What Is Paper Weight? GSM Explained
If you have ever ordered custom packaging, picked paper for a book print run, or tried to match a sample swatch across borders, you already know the problem. One supplier quotes “350 gsm art paper.” Another says “94 lb cover.” A third sends a spec sheet in points. Suddenly a simple purchasing decision turns into a math exam you did not sign up for.
This guide solves that. It gives you every conversion formula, every data table, and every practical recommendation you need to move between paper weight and thickness measurements without guessing. It covers six paper types, four different unit systems, and the real-world applications that tell you which numbers actually matter for your project.
GSM stands for grams per square meter . It is the most straightforward paper weight measurement in the world — and the one used everywhere outside North America.
Here is how it works: take one square meter of the paper. Weigh it in grams. That number is the GSM. A sheet of standard office copy paper weighs 80 grams per square meter, so it is called 80 gsm. Thin bible paper might be 30 gsm. A heavy business card stock could be 350 gsm. No conversion factor, no paper-type dependency, no ambiguity.
That last part matters more than it sounds. A number in GSM means exactly the same thing whether you are holding text paper, cover stock, or newsprint. There is no hidden variable hiding behind the label. That makes GSM the default language of international print buying — and the system every printer in Asia, Europe, and Australia uses by default.
GSM also correlates reasonably well with how a paper feels in your hand. A 60 gsm sheet is noticeably thinner and more translucent than a 200 gsm sheet. The relationship is not perfectly linear — paper density varies by fiber composition, coating, and manufacturing process — but GSM gets you close enough to make practical decisions without touching a sample.
A quick mental benchmark: copy paper at 80 gsm, a paperback cover around 250 gsm, a hardcover case wrap around 120-150 gsm wrapped over gray board. Get those three reference points locked in and you can estimate almost any print spec.
What Is Paper Thickness? PT & MM Explained
Paper weight tells you mass. Paper thickness tells you the physical distance between the front and back surfaces of a single sheet. The two are related but not interchangeable — a 300 gsm coated art paper is thinner than a 300 gsm uncoated offset sheet because coating compresses the surface.
Two units dominate thickness measurement.
PT (points) comes from the traditional printing and packaging industries, especially in North America. One point equals one-thousandth of an inch (0.001″). So a 12 pt cover stock is 0.012 inches thick, and a 24 pt rigid board is 0.024 inches. The system is simple — point values are small integers, easy to remember, easy to compare — which is why packaging designers and business card printers still swear by it.
MM (millimeters) is the metric equivalent. It is the standard for book spine calculation, precision engineering of packaging, and any manufacturing workflow that uses metric tooling. Converting between the two is straightforward:
1 pt = 0.0254 mm (exact)
1 mm = 39.37 pt (approximate)
The relationship is linear and precise because it derives from the definition of an inch: 1 inch = 25.4 mm, and 1 pt = 0.001 inch, so 1 pt = 25.4 × 0.001 = 0.0254 mm exactly. No rounding, no paper-type dependency, no ambiguity.
The key distinction to remember: GSM measures weight per area, PT and MM measure physical thickness. A dense, highly compressed paper can have high GSM with low thickness. A bulky, uncoated paper can have modest GSM with surprising thickness. For book spine calculation, use thickness in mm. For estimating paper feel and stiffness, use GSM. For packaging and rigid applications, use either — but know which one your manufacturer speaks.
Understanding Paper Weight Units
GSM (Grams per Square Meter)
GSM is the international standard for paper weight, and its logic is refreshingly simple. Cut a perfect one-meter-by-one-meter square from any paper stock. Put it on a scale. The reading in grams is the GSM. No hidden variables, no paper-category qualifications.
That universality is its main strength. When a Chinese printer quotes “157 gsm art paper” and an Indian supplier quotes the same number, you get the identical product (within manufacturing tolerance). The same cannot be said for pound-based measurements, where 80 lb text and 80 lb cover describe completely different physical sheets.
