You are looking at two prints. One lists 300gsm, the other 200gsm. The first costs more. You assume the heavier paper is better. You would be wrong about half the time.
GSM stands for grams per square metre—literally how much a square metre of that paper weighs. It tells you mass. It does not tell you stiffness, texture, whiteness, ink holdout or how the thing feels when you pick it up. A 200gsm cotton rag often feels more substantial than a 300gsm resin-coated photo paper, because structure matters more than weight.
What GSM actually predicts
Paper weight correlates loosely with three things: opacity, curl resistance and perceived value. Anything below 180gsm will usually show through to some degree under gallery lighting, which means you will see the wall colour or the back of the print influencing the image. Between 200gsm and 350gsm, opacity is rarely a problem unless the paper is unusually translucent by design.
Curl happens when one side of the paper takes on moisture differently than the other—inkjet prints curl because the printed side swells slightly. Heavier papers resist this better, but a 250gsm sheet with internal sizing will stay flat longer than a 400gsm sheet without it. If the manufacturer does not list whether the paper is internally sized, that is a problem bigger than the GSM number.
Perceived value is real. A 210gsm print feels like a poster. A 310gsm print feels like an object someone printed on purpose. The difference costs about forty cents per sheet at trade prices, which tells you something about markups when you see a thirty-dollar gap between weights.
The four specs that matter more
Caliper is thickness in microns or thousandths of an inch. Two sheets can weigh the same and measure differently, because one uses denser fibres or more coating. A 300gsm baryta paper might measure 350 microns. A 300gsm canvas substrate might measure 450 microns, because canvas weave traps air. Caliper predicts handling feel and frame-gap tolerance better than weight does.
Finish determines gloss, texture and how ink sits on the surface. Glossy papers have a resin or clay coating that keeps ink on top, producing high dmax—that is, deep blacks that measure above 2.0 on a densitometer. Matte papers let ink soak in slightly, trading maximum black depth for reduced glare and a softer appearance. Textured papers like cold-press watercolour add surface dimension that becomes visible in raking light, which you either want or you do not.
Brightness is measured on a 0-100 scale, where 100 is theoretical perfect white. Most fine art papers sit between 85 and 98. Natural white papers around 90 brightness look warm under incandescent light and neutral under daylight. Bright white papers above 96 often contain optical brighteners—fluorescent dyes that glow slightly under UV, making whites appear cooler and more intense. This affects colour accuracy. If you are printing work that depends on precise neutrals, you want brightness listed and you want to know whether brighteners are present.
Base material—alpha cellulose, cotton rag, wood pulp, polyester—affects longevity and feel more than any other factor. Cotton rag is acid-free, lignin-free and stable for centuries if stored properly. Alpha cellulose is chemically purified wood pulp, nearly as stable and less expensive. Resin-coated papers use a plastic base that resists humidity but cannot be called archival by any honest standard. At Printasso, the house stock is 230gsm cotton rag with a natural white point around 91 brightness, which means excellent longevity and a slight warmth that flatters most images.
When to ignore GSM entirely
If the paper is going behind glass in a frame, anything above 200gsm performs identically in practice. The glass holds it flat, the mat isolates it from the glazing, and you will never touch the surface after installation. Weight becomes a shipping concern, not a quality concern.
If the print will be mounted to a substrate—foam board, gatorboard, dibond—the mount provides all the rigidity. A 180gsm print on 3mm dibond is effectively stiffer than a 400gsm print hanging loose. You are paying for the mount, and the paper weight is now about ink compatibility and surface finish, nothing else.
For unframed prints that ship rolled, weight starts mattering again around 250gsm, because that is the threshold where most papers stop taking a permanent curl from the tube. Below that, you are counting on the recipient to flatten it under weight for a few days, and not everyone does.
The actual question to ask
Do not ask what the GSM is. Ask what the base is, what the caliper is, whether it is sized internally, and what the manufacturer’s archival rating is under ISO 9706 or equivalent standards. If the seller cannot answer those four questions, the GSM number is meaningless and possibly invented.
A properly specified 200gsm cotton rag will outlast and outperform a mystery 350gsm gloss sheet every time, because you know what you are getting and the chemistry is documented. Weight is one variable. It is not a proxy for the other twelve.
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