Paper Weight and Thickness Guide: GSM, Basis Weight, and Caliper Explained

Paper weight is one of the first decisions in any print project, yet it is also one of the most confusing. Ask a printer in London for "300 gsm silk" and you will get exactly what you expect. Ask a printer in Chicago for "100 lb gloss cover" and you will also get exactly what you expect -- but the two are describing nearly the same sheet using entirely different measurement systems. Add "caliper," "mils," and "points" to the mix, and the confusion multiplies.

Understanding paper weight and paper thickness is not an academic exercise. These specifications directly affect how a printed piece feels in the hand, whether it can survive postal handling, how it folds and binds, how much it costs to ship, and whether your press can even feed it. A business card printed on 80 gsm copy paper feels flimsy and disposable. The same design on 400 gsm cotton stock communicates permanence and quality -- without a single word of copy changing.

This guide untangles the three major paper measurement systems -- GSM, US basis weight, and caliper -- and gives you the practical knowledge to choose the right stock for every job. Whether you are preparing files in PDF Press for a saddle-stitched booklet or laying out business cards for gang-run printing, the weight and thickness of your paper will shape the final product.

The GSM system -- grams per square meter, also written as g/m² -- is the international standard for measuring paper weight. It is used universally outside of North America and is increasingly common within it. The concept is simple: take a single sheet of paper measuring exactly one meter by one meter, weigh it in grams, and that number is its GSM.

The beauty of GSM is its universality. A sheet of 120 gsm paper weighs 120 grams per square meter regardless of the sheet size it was cut from, the category of paper it belongs to, or the country it was manufactured in. There is no ambiguity and no need for a "basis size" reference. This makes gsm paper weight the simplest system to understand and communicate internationally.

Here are the common GSM ranges and their typical uses:

When you set up your imposition layout in PDF Press and choose a paper size, the GSM of your stock determines whether the finished piece will feel like a flyer or a premium invitation -- even if both use the same A5 trim size.

The US basis weight system measures paper weight in pounds (lb) per ream (500 sheets) of a "basis size" that varies by paper category. This is the source of most confusion: 80 lb text and 80 lb cover are not the same weight. They are measured against different reference sheet sizes, so the actual weight per square meter (or per square inch) is dramatically different.

The major paper categories and their basis sizes are:

So when someone says "80 lb paper," the natural follow-up question is: 80 lb of which category? An 80 lb text sheet weighs about 118 gsm. An 80 lb cover sheet weighs about 216 gsm -- nearly double. This discrepancy has caused countless misprints and job reruns. If you order "80 lb" without specifying the category, you are rolling the dice.

The US system persists because of legacy inventory systems, supplier catalogs, and entrenched industry vocabulary. If you work primarily with North American paper mills, you will encounter basis weight paper designations daily. The key is to always pair the number with the category ("80 lb cover" or "70 lb text") and, whenever possible, confirm the GSM equivalent.

While GSM and basis weight tell you how much the paper weighs, paper caliper tells you how thick it is. These are related but not identical. Two papers can have the same GSM but different calipers because of differences in fiber density, coating layers, and the degree of calendering (compression) during manufacturing.

Caliper is measured in several units depending on the region:

• Mils (thousandths of an inch): The standard in North America. A typical business card stock is 12-16 mils. Office copy paper is about 4 mils.
• Points (also thousandths of an inch): In paper measurement, 1 point = 1 mil. A "10 pt" card stock is 0.010 inches thick. Do not confuse this with typographic points (1/72 inch).
• Microns (μm or micrometers): The metric standard. 1 mil = 25.4 microns. A 300 gsm coated board might be about 350 microns (roughly 14 mils).
• Millimeters: Sometimes used for very thick boards. 1 mm = 1000 microns = approximately 39.4 mils.

Caliper matters enormously for several practical reasons:

• Spine width calculation: When producing perfect-bound books, you must know the caliper of the text paper to compute the spine width. A 200-page book on high-bulk 100 gsm paper will have a wider spine than the same page count on calendered 100 gsm paper.
• Postal regulations: Many postal services (USPS, Royal Mail) have minimum and maximum thickness requirements for letters, postcards, and self-mailers. Falling below the minimum means your piece may not survive automated sorting; exceeding the maximum bumps it to a higher postage class.
• Press feeding: Sheet-fed presses have minimum and maximum caliper specifications. A paper that is too thin may misfeed or double-feed; a paper that is too thick may jam or not make it through the fuser (on digital presses).
• Finishing operations: Die-cutting, scoring, embossing, and folding all behave differently as caliper changes. Thicker stock requires deeper scores, wider gutters, and sometimes modified grain direction planning.

When in doubt, request a caliper measurement (in mils or microns) from your paper supplier in addition to the weight designation. It is the most objective physical measurement you can get.