In practice, GSM maps to tactile experience in a fairly predictable way:
30 – 50 GSM : Tissue-thin, translucent — Bible paper, airmail paper, dictionary stock60 – 80 GSM : Light, slightly see-through — Office paper, notebook pages, mass-market paperback interiors90 – 120 GSM : Moderate body, minimal show-through — Quality stationery, magazine interiors, brochure stock128 – 170 GSM : Substantial, crisp — Premium brochures, flyers, art book interiors200 – 250 GSM : Heavy, rigid-feeling — Paperback covers, postcards, greeting cards270 – 350 GSM : Card-like stiffness — Business cards, hardcover case wraps, luxury packaging400+ GSM : Board-like, difficult to fold — Rigid boxes, hardcover boards, structural packaging
The ranges overlap because paper density varies. A 250 gsm coated art paper feels noticeably denser and stiffer than a 250 gsm uncoated offset sheet, even though both weigh the same per square meter.
LBS / Pounds — with Complete Basis Size Table
Pound-based paper weight is the system that trips up even experienced print buyers. The reason: “80 lb paper” means nothing until you know which paper type it refers to.
Here is how the pound system works. Paper mills take 500 sheets (one ream) cut to a specific “basis size” — and that basis size depends on the paper type. They weigh that ream in pounds. The result becomes the paper’s “lb” rating. So 80 lb text paper means: 500 sheets of text paper cut to 25″ × 38″ weigh 80 pounds.
The catch: every paper type uses a different basis size. A ream of text paper (25″ × 38″) covers 950 square inches per sheet. A ream of cover paper (20″ × 26″) covers only 520 square inches per sheet — nearly half the area. So 80 lb cover stock is a much heavier, thicker sheet than 80 lb text, because you are weighing the same number of sheets cut to a smaller size.
Here are the standard basis sizes for the six major paper categories:
Bond / Writing : 17 × 22 in (374 sq in) — Letterhead, stationery, office paperText / Book : 25 × 38 in (950 sq in) — Book interiors, brochures, magazine pagesCover : 20 × 26 in (520 sq in) — Book covers, postcards, presentation foldersIndex Bristol : 25.5 × 30.5 in (777.75 sq in) — Index cards, file folders, tab dividersMill Bristol : 22.5 × 28.5 in (641.25 sq in) — Art boards, signage, display materialNewsprint : 24 × 36 in (864 sq in) — Newspapers, comic books, directories
This table is the key to every conversion that follows. Notice that Text paper has the largest basis sheet — over 2.5 times the area of Bond — which is why “80 lb text” weighs one thing and “80 lb bond” weighs something completely different. Always confirm the paper type before interpreting a pound rating.
Understanding Paper Thickness Units
PT (Points)
In the paper and packaging trade, a point is not a typographic unit. It is a measurement of caliper — the physical thickness of a single sheet. One point = 0.001 inch.
The point system dominates packaging design because it maps directly to the tooling. Die-cutting rules, creasing channels, and folding tolerances are all specified in points. A folding carton made from 18 pt board needs a creasing rule width calibrated to that exact caliper. Change to 24 pt board and the entire tooling setup changes.
Common point references in the print industry:
8 – 10 pt : Magazine covers, lightweight brochures12 – 14 pt : Standard business cards, postcards16 – 18 pt : Premium business cards, greeting cards, folding cartons20 – 24 pt : Heavy packaging, rigid mailers, luxury card stock28 – 40 pt : Rigid boxes, hardcover book boards, structural packaging60+ pt : Gray board for hardcover cases, industrial packaging
Point measurements are most useful when you need to know how thick a finished piece will be — a stack of 200 pages at 0.12 mm each for spine calculation, a folded carton that must fit a specific shelf slot, or a card that must survive postal sorting machines.
MM (Millimeters)
Millimeters are the metric unit for paper thickness, and they dominate in every print market outside North America — China, Europe, India, Southeast Asia, Australia, and most of Africa.
The practical advantage of millimeters is precision. A book spine that is 5.8 mm wide needs a cover design with exactly 5.8 mm allocated for the spine panel. A box insert that is 2.3 mm thick must fit into a slot with at least 2.3 mm clearance. Millimeters give you that level of detail without decimal points.
