The short answer
A callout like 42/26/42 names three papers in pounds per 1,000 square feet. Outer liner, corrugating medium, inner liner.
Those three numbers do not add up to the board’s weight. The medium is fluted, so it carries more paper per square foot of board than its flat weight suggests. A C-flute board runs about a tenth heavier than the sum.
A board callout looks like a part number. It is closer to a bill of materials, and reading it tells a buyer what paper went into a sheet and roughly what that sheet costs.
Every page that explains the notation gets the definition right. Almost none of them do anything with it, which leaves the two questions a buyer actually has unanswered.
The first is arithmetic, since adding the three numbers gives a figure wrong by a predictable margin, and the correction depends on which flute the board runs.
The second is harder. A lighter set of papers is not automatically a weaker board, because board strength rests on how the fiber resists compression rather than on how much of it there is.
This reference covers the unit and the standard weights, then the take-up factor that fixes the arithmetic. It also covers why that factor is a range, and where paper weight is still written into the rules.
What Basis Weight Measures
Paper is sold by the weight of a given area, not by thickness. A box maker’s own explainer puts it plainly: basis weight is pounds per 1,000 square feet, so 42 means 1,000 square feet of that paper weighs 42 pounds.
The shorthand follows from there. A 42 pound liner gets written 42#, and a board callout strings three of those figures together in the order outer liner, medium, inner liner.
Doublewall adds two more numbers, since it carries three liners and two mediums. A five-figure callout is a doublewall board rather than a typing error.
The two components do different work. A box maker’s linerboard page describes the liners as the flat outer layers, and they are what a printing plate, a staple or a forklift tine meets first.
The medium never touches the outside, and it holds the liners apart at a fixed distance, which is what turns two thin sheets into a stiff panel.
The glued assembly carries its own name. Liners and medium together make combined board, and a callout describes its recipe rather than its properties.
The weights in use are not arbitrary. The same source lists linerboard at 26, 33, 38, 42, 47, 56, 69 and 90 pounds, with corrugating medium at 23, 26, 30, 33 and 40.
| Component | Common basis weights (lb/MSF) | Role in the board |
|---|---|---|
| Linerboard | 26, 33, 38, 42, 47, 56, 69, 90 | The flat facings that carry bending and puncture loads |
| Corrugating medium | 23, 26, 30, 33, 40 | The fluted core that spaces the liners apart |
Outside North America the same property gets quoted in grams per square metre, and the two are not interchangeable without converting.
The conversion is arithmetic anyone can check. A thousand square feet is 92.9 square metres and a pound is 453.6 grams, so one pound per 1,000 square feet works out to 4.88 grams per square metre.
Run it on a familiar number: a 42 pound liner comes to 205 grams per square metre, which is how the same paper gets quoted in a metric specification.
Going the other way, divide by 4.88, or multiply by 0.205. A 200 gram liner is a shade over 41 pounds, which is why 42# and 200 gsm turn up as near equivalents on supplier sheets.
The liners are the easy part. Everything interesting about a callout sits in the middle number.
Why the Numbers Do Not Add Up
A square foot of finished board does not contain a square foot of medium. The medium runs through the corrugator and comes out fluted, so it travels further than the liners glued to it.
Converters handle that with a take-up factor. Multiply the medium’s flat basis weight by the factor for its flute profile, then add the liners at face value.
One converter publishes the method and its factor set together. The factors run A flute 1.54, B flute 1.35, C flute 1.44, E flute 1.27 and F flute 1.22.
The method itself is one line, where weight per unit area equals the liners plus each medium multiplied by its own factor.
Put the C-flute figure to work on a common board. Two 42 pound liners give 84 pounds, and 26 pounds of medium at 1.44 gives 37.4, for a combined 121.4 pounds per 1,000 square feet.
Compare that against the sum of the printed numbers. Adding 42, 26 and 42 gives 110, which understates the board by about 10 percent.
Doublewall compounds the effect. A 42/26/26/26/42 board in BC flute carries three liners totaling 110 pounds, plus two mediums at their own factors.
Work the mediums separately, where the B medium gives 26 at 1.35, or 35.1 pounds, and the C medium gives 26 at 1.44, or 37.4 pounds.
Add the fluted paper and the gap widens. Those two mediums contribute 72.5 pounds, taking the combined board to 182.5 pounds per 1,000 square feet against a printed sum of 162.
