Why 80 lb cover is heavier than 80 lb text
Basis weight is the weight in pounds of 500 sheets of a paper cut to that grade's basis size — and every grade has a different basis size. Text and book papers are weighed at 25 × 38 in, cover at 20 × 26 in, bond at 17 × 22 in, index at 25.5 × 30.5 in, tag at 24 × 36 in and bristol at 22.5 × 28.5 in. A cover basis sheet is 520 in² against a text basis sheet's 950 in², barely more than half the area, so 500 cover sheets weighing 80 lb are made of a far denser sheet than 500 text sheets weighing the same 80 lb.
Run the numbers: 80 lb cover is 80 × 1406.139 ÷ 520 = 216.3 g/m², while 80 lb text is 80 × 1406.139 ÷ 950 = 118.4 g/m². The cover stock is 216.3/118.4 = 1.83 times the mass per unit area, from an identical-looking number on the label. That single fact is responsible for more wrong paper orders than any other convention in the trade.
Grammage has no such ambiguity. Grams per square metre is mass divided by area, so it describes the sheet itself rather than a commercial packaging convention. Convert everything to gsm first and comparisons become trivial: any two papers with the same gsm have the same mass per unit area, whatever their grade names say.
Where the conversion constant comes from
The constant 1406.139 is not a lookup value or a mill convention — it is a unit conversion you can derive in one line. Basis weight B is the mass of 500 basis sheets in pounds. Convert to grams by multiplying by 453.59237, and divide by the total area of those 500 sheets in square metres, which is 500 × A × 0.00064516 when A is the basis sheet area in square inches:
gsm = B × 453.59237 / (500 × A × 0.00064516) = B × 1406.139 / A
Both constants are exact by definition — there are exactly 453.59237 grams in a pound and exactly 0.00064516 square metres in a square inch — so the ratio 453.59237 / (500 × 0.00064516) = 1406.139 is exact too. Many published charts round it to 1406.5, which is close enough for a catalogue and not close enough to reconcile a freight invoice.
Going the other way is the same relation rearranged: B = gsm × A / 1406.139. Because the only thing that changes between grades is A, converting a paper from one grade name to another is one multiplication by the ratio of their basis areas. Text to cover, for instance, is 520/950 = 0.5474, which is exactly why 100 lb text and 55 lb cover sit next to each other in every equivalence chart.
M weight is the term a print estimator actually buys against: the weight in pounds of 1,000 sheets of the actual size ordered, not the basis size. It is what the mill prices from, what the freight bill is built from, and what determines how many sheets go on a skid. Once you know gsm, M weight is gsm × the real sheet area in square metres × 1000 ÷ 453.59237. When the sheet you order happens to be the basis size, the M weight comes out at exactly twice the basis weight, because a ream is 500 sheets and M weight counts 1,000.
Worked example: 70 lb text on a 25 × 38 sheet, 5,000 sheets
A job calls for 70 lb text (25 × 38 basis) and the paper is being bought at the basis size, 5,000 sheets.
- Grammage. 70 × 1406.139 ÷ 950 = 98,429.7 ÷ 950 = 103.61 g/m². That is why this stock is sold in Europe as "100 gsm" — close, but 3.6% heavier than a true 100 gsm sheet.
- Sheet area. 25 × 38 = 950 in², and 950 × 0.00064516 = 0.612902 m².
- One sheet. 103.61 × 0.612902 = 63.50 g.
- M weight. 63.50 × 1,000 ÷ 453.59237 = 140.0 lb per thousand sheets. Sanity check: the sheet ordered is the basis size, so M weight should be twice the 70 lb basis weight, and it is.
- Ream weight. Half of that, 70.0 lb per 500 — the basis weight again, by definition.
- Run weight. 63.50 g × 5,000 = 317,500 g = 317.5 kg, which is 317.5 × 2.20462 = 700.0 lb. Two skids' worth of paper for one modest job.
Now change the sheet size. Suppose the same 70 lb text is bought as a 23 × 35 press sheet instead. Area is 805 in² = 0.519354 m², so one sheet is 103.61 × 0.519354 = 53.81 g and the M weight is 53.81 × 1,000 ÷ 453.59237 = 118.6 lb. The basis weight has not moved — it is still 70 lb text — but the weight you pay for per thousand sheets falls to 118.6/140.0 = 85% of the basis-size figure, because 805 in² is 805/950 = 85% of the paper per sheet. Basis weight identifies the stock; M weight is what you pay for.
How to read the result
Use gsm to compare, use basis weight to order. If two vendors quote 70 lb text and 100 gsm respectively, the calculator tells you they are 103.6 and 100.0 g/m² — a 3.6% difference in mass, and possibly a visible difference in opacity and stiffness on a book job. If a designer specifies "100 lb cover" and the printer offers "270 gsm", those are the same sheet.
Read the equivalence table as one paper with six names. Every row is the same physical stock priced under a different grade convention. The row that matters is the one your supplier quotes in, but seeing all six at once catches the classic error of comparing a cover number against a text number.
Watch the M weight when the sheet size changes. Two quotes for the same basis weight at different sheet sizes are not comparable until you convert both to M weight, because paper is sold by weight and delivered by sheet.
Use the run weight for freight and skid planning. Freight classification for paper is driven by weight and density, and the run weight here is the input to that. If you are shipping the finished job rather than the raw stock, the dimensional weight shipping calculator tells you whether the carrier will bill on actual weight or on cube, and the freight density and class calculator handles the LTL side.
Grammage does not tell you thickness. Caliper depends on bulk, which varies with the furnish and the finish: an uncoated, high-bulk book paper is noticeably thicker than a coated gloss sheet at the same grammage. If thickness matters — for a spine, a folding score or a mailing thickness limit — ask for the caliper in points or microns, and use the book spine width calculator, which works from pages and caliper rather than from weight.
