Most packaging specifications haven't been reviewed since the person who wrote them left the company. The result is businesses paying for board grades they don't need, or destroying product with board grades that aren't adequate. Understanding what the codes on your spec sheet actually mean is the first step to fixing either problem.
What the flute letter tells you (and what it doesn't)
Corrugated board gets its properties from the fluted (wavy) middle layer, the medium, bonded between flat outer sheets called liners. The flute letter tells you roughly how thick that layer is and how it performs under load.
C flute (around 4mm) is the workhorse of UK transit packaging. It's used in probably 70% of standard shipper boxes because it balances stacking strength, cushioning and print surface adequately for most applications. If you've inherited a specification that doesn't specify flute type, it's almost certainly C flute.
B flute (around 3mm) is thinner and denser, which makes it better for printability and resistance to point-load crushing. It's commonly used for point-of-sale trays, smaller product boxes and retail-ready packaging where the carton needs to look good on shelf.
A flute (around 5mm) is the thickest standard flute and gives the best cushioning per unit of compression. Used for fragile goods, certain fresh produce packaging and some export applications. It uses more material than C flute for equivalent box dimensions.
E flute (around 1.5mm) is closer to cartonboard in feel. It accepts print very well and is widely used for cosmetics, giftware and premium retail packaging where appearance is the priority rather than transit strength.
BC double wall combines B and C flute layers between three liners. It substantially increases stacking and compression resistance and is the right choice for heavy goods or products stored in high-pallet stacks. The mistake I see regularly is specifying BC double wall on products that don't warrant it. The board costs more, weighs more, and adds to your EPR fee liability.
Why the liner grade changes everything
The flute code is only half the story. The liner (the flat paper sheets bonded either side of the medium) determines burst strength and edge crush performance as much as the flute does.
Liners are described by weight in grams per square metre (gsm) and fibre type. Kraft liner (typically 125–200gsm, brown) is made from virgin or semi-virgin fibre and outperforms test liner (recycled fibre) on strength at equivalent weight. A box made with 150gsm kraft liner will handle roughly 20-30% more compressive load than the same structure in 150gsm test liner.
Your specification should show both. A reference like 125T/C/125K means 125gsm test liner outer, C flute medium, 125gsm kraft inner. If your spec just says "single wall corrugated" with no liner detail, you have no way of knowing what strength you're actually getting, and your supplier has no obligation to maintain it between runs.
The over-specification error that's costing you money
The most common over-specification I see is BC double wall on goods that could be adequately protected with 150gsm kraft liner single wall C flute. BC double wall typically costs 25-35% more per square metre than equivalent single wall. A 150gsm kraft liner C flute box will often exceed the stacking performance of a 125gsm test liner BC double wall, because the liner quality matters more than doubling the flute layers.
The most common under-specification is running standard 125gsm test liner C flute on heavy goods stored three pallets high in a humid warehouse. If your product sits in cold storage, a high-humidity distribution centre or is palletised for extended periods, the board needs to be specified for that environment, not for a dry ambient warehouse at ground level.
If you're uncertain which side of the line your current specification falls on, ask your supplier for edge crush test (ECT) data, measured in kN/m, and compare it against the actual compressive load your bottom boxes face in the worst-case stack. If they can't provide ECT data, that tells you something about the relationship too.