Converting between GSM and US basis weight is one of the most common tasks in international prepress. Because the US system uses different basis sizes for each category, the conversion factor changes depending on the paper category. Here is a practical paper weight chart covering the most frequently encountered weights:

The conversion formulas are:

• GSM to Bond lb: GSM x 0.2660
• GSM to Text lb: GSM x 0.6756
• GSM to Cover lb: GSM x 0.3696
• Bond lb to GSM: lb / 0.2660
• Text lb to GSM: lb / 0.6756
• Cover lb to GSM: lb / 0.3696

Note that the caliper column shows a range because thickness depends on paper finish (coated vs. uncoated), bulk, and manufacturing process -- not just weight. A high-bulk uncoated 120 gsm paper can be thicker than a heavily calendered coated 148 gsm paper.

A common misconception is that heavier paper is always thicker. In reality, paper weight (GSM or basis weight) and paper thickness (caliper) measure different properties. Weight measures mass; thickness measures physical height. They correlate broadly, but several factors drive them apart:

• Coatings: Coated papers have thin layers of clay, latex, or calcium carbonate applied to the surface. These coatings add weight without adding proportional thickness. A 150 gsm coated sheet is typically thinner than a 150 gsm uncoated sheet.
• Bulk: Paper manufacturers describe "bulk" as the ratio of caliper to basis weight. A "high bulk" paper is thicker for its weight -- think of the difference between a fluffy blanket and a compressed felt pad of the same mass. Publishers love high-bulk papers because they make thin books appear thicker on the shelf.
• Fiber type: Papers made from softwood fibers (long, flexible) tend to be bulkier than those made from hardwood fibers (short, compact). Cotton-based papers are denser still.
• Calendering: Passing paper through heavy rollers compresses the fibers and reduces caliper without changing weight. Supercalendered papers used in magazines and catalogs are smooth, thin, and relatively heavy for their thickness.

This distinction matters practically. If you are calculating spine width for a book, you need the caliper (or PPI), not the GSM. If you are estimating shipping costs, you need the GSM (to compute total weight), not the caliper. Both numbers belong on your job specification.

Different print products demand different paper weights. Here is a practical reference for the most common applications, drawn from industry standards and supplier recommendations:

Business Cards

Standard: 300 - 400 gsm (110 - 148 lb cover) / 12 - 16 pt caliper. Premium cards use 350+ gsm with soft-touch lamination or specialty finishes. Anything below 300 gsm feels insubstantial. When you are imposing business cards for gang-run printing, the stock thickness also determines whether you need scoring on the cut edges.

Postcards and Direct Mail

USPS requires a minimum of 7 pt caliper (roughly 200 gsm uncoated) for a piece to qualify as a "card" and avoid the letter surcharge. Standard postcards use 270 - 350 gsm (100 - 130 lb cover). EDDM (Every Door Direct Mail) pieces must be at least 0.007" thick and no more than 0.25" thick.

Flyers and Brochures

Single-sheet flyers: 130 - 170 gsm (80 - 100 lb text) coated gloss or silk. Tri-fold brochures: 130 - 150 gsm for easy folding. Heavier stocks (170+ gsm) require scoring before folding to avoid cracking, especially on coated sheets. Use PDF Press to lay out your brochure panels and verify fold positions before sending to press.

Book Interiors (Text Pages)

Novels and trade paperbacks: 70 - 90 gsm (50 - 60 lb text) uncoated, often cream or natural white. Art books and catalogs: 130 - 170 gsm (80 - 100 lb text) coated gloss. Children's board books: 250 - 350 gsm laminated board, often glued in pairs to achieve a final "page" thickness of 1.5 - 3 mm.

Posters

Standard posters: 150 - 200 gsm (100 lb text to 80 lb cover) coated gloss. Gallery-quality fine art prints: 250 - 310 gsm cotton rag or heavyweight matte. Large-format posters that need to be rolled for shipping benefit from lighter stocks (150 - 170 gsm) that will not crack at the roll.

Packaging and Labels

Folding cartons: 270 - 400 gsm (100 - 148 lb cover) SBS (solid bleached sulfate) board. Labels: 70 - 90 gsm face stock on adhesive liner. Hang tags: 300 - 400 gsm uncoated. The cardstock weight guide for packaging must also account for die-cutting, creasing, and gluing performance.

Paper coatings are applied to improve ink holdout, color reproduction, and surface smoothness. They come in four primary finishes -- gloss, silk (satin), matte, and dull -- and each adds a measurable amount of weight and a smaller amount of thickness.

A typical clay coating adds 10 - 30 gsm per side. A sheet described as "150 gsm C2S" (coated two sides) might have a base sheet of about 100 - 110 gsm with 20 - 25 gsm of coating on each side. This means:

• Coated papers are heavier per unit thickness than uncoated papers of the same caliper. If you are weight-sensitive (postal costs, for example), be aware that a 170 gsm coated sheet is not the same "feel" as a 170 gsm uncoated sheet -- the coated version will be thinner and denser.
• Coated papers are smoother and less opaque at a given GSM. If you need high opacity (to prevent show-through in a book with printing on both sides), an uncoated sheet at the same GSM will usually perform better.
• Coated papers are less forgiving when folded. The coating layer can crack where the paper bends, exposing white fibers beneath a dark printed area. Scoring before folding is mandatory on coated stocks above about 170 gsm.

