Why 70 lb text and 70 lb cover are different papers
Basis weight is the weight in pounds of 500 sheets of a paper, measured at that grade's own parent size — and the parent size is different for every grade. Text and book stock is weighed at 25 by 38 inches, so 70 lb text is 500 sheets of 25 by 38 weighing 70 pounds, which works out at 104 gsm. Cover stock is weighed at 20 by 26, a much smaller sheet, so 70 lb cover has to be far heavier paper to reach the same 70 pounds over 500 sheets: 189 gsm, nearly twice as much.
Run the default 70 lb text through the table on the page and it comes out as 27.6 lb bond, 38.3 lb cover, 57.3 lb index, 47.3 lb bristol and 63.7 lb tag. Every one of those is the same sheet of paper. A quote that says "70 lb" and nothing else has not specified a stock.
M weight is the number that actually gets used
Shops order and freight on M weight, the weight of 1,000 sheets at the size being bought. It is useful precisely because it is tied to the real sheet rather than to a parent nobody is handling. Where the sheet you buy is the parent size, M weight is exactly twice the basis weight — 70 lb text at 25 by 38 has an M weight of 140 lb — and where it is not, the ratio is the area ratio and the arithmetic is worth doing rather than guessing.
Five thousand sheets of that stock weigh 700 pounds, which is under a third of a skid, and at 95 dollars a hundredweight it costs 665 dollars, or 13.3 cents a sheet. Paper priced per hundredweight is the norm in the trade and it is the reason two quotes at the same dollar figure can be for different paper.
Grammage does not have the problem
A gsm figure is grams in a square metre, full stop. No parent size, no grade, nothing to misread. If you are specifying a paper for anyone outside the US, or comparing two mill datasheets, work in gsm and convert at the end. The conversion is one line: gsm equals the basis weight times 1,406.1 divided by the parent area in square inches, and the only thing you can get wrong is which parent area belongs to the grade.
Weight and thickness are separate specifications
Bulk is something a mill controls independently of weight, so two papers at 104 gsm need not be the same thickness. That matters when the sheets are going into a book — 400 pages per inch and 300 pages per inch give spines of 0.625 and 0.833 inches on a 250-page book, a difference of 0.21 inches — and it matters when a piece has to fit an envelope or feed through a press. Substituting on grammage alone is the usual way a book cover comes back the wrong width.
The mailing question this page does not answer
It will weigh your piece and it will subtract that from a band you type in. It will not tell you which band applies. Carriers set their own bands and change them, and the classification of a mailing piece turns on thickness, flexibility, aspect ratio and how it is sealed as much as on what it weighs. Weigh a finished sample on a scale, take the band from the current rate table you are quoting against, and confirm both with the carrier before a run goes out.
Questions people ask
What is 70 lb text in gsm?
About 104 gsm. Text and book stock is weighed at a 25 by 38 inch parent, so 70 pounds per 500 sheets of that size works out at 104 grams per square metre. The same 70 lb written against cover stock, which is weighed at 20 by 26, is 189 gsm instead — a much heavier paper wearing the same number.
How do I convert basis weight to gsm?
Multiply the basis weight by 1,406.1 and divide by the parent area of that grade in square inches. Bond is 17 by 22 so the divisor is 374 and the factor comes out at 3.76 gsm per pound. Text is 25 by 38, area 950, so the factor is 1.48. Cover is 20 by 26, area 520, factor 2.70. The whole difference between grades is that one number.
What is M weight and why do printers use it?
M weight is the weight of 1,000 sheets at the size you are actually buying, so it maps straight onto what arrives on the skid and what the freight is charged on. Basis weight is tied to a parent size that may be nothing like the sheet in the job. If the sheet you buy is the parent size, M weight is twice the basis weight; otherwise scale it by the area ratio.
Can I substitute a paper on gsm alone?
Only if thickness does not matter to the job, which is rarer than it sounds. Caliper and grammage are separate specifications and mills bulk paper deliberately, so two sheets at the same gsm can be noticeably different thicknesses. Anything that gets bound, folded, stacked to a height or fitted into an envelope needs the caliper checked as well.
Will this tell me the postage on my mailer?
No. It weighs the piece and reports the difference against a band you type in yourself. It carries no rate table, does not know your carrier or class, and makes no claim that a piece qualifies for any rate. Weigh a finished sample, read the band off the current schedule you are quoting against, and confirm it with the carrier — thickness and shape matter as well as weight.