For book production specifically, spine width in mm is calculated as:
Spine Width (mm) = Number of Pages × Single Sheet Thickness (mm) + Cover Thickness × 2
So a 200-page paperback printed on 0.10 mm offset paper with a 0.30 mm cover has a spine of:
Spine = 200 × 0.10 + 0.30 × 2 = 20 + 0.6 = 20.6 mm
Get this number wrong and your cover design will not align. The spine text wraps onto the front or back cover, or a gap appears at the hinge. Millimeters are not optional for book production — they are mandatory.
Paper Weight Conversion: The Complete Guide
All conversions on this page are derived from first principles using the basis sizes listed earlier. The mathematical foundation:
1 pound = 453.59237 grams
1 square inch = 0.00064516 square meters
1 square meter = 1,550.0031 square inches
The general formula for converting GSM to LB for any paper type:
LB = GSM × (Basis Area in sq in × 500) / (1,550.0031 × 453.59237)
LB ≈ GSM × Basis Area in sq in / 1,406.17
And the reverse:
GSM = LB × 1,406.17 / Basis Area in sq in
GSM to LB (by Paper Type)
Using the formulas above, here are the exact conversion factors for all six paper types, derived from their basis sizes:
Text Paper (25″ × 38″ = 950 sq in)
LB Text = GSM × 0.676
GSM = LB Text × 1.480
Example: 128 gsm text paper = 128 × 0.676 = 86.5 → approximately 87 lb text.
Cover Paper (20″ × 26″ = 520 sq in)
LB Cover = GSM × 0.3695
GSM = LB Cover × 2.708
Example: 250 gsm cover stock = 250 × 0.3695 = 92.4 → approximately 92 lb cover. The reverse: 100 lb cover = 100 × 2.708 = 270.8 → approximately 271 gsm.
Bond Paper (17″ × 22″ = 374 sq in)
LB Bond = GSM × 0.266
GSM = LB Bond × 3.760
Example: 80 gsm bond (standard office paper) = 80 × 0.266 = 21.3 → approximately 21 lb bond. This checks out — standard US letter stock is indeed 20 lb bond.
Index Bristol (25.5″ × 30.5″ = 777.75 sq in)
LB Index = GSM × 0.553
GSM = LB Index × 1.808
Example: 180 gsm index stock = 180 × 0.553 = 99.5 → approximately 100 lb index.
Mill Bristol (22.5″ × 28.5″ = 641.25 sq in)
LB Mill Bristol = GSM × 0.456
GSM = LB Mill Bristol × 2.193
Example: 250 gsm mill bristol = 250 × 0.456 = 114 → approximately 114 lb mill bristol.
Newsprint (24″ × 36″ = 864 sq in)
LB Newsprint = GSM × 0.614
GSM = LB Newsprint × 1.629
Example: 48.8 gsm newsprint (standard) = 48.8 × 0.614 = 30 → 30 lb newsprint. Standard newsprint is indeed sold as “30 lb newsprint” in North America, confirming the math.
LB to GSM Formula
For quick reverse conversion, here are all six types in one table:
Text : LB → GSM = LB × 1.480Cover : LB → GSM = LB × 2.708Bond : LB → GSM = LB × 3.760Index Bristol : LB → GSM = LB × 1.808Mill Bristol : LB → GSM = LB × 2.193Newsprint : LB → GSM = LB × 1.629
The higher the multiplier, the smaller the basis sheet — so fewer grams are needed to reach the same pound rating. Bond paper, with the smallest basis sheet at 374 sq in, has the highest multiplier. Text paper, with the largest at 950 sq in, has the lowest.
GSM ↔ LB Quick Reference Chart
The chart below covers the GSM range from 60 to 400, showing equivalent pound ratings for Text and Cover — the two paper types most commonly encountered in book printing and commercial print work. Values are rounded to the nearest whole pound.
