A-flute, B-flute, C-flute, and the double-wall combinations built from them — what each one actually is, how thick it runs, and when it’s used. Reference guide, not a sales pitch.
Flute comparison
A flute is the wavy corrugated medium sandwiched between a board’s liners. Different flute profiles trade off thickness, cushioning, printability, and stacking strength — that’s the entire reason more than one flute type exists.
B-flute, single-wall — thinnest common flute (≈0.121")
The thinnest common flute, with the flattest surface for printing. Common in point-of-sale trays, canned goods, and printed retail cartons.
C-flute, single-wall — most common for shipping cartons (≈0.159")
The default single-wall flute for most shipping cartons (RSC) — balances stacking strength, material cost, and printability.
A-flute, single-wall (≈0.201")
The thickest common single-wall flute — best cushioning and stacking strength per given ECT. Used for wraps and heavier protective packaging.
B/C-flute, double-wall — most common double-wall combination (≈0.258")
The most common double-wall combination. Used when a single wall can't reach the required stacking strength.
A thicker, heavier-duty double-wall combination for the heaviest standard loads before triple-wall.
Nominal caliper values (B ≈0.121", C ≈0.159", A ≈0.201", BC ≈0.258", AC ≈0.338") are drawn from the National Motor Freight Classification Item 222 / Rule 41 caliper reference (200#/MSF combined-weight row) — the same values the Warehouse Stacking Calculator uses. Actual caliper varies by mill, combination, and combined board weight; treat these as typical values for comparison, not a substitute for your supplier’s spec sheet.
Why flute matters
Stacking strength
Flute caliper is a direct input to the McKee formula, which predicts box compression strength — a thicker flute reaches a given stacking-strength requirement at a lower ECT grade. The Warehouse Stacking Calculator uses these same nominal caliper values to suggest a standard ECT grade live from your own stack configuration.
Material and pricing
A thicker flute uses more combined-board material for the same box size, which shows up in basis weight and price per MSF. The Price per MSF Calculator converts $/M, $/MSF, and $/lb for any sheet size, so you can check that trade-off against a real quote.
Flute caliper is only half the strength story — the other half is which certification you’re reading it against. See ECT vs. Mullen (Burst Test) for the difference.
Flute type questions
What's the difference between B-flute and C-flute?
B-flute runs about 0.121" thick, C-flute about 0.159". C-flute is the more common choice for shipping cartons because its extra thickness gives more stacking strength for a given ECT; B-flute's thinner, flatter profile is more common in printed retail cartons and point-of-sale trays, where appearance and space matter more than maximum stacking strength.
Which flute is most common for shipping cartons?
C-flute. It's the default single-wall flute for most Regular Slotted Containers (RSC) because it balances stacking strength, material cost, and printability better than A- or B-flute for general shipping use.
Does flute type affect box price?
Indirectly, yes. A thicker flute uses more combined-board material for the same box size, which shows up in basis weight (lb/MSF) and therefore price per MSF. It can also let you use a lower ECT grade to hit the same stacking strength, which cuts the other direction. The Price per MSF Calculator and Warehouse Stacking Calculator on this site let you check both sides of that trade-off for a specific box.
What is double-wall board, and when is it needed?
Double-wall board combines two flute layers into one board — most commonly B/C, sometimes A/C for heavier duty. It's used when a single-wall flute can't reach the required compression strength for the load, storage time, or stack height involved, without pushing the ECT grade so high it stops being cost-effective or available. The Warehouse Stacking Calculator flags this automatically if your required strength exceeds what standard single-wall grades can deliver.