That is an understatement of nearly 13 percent, and the more mediums a construction carries, the further the printed figures drift from the real one.
| Flute | Take-up factor | 42/26/42 combined (lb/MSF) |
|---|---|---|
| A | 1.54 | 124.0 |
| B | 1.35 to 1.43 | 119.1 to 121.2 |
| C | 1.44 to 1.46 | 121.4 to 122.0 |
| E | 1.27 | 117.0 |
| F | 1.22 to 1.25 | 115.7 to 116.5 |
The medium’s share surprises people. At face value 26 of 110 reads as under a quarter of the board, while after take-up it is 37.4 of 121.4, which is nearer a third.
That matters for cost as much as for weight. Lightening the medium by a pound removes about a pound and a half of paper from every thousand square feet. The middle number is the most leveraged one in the callout.
Why the Factor Is a Range
A take-up factor is not a constant of nature. It follows from how many flutes the corrugator puts into a given length, and that count varies inside each profile.
A second converter’s flute table shows the mechanism. It gives flute counts per linear metre as ranges rather than single figures, alongside a nominal height for each profile.
| Flute | Nominal height | Flutes per linear metre |
|---|---|---|
| A | 5.0 mm | 108 to 120 |
| C | 4.0 mm | 128 to 140 |
| B | 3.0 mm | 154 to 166 |
| E | 1.6 mm | 295 to 310 |
| F | 0.8 mm | 420 to 440 |
Shorter flutes pack closer together, so an F flute puts four hundred or more flutes into a metre of board where an A flute manages barely a hundred.
More flutes per metre means more paper consumed per metre of board. A profile quoted as a range of flute counts therefore has a range of take-up factors behind it.
The published values reflect that. The same table lists A at 1.54, B at 1.43, C at 1.46, E at 1.27 and F at 1.25.
Set the two sets side by side and the pattern is clear, because they agree on A and E while diverging on B, C and F.
The B flute gap is the widest. Running 1.35 against 1.43 moves the combined weight of a 42/26/42 board by about 2 pounds per 1,000 square feet.
That is small on one sheet and real across a year of orders, and it is also the difference between a freight estimate that lands and one that drifts.
So the practical rule is to treat these figures as planning numbers. A quotation, a freight estimate or a cost model should use the factor the converter running the board actually applies.
Weight Is a Proxy, Not the Property
Basis weight is easy to measure and easy to compare, which is why it ended up doing work it was never designed for.
The link to strength is real. One industry treatment calls basis weight and flute caliper the two most important performance indicators of raw material quality.
It goes further than correlation, since that treatment states basis weight as measured in pounds per 1,000 square feet directly impacts paper strength.
That holds within a furnish and a process, so take the same pulp, the same refining and the same machine, and more fiber per square foot gives a stronger sheet.
It stops holding across them, because what a board’s edge crush rests on is how well its components resist compression in the plane of the sheet. That property is measured in its own right.
Two tests measure it. Ring crush forms a 12.7 mm strip into a ring and compresses it at 12.5 mm per minute until it buckles, under ISO 12192 and TAPPI T822.
Results come back by direction, since the test reports separately along and across the machine direction, and paper is stiffer along the grain than across it.
The newer method narrows the span. Short-span compression clamps the specimen to a 0.7 mm free span under ISO 9895 and TAPPI T826, for papers up to 400 grams per square metre.
That short span is the whole point. With almost no free length, the sheet cannot buckle as a structure.
The reading therefore reflects load-bearing fiber content rather than sheet geometry, which is the reason short-span compression is steadily replacing ring crush.
Which explains how a lighter callout can match a heavier one. Stronger fiber, better orientation or a different refining strategy raises the compressive strength per pound, so the same edge crush arrives on fewer pounds of paper.
It also explains why swapping callouts on price alone goes wrong. Two 42 pound liners from different mills are the same weight and not necessarily the same paper.
Where Weight Is Still the Rule
None of that removes weight from the specification, because carriers wrote weight into their own requirements decades ago and it stayed.
Those requirements pair a strength figure with a paper figure. The same industry treatment reproduces carrier tables that set a minimum combined weight of facings next to the burst or edge crush value.
One row shows the shape of it: a box rated for a 20 pound maximum contents weight carries a 52 pound minimum combined facing weight.