60 GSM : 41 lb Text / 22 lb Cover70 GSM : 47 lb Text / 26 lb Cover80 GSM : 54 lb Text / 30 lb Cover90 GSM : 61 lb Text / 33 lb Cover100 GSM : 68 lb Text / 37 lb Cover105 GSM : 71 lb Text / 39 lb Cover115 GSM : 78 lb Text / 42 lb Cover120 GSM : 81 lb Text / 44 lb Cover128 GSM : 87 lb Text / 47 lb Cover135 GSM : 91 lb Text / 50 lb Cover148 GSM : 100 lb Text / 55 lb Cover150 GSM : 101 lb Text / 55 lb Cover157 GSM : 106 lb Text / 58 lb Cover170 GSM : 115 lb Text / 63 lb Cover176 GSM : 119 lb Text / 65 lb Cover180 GSM : 122 lb Text / 67 lb Cover200 GSM : 135 lb Text / 74 lb Cover210 GSM : 142 lb Text / 78 lb Cover216 GSM : 146 lb Text / 80 lb Cover230 GSM : 155 lb Text / 85 lb Cover250 GSM : 169 lb Text / 92 lb Cover256 GSM : 173 lb Text / 95 lb Cover270 GSM : 183 lb Text / 100 lb Cover280 GSM : 189 lb Text / 103 lb Cover300 GSM : 203 lb Text / 111 lb Cover310 GSM : 210 lb Text / 115 lb Cover325 GSM : 220 lb Text / 120 lb Cover350 GSM : 237 lb Text / 129 lb Cover400 GSM : 270 lb Text / 148 lb Cover
To read this chart: find your GSM in the left column. Cross-reference with the paper type you need. If your supplier says “100 lb cover,” find 100 lb in the LB Cover column → the equivalent GSM is approximately 270.
PT ↔ MM Conversion Chart
Since 1 pt = 0.0254 mm exactly, converting between points and millimeters is a straightforward multiplication or division. The chart below provides quick reference for the most commonly used caliper values in printing and packaging.
4 pt : 0.102 mm — Tissue6 pt : 0.152 mm — Light text8 pt : 0.203 mm — Light cover10 pt : 0.254 mm — Standard cover12 pt : 0.305 mm — Card stock14 pt : 0.356 mm — Premium card16 pt : 0.406 mm — Heavy card18 pt : 0.457 mm — Packaging board20 pt : 0.508 mm — Rigid board24 pt : 0.610 mm — Extra-rigid board
For values not listed, multiply points by 0.0254 to get mm. Divide mm by 0.0254 to get points. The relationship is exact — no approximations needed.
GSM to PT/MM Reference Table
Paper thickness at a given GSM depends on the paper type. Coated papers are compressed during manufacturing and are thinner than uncoated papers of the same weight. The table below provides approximate thickness values; actual measurements vary by manufacturer, fiber blend, and coating process.
Offset Paper (Uncoated)
60 GSM : 0.08 mm (~3.1 pt)70 GSM : 0.09 mm (~3.5 pt)80 GSM : 0.10 mm (~3.9 pt)100 GSM : 0.12 mm (~4.7 pt)120 GSM : 0.14 mm (~5.5 pt)140 GSM : 0.16 mm (~6.3 pt)160 GSM : 0.18 mm (~7.1 pt)180 GSM : 0.22 mm (~8.7 pt)200 GSM : 0.24 mm (~9.4 pt)230 GSM : 0.28 mm (~11.0 pt)250 GSM : 0.29 mm (~11.4 pt)300 GSM : 0.35 mm (~13.8 pt)
Glossy Art Paper (Coated Gloss)
80 GSM : 0.06 mm (~2.4 pt)105 GSM : 0.09 mm (~3.5 pt)128 GSM : 0.12 mm (~4.7 pt)157 GSM : 0.14 mm (~5.5 pt)200 GSM : 0.18 mm (~7.1 pt)250 GSM : 0.25 mm (~9.8 pt)300 GSM : 0.32 mm (~12.6 pt)350 GSM : 0.36 mm (~14.2 pt)
Matte Art Paper (Coated Matte)
80 GSM : 0.08 mm (~3.1 pt)105 GSM : 0.10 mm (~3.9 pt)128 GSM : 0.13 mm (~5.1 pt)157 GSM : 0.16 mm (~6.3 pt)200 GSM : 0.20 mm (~7.9 pt)250 GSM : 0.26 mm (~10.2 pt)300 GSM : 0.29 mm (~11.4 pt)
Coated One Side (C1S)
210 GSM : 0.28 mm (~11.0 pt)250 GSM : 0.35 mm (~13.8 pt)300 GSM : 0.42 mm (~16.5 pt)350 GSM : 0.49 mm (~19.3 pt)400 GSM : 0.56 mm (~22.0 pt)
Kraft Paper
60 GSM : 0.10 mm (~3.9 pt)80 GSM : 0.12 mm (~4.7 pt)120 GSM : 0.17 mm (~6.7 pt)
Bible Paper
30 GSM : 0.04 mm (~1.6 pt)32 GSM : 0.04 mm (~1.6 pt)35 GSM : 0.05 mm (~2.0 pt)
Gray Board / Chipboard
Approximate — this material is typically specified by thickness, not GSM.
