Paperboard Thickness Guide: Points, GSM and Caliper Explained

Paperboard thickness is measured in points in the US, where one point (pt) equals one thousandth of an inch, or about 0.0254 mm. An 18 pt carton board is 0.018 in, roughly 0.46 mm thick. GSM (grams per square meter) is not a thickness at all; it is a weight, so the same 350 gsm can be thinner or thicker depending on how dense the board is.

That difference explains most of the confusion when buyers compare quotes from US, UK and overseas suppliers. This guide gives you the conversions that do hold (points, inches, millimeters and microns), the approximate ones that depend on board type (pt vs gsm), and a practical way to choose the right caliper for a product box.

Key takeaways

• 1 pt = 0.001 in = 0.0254 mm. That conversion is exact. 14 pt is about 0.36 mm, 18 pt about 0.46 mm, 24 pt about 0.61 mm.
• GSM measures weight per area, not thickness. Converting gsm to points needs the board's density, so any pt vs gsm chart is an approximation.
• For bright white SBS carton board, 18 pt usually lands somewhere around 300 to 350 gsm, but mills vary.
• Bending stiffness rises roughly with the cube of thickness, so a few points make a large difference to how a carton feels and resists bulging.
• Most retail product boxes use 14 to 24 pt. Small, light items sit at the low end; larger panels and heavier products need the high end or a move to micro-flute corrugated.
• Box Sense prints paperboard cartons from 10 units on 12 to 24 pt SBS and 14 to 24 pt kraft board, with free structural samples so you can feel the caliper before ordering.

Three ways to describe board: points, caliper and gsm

Paperboard specs use three different kinds of number, and only two of them describe thickness.

Caliper is the physical thickness of the sheet, measured with a micrometer under a set pressure. It can be expressed in any length unit.

Points are the US way of stating caliper. One point is 0.001 in, also called a mil or a thou in other trades. A "16 pt board" simply means the board is 0.016 in thick.

Microns and millimeters are the metric way of stating caliper. Suppliers in the UK, Europe and the Middle East often quote carton board in microns (µm), such as 450 µm.

GSM (grams per square meter), also called grammage, tells you how much a square meter of the board weighs. It is the standard way paper and board are sold internationally, and it tells you about fiber content and cost, but not directly about thickness.

Basis weight is the US equivalent of grammage for paperboard, usually stated in pounds per 1,000 square feet. You will see it on mill data sheets more often than on a packaging quote.

Why suppliers use different units

Mills sell board by weight because that is how fiber is bought and priced. Converters and designers care about thickness because it drives stiffness, fold behavior and dieline allowances. A US carton printer will almost always talk in points; an overseas supplier may quote gsm only. When comparing, ask each supplier for the caliper in points or microns, not just the weight.

Points to inches, millimeters and microns

These conversions are exact, so you can use this table with confidence.

For comparison, E-flute corrugated is about 1.5 mm, which is roughly 60 pt, and a 2 mm rigid greyboard is roughly 79 pt. That jump explains why a mailer or rigid box feels so different from a folding carton. See the corrugated flutes guide for corrugated calipers.

PT vs GSM: why there is no fixed conversion

Grammage is caliper multiplied by density. A dense board packs more fiber into each point of thickness, so it weighs more per square meter at the same caliper. A bulky board does the opposite.

The practical rule: gsm ≈ caliper in microns × density in g/cm³. An 18 pt board is 457 µm. At a density of 0.7 g/cm³ it weighs about 320 gsm; at 0.6 it weighs about 275 gsm. Same thickness, noticeably different weight.

Approximate gsm for common SBS calipers

Solid bleached sulfate (SBS) board is the white carton board most US product boxes use. These ranges reflect typical published mill data, and a specific mill's grade can sit outside them.

How board type shifts the conversion

Different board families have different densities, so 350 gsm means a different thickness in each.

This is why a UK supplier quoting a 350 gsm folding boxboard and a US printer quoting 18 pt SBS can be offering boxes that feel quite similar, while a 350 gsm recycled board may feel flimsier. The board grades themselves are compared in our guide to SBS vs kraft vs CUK paperboard.

Watch out: if a quote lists only gsm, do not assume it matches the points on another quote. Ask for caliper in points or microns and the board type, then compare like with like.

Basis weight and cardstock thickness for boxes

People searching for cardstock thickness for boxes often have a printing-paper spec in mind, such as 100 lb cover. Cover weight is a different US system based on the weight of 500 sheets of a 20 x 26 in basis size. It is useful for small DIY boxes and inserts, but most commercial cartons are specified in points instead.

