The Box Files · File 02
File 02Flutes & ECT,
explained
Every corrugated quote mentions a flute letter and an ECT number. Here's what they mean and which ones you actually need.
What a flute is
Cut a corrugated box open and you'll see a wavy layer running between the flat outer sheets. That wave is the flute. Its size — how tall and how tightly packed the arches are — determines how the board behaves: thinner flutes print better and fold tighter; thicker flutes cushion better and stack stronger.
The three flutes that matter
| Flute | Thickness | Best at | Typical use |
|---|---|---|---|
| E | ~1/16 in (1.5 mm) | Crisp printing, tight folds | Mailer boxes, retail-facing packaging |
| B | ~1/8 in (3 mm) | Balance of print and protection | Heavier mailers, product shippers |
| C | ~5/32 in (4 mm) | Cushioning and stacking strength | Standard shipping boxes (RSC) |
There are others (A, F, double-wall combinations), but for small-business packaging, E, B, and C cover nearly every job.
What ECT means
ECT stands for edge crush test — a lab measurement of how much stacking force the board withstands before its edge buckles. It's the standard strength rating for corrugated board, and it's the number that tells you whether a box will survive a pallet stack or a courier van floor.
- 32 ECT — the workhorse. Handles most e-commerce parcels up to roughly 30 lb (13 kg).
- 44 ECT — for heavier products, or boxes that get stacked high in storage or freight.
- Double-wall grades — two fluted layers, for genuinely heavy or fragile freight.
E-flute for mailers people see, C-flute at 32 ECT for shippers they don't. Upgrade the ECT before you upgrade the flute if weight is the concern.
Why not just order the strongest?
Because you'd pay for strength you don't use. Heavier board costs more per box, weighs more (which can raise your shipping rates), and folds less cleanly. The right grade is the lightest one that reliably survives your product's journey — that's the number a good quote should land on.
Put it to work
Tell us what you're packing — product, rough dimensions, quantity — and we'll send back a straight quote, including a cheaper alternative if one exists. Minimum order: 500 units.
Request a quote