Combined weight of facings means the two liners added together, with the medium excluded. A 26/26/26 board has 52 pounds of facings, which is how the numbers in those tables line up.
Grades get written as callouts in the tables for the same reason. Constructions such as 26/26/26 and 33/26/33 appear directly against their minimum burst figures.
The tables themselves are paid documents, so they are described here rather than reproduced. Anyone writing a certificate or settling a damage claim needs the controlling classification text rather than a summary.
The practical consequence is that two tests look at different parts of the board. Burst responds mainly to the facings, which is why the rule is written against their combined weight.
Edge crush answers to the whole glued structure instead, as the ECT grade chart sets out.
That same treatment notes burst correlates poorly with stacking strength, which is the honest reason the industry moved toward edge crush for stacking questions in the first place.
What a Callout Leaves Out
Three numbers describe three papers. A board has several other properties that decide how it behaves, and none of them appear in the callout.
Flute profile is the first omission. A 42/26/42 in B flute and the same callout in C flute are different boards. Caliper, stacking behavior and take-up all change, as the corrugated flute types reference covers.
Wall count is the second. Reading a five-number callout as a long singlewall misses that the board is doublewall, with an extra liner and an extra medium.
Furnish is the third and least visible. Recycled and virgin liners at the same basis weight differ in fiber length and in compressive strength, which is exactly the gap the component tests expose.
Adhesive and treatments do not show either. Water-resistant adhesive, wax alternatives and coatings change performance in humidity without changing a single digit of the callout.
Moisture is the quiet one. Paper gains and loses water with the air around it, so a board’s measured basis weight moves with conditioning, which is why test labs condition samples before they measure anything.
Ordering by Weight or by Performance
A purchase specification can anchor on either, and the choice has consequences worth naming.
Specifying the callout locks the paper. The supplier has to deliver those basis weights, which gives repeatability and blocks a quiet substitution to lighter stock.
Specifying performance locks the outcome instead. Calling for an edge crush value, a burst value or a box compression result leaves the mill free to hit it with whatever construction works, including a lighter one.
Many buyers end up writing both, with the performance figure as the requirement and the callout as a reference construction. That combination is also how a substitution request becomes a conversation rather than a surprise.
Whichever route a specification takes, two questions are worth putting to a supplier. Which take-up factor does the plant apply to this profile, and what component compression figures does the mill report for these liners?
Both answers already exist, because a corrugator runs to a known flute count and a mill tests its own output, so neither question asks anyone to invent a number.
Common Questions
What does 42/26/42 mean on a corrugated spec?
Three papers in pounds per 1,000 square feet: a 42 pound outer liner, a 26 pound corrugating medium and a 42 pound inner liner. The order runs outside to inside, and a five-number callout means doublewall rather than a typing error.
Is a heavier board always stronger?
Not across different papers. More fiber per square foot helps within one furnish and process. What edge crush actually tracks is the compressive strength of the components, which short-span compression measures directly, so a lighter liner with stronger fiber can match a heavier one.
How do pounds per 1,000 square feet convert to grams per square metre?
Multiply by 4.88, since a thousand square feet is 92.9 square metres and a pound is 453.6 grams. A 42 pound liner is 205 grams per square metre. Going back the other way, multiply grams per square metre by 0.205.
Why does the combined board weigh more than the three numbers added together?
Because the medium is fluted and consumes more paper per square foot of board than its flat weight implies. Multiplying the medium by a take-up factor of roughly 1.44 for C flute takes a 42/26/42 board from an apparent 110 pounds to about 121.
Where these figures come from and where they stop. Basis weight lists and take-up factors are published by converters and vary between them, so a cost model should use the figure from the plant running the board. Carrier classification tables are paid documents and appear here only as described, with one attested row. Measured basis weight also depends on conditioning, so a comparison between two reported figures only holds if both were conditioned the same way.
Key takeaways
- A callout names three papers in pounds per 1,000 square feet, running outer liner, medium, inner liner, with five numbers meaning doublewall.
- The three numbers do not sum to the board’s weight, because the fluted medium needs a take-up factor of roughly 1.44 for C flute.
- Take-up is a range rather than a constant, since flute counts per metre vary inside each profile and published factors disagree most on B flute.
- Weight predicts strength only within one furnish and process; ring crush and short-span compression measure the property board performance actually rests on.
- Carrier requirements still set a minimum combined weight of facings, which counts the two liners and excludes the medium.