1.0 mm : ~600 gsm (39 pt)1.5 mm : ~900 gsm (59 pt)2.0 mm : ~1,200 gsm (79 pt)2.5 mm : ~1,500 gsm (98 pt)3.0 mm : ~1,800 gsm (118 pt)
For gray board, thickness is the primary specification and GSM is derived. Most hardcover book cases use 1.5 mm to 3.0 mm gray board depending on the trim size and desired rigidity. A standard 6″ × 9″ hardcover typically uses 2.0 mm board.
Common Paper Weights by Application
Choosing the right paper weight for a project means balancing feel, durability, cost, and printability. The table below provides starting-point recommendations based on what printers and publishers actually use.
Bible / thin dictionary pages : 30 – 50 GSM (Bible/offset) — Minimal bulk for high page counts; requires opaque fiber to reduce show-throughMass-market paperback interior : 60 – 80 GSM (Offset/novel) — Light, cost-effective, acceptable opacity at 300+ pagesTrade paperback / novel interior : 70 – 100 GSM (Offset/cream book) — Noticeably better feel than mass-market; standard for literary fiction and nonfictionMagazine interior : 80 – 128 GSM (Glossy art/LWC) — Thin enough for saddle-stitching, glossy enough for photo reproductionFlyer / leaflet : 100 – 170 GSM (Offset/art paper) — Lightweight for distribution, enough body to feel legitimateBrochure (bi-fold / tri-fold) : 128 – 200 GSM (Glossy or matte art) — Must fold cleanly without cracking; coating prevents ink smudgingPoster : 128 – 200 GSM (Art paper/offset) — Hangs flat, resists curling, handles ink saturationCatalog interior : 80 – 128 GSM (Glossy art) — High page count needs thin stock; coating carries photo detailPaperback cover : 200 – 300 GSM (C1S/art board) — Flexible enough to laminate, rigid enough to protect interior pagesPostcard : 200 – 350 GSM (Cover/art board) — Must survive postal handling without bending or tearingGreeting card : 250 – 350 GSM (Cover/art board) — Stands upright, feels substantial, takes scoring and foldingBusiness card (standard) : 250 – 350 GSM (Art board/cover) — Standard for most markets; 300 gsm is the industry sweet spotBusiness card (premium) : 350 – 400 GSM (Art board/duplex) — Thicker, more memorable; often laminated for extra rigidityHardcover case wrap : 120 – 157 GSM (Coated art paper) — Wraps around gray board; must laminate cleanly without bubblingHardcover cover board : 1,200 – 1,800 GSM (Gray board) — 1.5–3.0 mm thickness; provides the rigid structure of a hardcover bookDust jacket : 150 – 200 GSM (Coated art paper) — Thin enough to fold flaps, heavy enough to resist tearingPackaging box (light) : 300 – 400 GSM (C1S/art board) — Small product boxes, cosmetic cartons, folding cartonsPackaging box (heavy) : 400 – 600+ GSM (C1S/gray board) — Shipping boxes, rigid gift boxes, luxury packaging
A note on paper type selection: GSM is only half the story. A 250 gsm uncoated offset sheet and a 250 gsm glossy art paper weigh the same but feel completely different. Coated papers feel slicker, reproduce images more sharply, and are thinner at equal GSM. Uncoated papers feel warmer, more tactile, and work better for heavy text. Match both the weight and the finish to the project.
Frequently Asked Questions
What does 80 lb paper actually mean?