When comparing paper samples, always compare coated-to-coated and uncoated-to-uncoated at the same GSM. The "hand feel" (how the paper feels when held) differs dramatically, and no conversion chart can substitute for holding both sheets side by side.

Paper weight and thickness have direct consequences for imposition planning. When you set up a job in PDF Press, the choice of stock weight influences several layout decisions:

• Creep compensation: In saddle-stitched booklets, thicker paper causes inner pages to extend beyond the trim edge (shingling). The thicker the stock, the more creep compensation you need to apply. A 32-page booklet on 170 gsm coated stock will exhibit significantly more creep than the same booklet on 90 gsm uncoated.
• Spine width and cover sizing: For perfect-bound books, the caliper of the text stock determines the spine width, which determines the cover flat size. Get this wrong and the cover will not wrap correctly.
• Signature planning: Heavier stocks produce thicker signatures. A 32-page signature on 150 gsm stock may be too bulky to fold cleanly; splitting into 16-page signatures might be necessary. This directly affects your imposition layout.
• Press sheet utilization: Heavier stocks cost more per sheet and are less forgiving of waste. Efficient imposition -- minimizing trim waste and maximizing the number of pages per press sheet -- becomes more important as the paper cost per sheet increases.
• Gripper and guide edge: Digital presses in particular have caliper limits. If your stock is near the maximum, you may need to leave wider gripper margins, which affects your imposable area.

The takeaway: paper weight is not just a specification to pass along to the paper supplier. It is an input to your imposition calculations. Factor it in early, or pay for it in reprints later.

Even experienced print buyers and prepress operators make paper weight errors. Here are the most frequent mistakes and their remedies:

1. Confusing "text" and "cover" basis weights. Ordering "80 lb" without specifying the category is the single most common error. An 80 lb text sheet (about 118 gsm) is suitable for a flyer. An 80 lb cover sheet (about 216 gsm) is suitable for a postcard. Always specify the full designation: "80 lb cover, C2S gloss."
2. Assuming GSM equals thickness. As discussed, a 170 gsm coated sheet is thinner than a 170 gsm high-bulk uncoated sheet. If thickness matters (for postal requirements, spine calculations, or rigidity), request the caliper in mils or microns.
3. Ignoring opacity. Thin papers allow printed images on the reverse side to show through ("ghosting" or "show-through"). For double-sided printing, check the paper's opacity rating (expressed as a percentage -- 90%+ is generally acceptable for text pages with normal coverage).
4. Forgetting about the paper-to-ink relationship. Heavier coating coverage improves ink holdout and color vibrancy but also increases drying time. If your job has heavy ink coverage and a quick turnaround, you may need to use a faster-drying ink or allow extra drying time before finishing. This is not a weight specification per se, but it is directly related to the coating decisions driven by weight choices.
5. Not requesting a paper dummy. For any book, catalog, or multi-page product, ask your paper supplier or printer to produce a blank dummy at the correct page count and paper stock. Hold it in your hands. Feel its weight. Flip through the pages. No specification sheet can replace this tactile evaluation.

Choosing the right paper weight is also an environmental decision. Heavier papers require more raw material, more energy to transport, and generate more waste at the trim. Here are practical ways to reduce your environmental footprint through smart paper selection:

• Right-size the weight. If your internal newsletter is printed on 120 gsm when 80 gsm would work perfectly, you are using 50% more material than necessary. Match the weight to the function, not the aspiration.
• Use high-bulk stocks. A high-bulk 90 gsm paper can provide the perceived thickness of a standard 110 gsm paper while using less fiber. This is particularly effective for book publishing where "shelf presence" matters.
• Consider recycled and FSC-certified stocks. Many recycled papers now match the quality and printability of virgin stocks. FSC (Forest Stewardship Council) certification ensures responsible forestry practices.
• Optimize your imposition. Efficient imposition with PDF Press reduces the number of press sheets needed, which directly reduces paper consumption. Even a 5% improvement in sheet utilization on a 50,000-sheet run saves 2,500 sheets.
• Reduce trim waste. Choosing a paper size that minimizes trim waste for your finished size is a key prepress decision. Use the paper sizes guide alongside this weight guide to find the optimal sheet size and weight combination.

Use this paper weight chart as a quick starting point when specifying stock for common print products. Always confirm with your printer and request paper samples before committing to a large run.

For a deeper dive into sheet dimensions, see our paper sizes for print guide. And when you are ready to impose your chosen stock, PDF Press handles the layout math so you can focus on choosing the right paper.