Guides
Paper Weight lb to GSM: A Print Shop Reference for Customers
Paper weight lb to gsm conversion is the first step in any paper spec. Here is how US print shops convert basis weight to GSM and catch the errors that follow.
What to take away
- Basis weight is the weight of 500 sheets in that grade's own parent size, so 20 lb bond and 20 lb cover are not the same paper.
- The working formula is GSM = lb x 1406 / (length x width in inches), and the divisor changes with every grade.
- Bond, offset and cover each have a different parent sheet, which is why the same GSM lands on three different lb numbers.
- A conversion is only right when the caliper and the feel agree with the number, so verify with a micrometer before you commit a run.
- The usual failure is quoting a cover weight from a bond table; the fix is to re-convert from the correct parent size before the job is priced.
Why the US basis weight system confuses buyers
In the US, paper sells by basis weight: the weight of 500 sheets of that grade's parent size. That parent size is not the sheet you buy.
Bond paper uses a 17 x 22 inch parent, offset 25 x 38, and cover 20 x 26. A ream of 20 lb bond and a ream of 20 lb cover share a number and nothing else.
Parent sizes by grade
Bond
- Parent size
- 17 x 22
- Sold by
- lb
- Same number
- 20 lb
- Sheet differs
- yes
Offset
- Parent size
- 25 x 38
- Sold by
- lb
- Same number
- 20 lb
- Sheet differs
- yes
Cover
- Parent size
- 20 x 26
- Sold by
- lb
- Same number
- 20 lb
- Sheet differs
- yes
The rest of the world sells paper by grammage, or GSM, which is the weight of one square meter in grams. That number is fixed to the sheet, not a parent size, so it travels across grades unchanged.
Print buyers who spec a job in GSM and then order in lb end up with a lighter sheet than the proof. The explanation of grammage covers how the metric definition works. The paper size reference shows which North American sizes the basis system attaches to.
What you need before starting
- The grade name, not just the weightbond, offset, text, cover or index
- The parent sheet dimensions for that grade
- The target GSM, or the target lb if you are going the other way
- A micrometer or caliper for the doneness check
- The mill or merchant spec sheet for the exact stock
The conversion sequence
- Identify the grade. Bond, writing, offset, text, cover and index all carry different parent sizes, and the grade drives every later step.
- Write down the parent sheet in inches. Bond is 17 x 22, offset and text are 25 x 38, cover is 20 x 26, index is 25.5 x 30.5.
- Multiply the two parent dimensions to get the area in square inches.
- Divide 1406 by that area. The result is the multiplier for that grade alone.
- Multiply the basis weight by the multiplier to get GSM. For 20 lb bond, 1406 divided by 374 is 3.76, and 20 times 3.76 is 75 GSM.
- Reverse the steps to go from GSM back to lb, using the same grade multiplier.
A worked conversion table
The table below uses the standard parent sizes. Treat the GSM figures as illustrative to the nearest whole number, because mills vary by a point or two.
Grade, parent size, GSM
Bond
- Parent size
- 17 x 22
- Basis weight
- 20 lb
- Approx. GSM
- 75
- Caliper feel
- light
Offset
- Parent size
- 25 x 38
- Basis weight
- 50 lb
- Approx. GSM
- 74
- Caliper feel
- light
Cover
- Parent size
- 20 x 26
- Basis weight
- 80 lb
- Approx. GSM
- 216
- Caliper feel
- stiff
| Grade | Parent size (inches) | Basis weight | Approx. GSM |
|---|---|---|---|
| Bond | 17 x 22 | 20 lb | 75 |
| Bond | 17 x 22 | 24 lb | 90 |
| Offset | 25 x 38 | 50 lb | 74 |
| Offset | 25 x 38 | 70 lb | 104 |
| Cover | 20 x 26 | 80 lb | 216 |
| Cover | 20 x 26 | 100 lb | 270 |
| Index | 25.5 x 30.5 | 90 lb | 163 |
A 80 lb cover at 216 GSM and a 50 lb offset at 74 GSM sit close together in stiffness once you account for caliper, which is why the number alone never settles a spec. The paper density overview explains why two sheets of equal GSM can feel different in the hand.
Convert from the grade you are actually buying. A conversion done from the wrong parent size is not an approximation, it is a different paper.
Where judgement replaces the recipe
Merchants round. A stock sold as 80 lb cover may measure anywhere from about 210 to 220 GSM depending on the mill and the finish. Coated sheets run denser than uncoated at the same weight, so a coated 100 lb text can feel heavier than its GSM suggests.
Bulk matters as much as weight. A 60 lb high-bulk offset can caliper like a 70 lb sheet, changing how it feeds through a folder and how it scores. When a customer asks for a specific GSM because a European parent company specified it, match caliper first and weight second.
Pricing the job? The sample quotes and template shows how paper grade lines are written so a substitution never becomes a dispute.
How to tell it is right
Measure the sheet with a micrometer in three places, then average them. A 20 lb bond should caliper near 0.004 inches; 80 lb cover near 0.010 inches. If caliper is close but GSM is off by more than five points, suspect the grade, not the paper.
Fold a corner by hand and compare the crack against a known sheet of the same grade. The feel test catches a wrong-grade substitution faster than arithmetic does.
Fixing it when it is not
Converting a cover weight with a bond multiplier is the common failure, understating GSM by roughly a factor of three.
Stop the quote, pull the mill spec sheet, and re-run the sequence from step two with the correct parent size. Tell the customer the corrected weight and price difference before the stock is ordered.
If the stock is already on the floor, the stock control checkpoints give you a way to flag the ream before it reaches the press.
For jobs that cross borders, the weight is only one line on the spec. The tax treatment of finishing work is worth reading before you promise a delivered price, because paper and labour are taxed differently by state.