It depends entirely on the paper type. 80 lb text paper (basis size 25″ × 38″) is a lightweight sheet equivalent to about 118 gsm — suitable for book interiors. 80 lb cover paper (basis size 20″ × 26″) is a heavy card stock equivalent to about 217 gsm — suitable for postcards and brochure covers. Same number, completely different physical product. Always confirm whether a pound rating refers to text, cover, bond, or another paper type.
Why do coated papers feel thinner than uncoated papers of the same GSM?
Coating involves applying a mineral layer (typically calcium carbonate or kaolin clay) to the paper surface and calendering it under high pressure. This process compresses the paper fibers and fills surface voids, creating a smoother, denser sheet. The result: a 157 gsm glossy art paper is approximately 0.14 mm thick, while a 160 gsm uncoated offset paper is approximately 0.18 mm — about 29% thicker. The GSM is identical. The caliper is not.
How do I calculate book spine width?
Take the number of interior pages, multiply by the single-sheet thickness of your chosen paper (from the GSM to PT/MM table above), then add twice the cover thickness. Use millimeters for precision. Example: a 240-page novel on 80 gsm offset paper (0.10 mm per sheet) with a 250 gsm C1S cover (0.35 mm) needs a spine of 240 × 0.10 + 2 × 0.35 = 24.7 mm. Round to one decimal place. Submit this number to your cover designer before they start work.
Is GSM or PT more important for business cards?
Both matter, but for different reasons. GSM determines the perceived quality — a 350 gsm card feels premium in the hand. PT determines whether the card fits standard holders, wallets, and sorting equipment. A 14–16 pt card stock (roughly 300–350 gsm depending on paper type) is the standard for most markets. If your client has a specific “16 pt” requirement, convert to the closest available GSM using the reference tables above and confirm with your printer that the caliper matches.
What is the difference between basis weight and GSM?
Basis weight (expressed in pounds) is the weight of 500 sheets cut to a specific size that varies by paper type. GSM is the weight of a single square meter of any paper, regardless of type. Basis weight requires you to know the paper category before the number means anything. GSM is universal. This is why international print buying uses GSM almost exclusively — it eliminates the category variable.
Why does mill bristol use two different basis sizes?
Mill Bristol historically comes in two standard sheet sizes: 22.5″ × 28.5″ for regular stock and 22.5″ × 35″ for double-thick. The double-thick variant produces a higher area basis sheet and therefore a slightly different conversion factor. Most commercial printing uses the 22.5″ × 28.5″ basis. If your supplier quotes a mill bristol weight that does not match your calculation, ask which basis size they are using.
Can I use the GSM → LB formula for any paper type?
Yes, if you know the basis size. The general formula LB = GSM × Basis Area (sq in) / 1,406.17 works for any rectangular basis sheet. The six specific formulas listed in this guide cover the paper types used in 99% of commercial printing. If you encounter a non-standard type, measure the basis area and plug it into the formula.
How accurate are the thickness values in the reference tables?
The GSM to PT/MM tables list approximate values. Paper thickness varies by manufacturer, fiber source (hardwood vs. softwood pulp), filler content, and finishing process. Two different mills can produce 128 gsm art paper with calipers that differ by 0.01–0.02 mm. These tables are reliable for estimation and specification writing. For precision work like spine calculation on a large print run, request a sample swatch and measure the caliper directly.
Conclusion
Paper weight conversion does not have to be complicated. The entire field reduces to three things: know your units, know your basis sizes, and know which paper type you are working with.
GSM is universal. Use it as your anchor reference. If you only memorize one conversion factor, make it the Text paper GSM-to-LB: multiply by 0.676 (or divide by 1.48 to go the other way). That covers most book interiors and commercial print jobs.
PT and MM are linear and exact. One point equals 0.0254 millimeters. The conversion has no rounding, no paper-type dependency, no hidden factors. PT handles packaging and card stock; MM handles book spines and precision engineering. Both are equally valid.
Paper type is the variable most people miss. A pound rating means nothing until you know whether it is text, cover, bond, bristol, or newsprint. Always confirm the type before interpreting a weight — and when in doubt, ask for the GSM.
If you are planning a print project and need precise paper specifications, book a consultation with our team. We will match your content and budget to the right stock, calculate your spine width, and make sure the final product feels exactly how you pictured it.
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