The gsm figures use exact unit conversions (1 lb cover is about 2.708 gsm). The calipers vary with coating and finish, because coated cover stocks are dense.

Converting US basis weight

Paperboard mill sheets often list basis weight in pounds per 1,000 square feet. To convert, multiply by about 4.88 to get gsm, or multiply gsm by about 0.205 to get lb per 1,000 sq ft. A 320 gsm board is roughly 66 lb per 1,000 sq ft.

Why a few points matter: stiffness and feel

Thickness has an outsized effect on how a carton performs. In simple beam terms, the bending stiffness of a sheet rises with roughly the cube of its thickness when the material is the same. Real paperboard is layered and not perfectly uniform, but the principle holds well enough to guide decisions.

Compare 14 pt and 18 pt board of the same type: (18 ÷ 14)³ is about 2.1, so the thicker board can be roughly twice as stiff. Going from 18 pt to 24 pt again more than doubles it. That is why a 24 pt carton feels solid while a 14 pt carton of the same size flexes when squeezed.

What stiffness does for a box

• Resists bulging when a product presses against the panels, especially on tall cartons.
• Holds shape on shelf so front panels stay flat and graphics look sharp.
• Survives handling in retail backrooms and during packing.
• Carries weight through the bottom, which matters with jars and bottles.

Stiffness is measured with instruments such as a Taber-type stiffness tester, and mills publish those values for each grade. If two boards have the same caliper but very different stiffness figures, the stiffer one will perform better as a carton.

Choosing paperboard thickness for your box

Weight is one factor, but panel size matters as much, because large unsupported panels bow before small ones do. Use the largest panel dimension and the product's behavior together.

Style changes the answer

Some structures need more caliper than their size suggests. Straight tuck end boxes rely on friction at the tuck flap, which holds better in 18 pt than 14 pt. Auto lock bottom boxes spread weight across a locked base, so they carry heavier products at a given caliper. Window cartons lose stiffness where the window is cut, so step up a few points. Hang tab cartons need enough board to stop the hole from tearing when shoppers pull the box off a peg.

Where the extra points cost you

Thicker board uses more fiber per box and takes more space when stacked flat. It can also crack along folds, particularly with dark flood colors that make white fiber visible at a split crease. Going thicker than you need does not make a box "premium" by itself; finishes and structure often do more. For a genuinely weighty feel, a rigid box is the next category up.

How thickness affects dielines, printing and finishes

Caliper is built into the dieline, so choose it before artwork starts.

Dieline allowances

Each fold consumes a little length, roughly one board thickness per fold depending on the crease. Tuck flaps, dust flaps and glue panels are offset to account for it. At 18 pt, the difference between internal and external dimensions across a carton is only a few hundredths of an inch, but on a tight-fitting product that decides whether it slides in or jams. Our explainer on what a dieline is covers how allowances work.

Creasing and cracking

Thicker board needs a deeper, wider crease. If creasing is not matched to the caliper, the outer surface can crack at the fold and expose fiber. This is more visible on coated white board with heavy ink coverage, and on kraft board with dark ink over white.

Inside printing and white ink

Printing both sides of a carton, or printing opaque white on kraft, adds ink layers that slightly change how the board folds. Neither changes caliper meaningfully, but both make a well-matched crease more important. Our guides to inside printing and white ink on kraft cover the design side.

Lamination and coatings

A lamination film adds a small amount of thickness, generally on the order of a thousandth of an inch or so including adhesive, and it adds stiffness and scuff resistance. Aqueous coatings add almost nothing measurable. Neither is a substitute for choosing the right base caliper.

How to measure board thickness yourself

If you have a sample box from a current supplier and want to match it, measure it before requesting a quote.

1. Use a micrometer or digital caliper. A paper micrometer is best because it applies consistent pressure, but a digital caliper that reads to 0.01 mm or 0.001 in works for a rough check.
2. Cut a flat piece away from folds. Creases and glue areas read thicker or thinner than the sheet.
3. Measure in several spots. Take at least five readings across the panel and average them.
4. Convert. Divide millimeters by 0.0254 to get points. A reading of 0.46 mm is about 18 pt.
5. Check for lamination. Peel a corner. If there is a film, subtract a small allowance or tell the supplier the box is laminated.
6. Note the board type. White on both sides, white with a grey back, or brown throughout tells you whether it is SBS, recycled board or kraft.
7. Record the stiffness feel. Squeeze the largest panel. If it flexes more than you want, ask for a few points more or a stiffer grade.

