustainability targets.
Quick Checks Before You Roll Out a New Box
Sample acceptance checklist
Run these simple checks on every new cardboard postal box:
- Board grade declared, including flute and liner weights, plus ECT rating with units.
- BCT claim for a box close to your size.
- Squeeze test. Firm two hand squeeze on side panels. No instant crease or cracking sound.
- Corner rigidity. Corners should not twist easily.
- Top load check. Stack 10 to 15 kg on a packed sample for 24 hours. No panel collapse or tape tearing.
- Drop tests. 1 metre drop on corner, edge and flat. No product damage with your insert.
- Humidity check. If you dispatch in humid conditions, let samples sit for 24 hours before testing.
Audit loop: damage reasons to spec tweak
Record every crush failure. Panel, corner or seam.
- If panels bow, add a partition or step up one ECT tier.
- If corners buckle, consider double wall or reduce box height.
- If seams fail, review glue, tape and slot depth with your manufacturer.
Spec Checklist for Buying
Add this to your RFQ:
- Box style and internal size in mm.
- Target weight per parcel in kg.
- Route risk: Stacking, pallets, returns, storage time.
- Board grade, flute and liner weights, plus ECT rating with units.
- Target outcome: For example, pass 15 kg top load for 24 hours and survive three way drop with pulp insert.
- Evidence: Recent edge crush test and box compression test reports for the proposed grade.
- Sustainability: Recycled content percentage, FSC claim, aqueous coating if required.
If you are reviewing specs for postal boxes cardboard, explore the full range on our website.
FAQs
What is the Edge Crush Test?
A lab test that measures a corrugated board’s edgewise compressive strength. In simple terms, how well it resists being squashed on its edge.
How do you check an Edge Crush Test rating when buying boxes?
Ask suppliers to show the ECT rating with units on quotes and samples. Keep units consistent across all suppliers.
How do you calculate Edge Crush Test?
You do not calculate ECT in house. Labs test small strips under controlled load. Use the published value to compare boards, then verify BCT on your chosen box.
How do you perform an Edge Crush Test?
In the standard edge crush test method, a lab cuts a board strip, stands it on edge and compresses it between plates at a set speed until failure. The peak force per width gives the ECT rating.
What is the unit of measure for the Edge Crush Test?
kN per metre in the UK and EU, or lb per inch in US style ratings such as 32 ECT.
What is the unit of measure for the box compression test?
kN or N of peak top load before collapse.
What is the difference between burst strength and Edge Crush Test?
Burst strength measures resistance to puncture under hydraulic pressure. Edge crush test measures edgewise compression. For postal boxes cardboard, ECT and BCT are usually more predictive of stacking survival.
What is ECT 32 and what does it mean?
ECT 32 is a common single wall board rating. It means 32 lb per inch of edge crush strength, roughly 5.6 kN per metre. It is a board property, not a guarantee for any box size. Always confirm BCT on your finished box.
