Board grades, calculated - not guessed

Three things: the box size (length x width x depth in mm), the gross mass of one packed box in kg, and how high it stacks - counting the bottom box, so a stack of 5 means the bottom one carries 4 above it. Quote your product's dimensions if that is what you have; clearance is added to get to the inside size of the box. If any of the three is missing the assistant asks for it rather than assuming, because a specification built on an invented number looks finished and is worthless.

Say where it ships and it infers them. Ocean or intercontinental freight is treated as humid with medium-term storage; a tropical destination or refrigerated chain as tropical; an ordinary unconditioned warehouse as temperate; only a climate-controlled facility counts as controlled. Storage bands are under 10 days, 10-30 days, 30-100 days and beyond. Stacking defaults to column-aligned - tell it if you stack interlocked, because that pattern trades roughly half the compression strength for pallet stability. Mention water or moisture exposure and moisture-resistant liners become a hard requirement rather than a cost trade-off.

You get a grade code such as SW-E-26 or DW-AC-82-MR - single or double wall, the flute, the ECT value, and MR if the liners are moisture-resistant. The margin is the headroom: 1.0 is exactly at failure, so 1.27 means the board clears the applied load by 27 percent. If you ask for the working, it will show the laboratory compression strength, the derating factor applied for your conditions, and the effective strength that remains - the last of those is the number that matters, because laboratory strength is not warehouse strength.

Because the compression engine actually ran for that answer. The assistant has one calculation tool; when it has your box size, mass and stack height it calls it, and the tool runs the McKee formula and applies the derating. Answers carrying the Computed badge contain figures that were calculated. Answers without it are ordinary prose - the assistant is not permitted to state a grade, ECT value or margin that did not come from the tool, so if it has not run, it asks instead of guessing.

That is a real answer, not a failure, and you get the detail behind it: the strongest catalogued grade, how far short it falls, the ECT that would actually be needed, and the stack height that would work. It also gives you the ways out - lower the stack, carry the load on pallet racking so the bottom box is not holding the column, lighten the box, or improve the storage climate. Changing the footprint alone rarely closes a large gap, because compression scales with the square root of the perimeter.

It returns the first grade in the catalogue that carries the load, which is the cheapest one that passes. Material is the dominant cost in corrugated and it is paid on every box you ever ship, so a board specified two grades above what the conditions call for is a permanent, recurring loss. Anything stronger than the recommendation is over-specification that your conditions do not justify - if you want margin beyond that, ask for it deliberately.

It does not quote prices, lead times, die-lines or a full specification - for those, use the contact page and an engineer will pick it up. The calculation covers boxes from 30 to 2000 mm per side, up to 500 kg gross, stacked up to 30 high; outside that range it will tell you it is out of scope rather than answer anyway. Messages are capped at 2000 characters, and the service allows 10 messages every 10 minutes - it is free and shared, so the limit keeps it available. If the calculator is ever unreachable the page says so and will not quote a grade.