If you have shopped for shipping boxes, you have seen ratings like 32 ECT, 200# test, or C-flute printed on spec sheets and stamped inside box flaps. These numbers tell you exactly how strong a box is, but nobody explains them in plain language. Here is what they mean, how they compare, and how to know whether 32 ECT is right for what you ship.

ECT: The Stacking Strength Rating

ECT stands for Edge Crush Test. A lab compresses a small section of corrugated board on its edge, the same direction a box gets squeezed when boxes are stacked on top of it, and measures how many pounds of force per inch it withstands before crushing.

A 32 ECT board withstands 32 pounds per inch of edge. That does not mean the box holds 32 pounds of product. It means the walls resist stacking pressure, which is what matters in a warehouse, on a pallet, and in a courier truck where your box spends most of its life underneath other boxes.

ECT became the standard rating for modern shipping because most box failures are stacking failures. Boxes rarely burst outward from their contents. They crush when the pallet above them is heavy, the humidity is high, or they sit at the bottom of a stack for weeks.

ECT vs Burst Test: The Old Rating You Still See

Before ECT, boxes were rated by the Mullen burst test, written as 200# test or 200 lb test. That test measures how much pressure it takes to rupture the box wall from inside, which mattered more in the era of loose rail freight when boxes were tossed and snagged.

The two ratings measure different things, so there is no exact conversion, but they are treated as rough equivalents:

For most businesses shipping everyday products, 32 ECT is the workhorse. It is the most common board grade in Canada for a reason: strong enough for parcel networks and pallet stacking, light enough to keep material and shipping costs down.

What About the "C" in 32C?

You will sometimes see our boxes described as 32C. The C refers to the flute, the wavy layer of paper sandwiched between the flat liners. Flutes come in sizes:

• A-flute is the tallest wave, maximum cushioning, used for fragile goods
• B-flute is a fine wave, good print surface, common for retail packaging
• C-flute sits between them and is the general-purpose standard, balancing crush resistance, cushioning, and stackability
• E and F flutes are very fine, used for small retail boxes and mailers

So 32C means 32 ECT board built with C-flute: standard strength, standard flute, the default choice for shipping and storage. That is what every stock box we make is built from.

How to Know If 32 ECT Is Enough

Ask three questions:

How heavy are your contents? As a general guideline, 32 ECT single wall handles contents up to about 65 lb in a properly sized box. Most e-commerce orders, packaged goods, documents, apparel, and household items fall well under that.

How high will boxes stack? Stacking is where ECT earns its keep. If your boxes sit two or three high in a stockroom, 32 ECT is comfortable. If you stack pallets of dense product to the ceiling for months, talk to us about heavier board.

How is the box filled? A well packed box is dramatically stronger than a half empty one, because contents help support the walls. If your product leaves empty space, choose a smaller box before you choose a stronger one. Our earlier guide on choosing the right box size covers this in detail.

If your product is unusually heavy, sharp edged, or stacked in demanding conditions, that is when custom board grades make sense. We manufacture custom corrugated boxes in heavier grades and double wall at our Scarborough facility.

Where to Find the Rating on Any Box

Flip a corrugated box over and look for the round Box Maker's Certificate stamped on a bottom flap. It lists the board grade (ECT or burst test), the wall construction, and size limits. If you are ever comparing supplier quotes, check the certificate rating rather than taking "heavy duty" at face value. Two boxes that look identical can be very different board grades, and price differences usually trace back to exactly this.

The Short Version

• ECT measures stacking strength, the way boxes actually fail
• 32 ECT is the Canadian standard for general shipping and storage, roughly equivalent to the old 200# burst test
• C-flute is the standard middle layer, so 32C means standard strength with standard flute
• Choose a heavier grade only if your contents are very heavy or your stacking is extreme; otherwise 32 ECT keeps costs down without giving up protection

Every in-stock box at Prime Carton is 32 ECT kraft, available in 12+ sizes with volume pricing and delivered flat across the GTA. Browse the price list or request a custom quote if your product needs something built to spec.

Choosing the right corrugated box size is one of the most practical decisions you'll make for your business. The wrong size means wasted void fill, higher shipping costs, and products that shift in transit. The right size means a clean, professional shipment and lower per-unit packaging costs.

Here's how to figure out exactly what you need.

How to Measure Your Product

Before picking a box, measure your product at its widest points — length, width, and height. These are the interior dimensions you need the box to accommodate. Always measure in the order Length × Width × Height (L×W×H).

Add a small buffer — typically 1 to 2 inches on each side if you're using void fill or wrapping material. If your product is a rigid item going straight into the box with no padding, you can go with a tighter fit.

Understanding Corrugated Box Sizes

Corrugated boxes are measured by their interior dimensions. A box listed as 12×10×8 means the inside of the box is 12 inches long, 10 inches wide, and 8 inches deep. This is the space your product needs to fit into.

When browsing sizes, keep in mind:

• Small boxes (6" to 11") — best for compact items, small parts, cosmetics, and retail goods. Light and easy to handle.
• Medium boxes (12" to 15") — the most versatile range. Good for e-commerce orders, food packaging, and general retail shipping.
• Large boxes (16" to 20") — suited for bulkier items, multi-product orders, and larger retail goods.
• Oversized boxes (21" to 36") — moving, furniture parts, industrial equipment, and anything that doesn't fit a standard range.

What Is 32C Kraft Corrugated?

Most standard shipping boxes — including all of Prime Carton's in-stock sizes — are made from 32 ECT (Edge Crush Test) single wall Kraft corrugated board. This is the industry standard for everyday shipping and storage. It handles the typical bumps, stacking, and humidity of regular transit without being unnecessarily heavy or expensive.

For heavier items or anything being stacked under significant weight, double wall corrugated (which uses two layers of fluting) offers significantly more crush resistance.

Choosing Based on Use Case

E-commerce and order fulfillment: Go for a snug fit. Oversized boxes increase dimensional weight charges from couriers. A box that fits your product with 1–2 inches of buffer on each side is ideal.

Moving and storage: Use medium to large boxes for most household items. Keep heavy items like books in small boxes — a large box packed with books becomes impossible to lift safely.

Retail and distribution: Your box size should fit your product plus any retail packaging or inserts. Consistency across your product line makes palletizing and warehouse stacking much more efficient.

Food and beverage: Size for your product configuration — whether that's a single item, a pack, or a case. If your product is temperature-sensitive, a tighter fit helps maintain pack integrity.

A Practical Sizing Example

Say you're shipping a product that measures 9×7×5 inches. You'd want a box with interior dimensions of roughly 11×9×7 — giving you about an inch of clearance on each side for light void fill. That falls squarely in Prime Carton's 12"–15" size range.

Custom Sizes vs. Stock Sizes

If your product fits one of our standard sizes, in-stock boxes are the fastest and most cost-effective option. They ship flat, are available in bundle quantities, and can be ordered online.

If your product has unusual dimensions — very flat, very long, or an irregular shape — a custom corrugated box manufactured to your exact specifications will reduce void fill costs and give your product a cleaner fit. Prime Carton manufactures custom sizes at our Scarborough facility.

Need Help Choosing?

Browse our corrugated boxes by size range — 6" to 11", 12" to 15", 16" to 20", and 21" to 36" — or request a custom quote if you need something built to spec. We deliver to businesses throughout the Greater Toronto Area from our Scarborough facility.