For formal testing, mills measure caliper under a standardized method and publish a tolerance. Expect normal variation of a few percent within a production lot, and a little more between mills.

Tip: the easiest way to choose a caliper is to hold it. Ask for an unprinted structural sample in two thicknesses, pack your real product, and compare how each feels in the hand and on a shelf.

Ordering paperboard boxes from Box Sense

We print custom product boxes and cosmetic boxes on 12 to 24 pt SBS paperboard and 14 to 24 pt kraft paperboard. If you are not sure which caliper to choose, send the product details and we will recommend one.

What to send

• Product dimensions, weight and whether it is glass, liquid or fragile
• Box style, such as straight tuck, reverse tuck or auto-lock bottom
• Preferred caliper in points, microns or gsm, or a sample box to match
• Board type: bright white SBS or natural kraft
• Print coverage, including any inside printing
• Finishes such as lamination, foil or spot UV
• Quantities to compare

Indicative pricing by caliper and spec

These per-unit figures include free US shipping. The rows differ in size and finish as well as board, so use the price estimator to compare calipers for your own dimensions.

Proofs, samples and timing

Design support is free: we build the dieline for your chosen caliper, set up artwork and send a 3D digital proof with two revision rounds. Unprinted structural samples are free, and a printed sample of approved artwork is free on orders of 100 or more. Quotes are confirmed within 24 hours. Standard production takes 8–12 business days after proof approval, with rush production in 5 business days on most paperboard styles. The minimum order is 10 boxes, US shipping is free, and there are no die or plate charges; foil, embossing, spot UV and custom inserts carry one-time tooling. The folding cartons style page shows the structures we make, and reverse tuck end boxes are a cost-efficient option for small products.

Frequently asked questions

How thick is 18 pt paperboard?

18 pt paperboard is 0.018 inches thick, which is about 0.46 mm or 457 microns. One point equals one thousandth of an inch, so the conversion is exact. 18 pt is the most common caliper for everyday retail cartons such as cosmetics, supplements and small candles, because it balances stiffness, clean folding and cost.

How do I convert gsm to pt?

You need the board's density, because gsm measures weight and points measure thickness. Divide gsm by density in grams per cubic centimeter to get microns, then divide microns by 25.4 to get points. For typical SBS carton board, 300 to 350 gsm is roughly 18 pt, but bulky and dense boards differ, so ask the supplier for the actual caliper.

Is 350 gsm thicker than 18 pt?

It depends on the board. For typical SBS, 350 gsm is around 18 to 20 pt, so it is similar or slightly thicker. A bulky folding boxboard at 350 gsm can be over 21 pt, while a dense recycled board at the same weight may be only 16 to 18 pt. Compare quotes by caliper in points or microns, not weight alone.

What thickness of cardstock is best for product boxes?

Most commercial product boxes use 14 to 24 pt carton board rather than cardstock sold by cover weight. Small, light items such as lip balm work in 14 pt, most cosmetics and supplements use 16 to 18 pt, and larger or heavier products need 20 to 24 pt. For DIY boxes, 130 lb cover stock is roughly the lightest practical choice.

What is the difference between caliper and basis weight?

Caliper is the physical thickness of the board, measured in points, millimeters or microns. Basis weight is how much a given area weighs, stated in pounds per 1,000 square feet in the US or as gsm internationally. Two boards with the same caliper can have different basis weights if one is denser, which affects stiffness, cost and how it prints.

Does thicker paperboard make a box more premium?

Up to a point. Moving from 14 pt to 18 pt makes a carton noticeably stiffer and less flimsy, which customers perceive as quality. Beyond what the product needs, extra thickness adds cost and can crack at folds without looking more luxurious. Finishes such as soft-touch lamination, foil and embossing, or a rigid box, usually do more for a premium feel.

When should I switch from paperboard to corrugated?

Consider corrugated when the product weighs more than roughly 3 lb, when the largest panel exceeds about 12 in, or when the box ships through parcel carriers on its own. Micro flutes such as F or E flute keep a smooth print surface while adding crush resistance. For the strength ratings used on shipping boxes, see the ECT vs burst strength guide.

Next steps

Pick a starting caliper from the size table, then check the numbers in the product box price estimator. When you are ready, request a quote with your product dimensions and preferred board, and we will confirm the caliper, send a dieline and price your run within 24 hours. For shipping boxes rather than cartons, our ECT vs burst strength guide explains corrugated ratings.