Basis sheet sizes and the conversion factor for each grade
| Grade | Basis sheet size | Basis area (in²) | gsm per lb of basis weight |
|---|---|---|---|
| Bond / Writing / Ledger | 17 × 22 in | 374 | 3.760 |
| Text / Book / Offset | 25 × 38 in | 950 | 1.480 |
| Cover | 20 × 26 in | 520 | 2.704 |
| Index | 25.5 × 30.5 in | 777.75 | 1.808 |
| Tag | 24 × 36 in | 864 | 1.627 |
| Bristol | 22.5 × 28.5 in | 641.25 | 2.193 |
The last column is 1406.139 ÷ basis area. It also shows why the grade names are not interchangeable: a pound of bond basis weight is 3.760/1.480 = 2.54 times as much grammage as a pound of text basis weight.
Common US paper weights converted to grammage
| Grade and basis weight | g/m² | Grade and basis weight | g/m² |
|---|---|---|---|
| 16 lb bond | 60 | 65 lb cover | 176 |
| 20 lb bond | 75 | 80 lb cover | 216 |
| 24 lb bond | 90 | 100 lb cover | 270 |
| 28 lb bond | 105 | 90 lb index | 163 |
| 32 lb bond | 120 | 110 lb index | 199 |
| 60 lb text | 89 | 140 lb index | 253 |
| 70 lb text | 104 | 100 lb tag | 163 |
| 80 lb text | 118 | 125 lb tag | 203 |
| 100 lb text | 148 | 100 lb bristol | 219 |
Note 90 lb index and 100 lb tag both land at 163 g/m² — the same sheet weight under two grade names, because 90/777.75 and 100/864 are within 0.03% of each other.
Mistakes that put the wrong paper on the skid
- Comparing basis weights across grades. 80 lb text and 80 lb cover differ by a factor of 216.3/118.4 = 1.83. Convert both to gsm before deciding anything.
- Assuming 'text' and 'book' and 'offset' differ. They share the 25 × 38 basis size, so their weight numbers are directly comparable to each other — the names describe finish and furnish, not weight.
- Quoting M weight without stating the sheet size. M weight is per 1,000 sheets of the actual size ordered. The same stock at 23 × 35 and at 25 × 38 has M weights 15% apart.
- Treating gsm as thickness. Bulk varies with furnish and finish. Two 150 gsm sheets can differ measurably in caliper, which matters for spines, scores and mailing thickness rules.
- Using the rounded 1406.5 constant on a big run. It is fine for a chart and wrong by a few kilograms on a truckload, which is enough to argue about on a freight invoice.
- Forgetting spoilage in the sheet count. The run weight is only as good as the sheet count you put in, and the sheets that matter for freight are the ones ordered, not the ones that survive to be delivered.
- Ignoring grain direction when converting sheet sizes. Grammage is unaffected by grain, but the sheet you can actually cut from a parent is, and that changes the sheet count that drives the run weight.
Metric grades have their own conventions too
Grammage removes the basis-size ambiguity, but it does not remove every ambiguity. Mills quote grammage to ISO 536, which specifies conditioning at 23 °C and 50% relative humidity before weighing — paper is hygroscopic, and a sheet weighed straight off a humid loading dock reads heavier than its specification. Board is often specified by caliper or by both grammage and caliper together, because for a folding carton the stiffness that comes from thickness matters more than the mass. And a metric sheet size such as SRA3 or B1 has no relationship to any US basis size, so when you switch to metric stock, switch to grammage and sheet area and stop translating pounds entirely.
Ordering paper, and checking what arrives
Specify three things and the order cannot be misread: grade, basis weight and sheet size. "70 lb text, 25 × 38" is unambiguous. "70 lb" alone is not, and "70 lb cover" is a completely different sheet at 189.3 g/m² rather than 103.6. When you are dealing with a mill that quotes metric, add the grammage as a cross-check — if the two figures the calculator produces do not both appear on the acknowledgement, somebody has translated and you should find out how.
Reconcile the delivery by weight, not by count. Paper is made and sold by weight, so a skid is the most reliable thing to check. Multiply the M weight by the thousands of sheets delivered and compare with the bill of lading: 12 cartons of 500 sheets at a 140 lb M weight should weigh 6 × 140 = 840 lb of paper before packaging. A discrepancy of a few per cent is usually moisture or a rounded conversion constant; a discrepancy of ten per cent is usually the wrong stock.
Watch the mill tolerance. Grammage is a target with a permitted variation, not an exact value, and that variation is agreed between mill and buyer rather than fixed universally. On a long run the practical consequence is that your calculated run weight and the delivered weight will differ slightly in either direction, which is normal; what is not normal is a consistent bias in one direction across several deliveries.
Then plan downstream from the sheet, not from the ream. The imposition decides how many sheets the job needs, and the sheet count decides the weight — the press sheet imposition calculator handles the first step and feeds the sheet count straight into this one.
Key terms
- Basis weight
- The weight in pounds of 500 sheets of paper cut to that grade's basis size. Meaningless without knowing the grade.
- Basis size
- The reference sheet dimensions at which a grade's basis weight is measured: 25 × 38 in for text, 20 × 26 in for cover, and so on.
- Grammage (gsm)
- Mass per square metre. Independent of grade, sheet size and packaging convention, which is why it is the right unit for comparison.
- M weight
- The weight in pounds of 1,000 sheets of the actual size ordered. The figure paper is priced and freighted on.
- Ream
- 500 sheets. A ream of the basis size weighs exactly the basis weight, which is the definition the whole system rests on.
- Caliper
- Sheet thickness, in thousandths of an inch (points) or microns. Related to grammage through bulk, but not derivable from it.