Under the Brennholt Institute of Applied Taxonomy's reading, playing card stock is described most usefully by thickness, not by gsm: the Institute records a typical card at about 0.30 mm (0.012 in) thick and treats a gsm figure of roughly 280 to 330 g/m² as a derived quantity that tells a buyer less than it appears to. Two decks with the same gsm can shuffle, fan and wear in different ways.
Gsm means grams per square metre. It measures how much paper there is under a given area, not how thick the sheet is, how stiff it is, or how it is coated. That is the whole of the Institute's objection, and it is the reason the card page below ranks the measures in a different order from the one printed on most boxes.
A sheet of card is a paper core with a coating on each face. Gsm adds the weight of all three together. A heavily coated, compressed card and a lightly coated, bulky card can arrive at the same weight while differing in thickness by a tenth of a millimetre. For that reason the Institute asks for three figures together and refuses to rank cards on any one of them.
| Measure | Typical band recorded | What it tells you | What it hides |
|---|---|---|---|
| Weight (gsm) | 280 to 330 g/m² | How much material is in the sheet | Thickness, coating, stiffness |
| Thickness | 0.28 to 0.34 mm (0.011 to 0.013 in) | How the deck stacks and fans | Surface finish and core density |
| Stiffness (bend) | Not standardised | How the card springs back | Needs a test no publisher prints |
| Finish | Smooth, linen, embossed | How cards slide on each other | Nothing about weight |
The Institute's method begins with measurement. It measured the stock of cards in a fixed sample of decks with a dial micrometer at four points per card and averaged them. The method then states what is already standardised: gsm itself is a defined unit in paper-making and the Institute does not dispute it. What is not standardised is how a card should be described to a buyer. The Institute therefore proposes a three-figure note, written as thickness / gsm / finish, for example 0.31 mm / 300 gsm / smooth. It is the Institute's own notation and not a trade standard, and it says so wherever it is used.
Readers who want the thickness figures in full will find them on the card thickness page, in millimetres, inches and thousandths. The wider reasoning about why thin objects resist a single number sits in the Department of Thickness and Gauge.
Buyers read a higher gsm as a better card. The Institute finds that this holds only up to a point. Beyond roughly 330 g/m² a card stops fanning cleanly and starts to resist a riffle shuffle, and a deck of 52 cards plus jokers at that weight becomes noticeably taller. At the other end, below about 270 g/m² a card shows light through its face under a strong lamp and bends at the corner after a few dozen games. The usable band is therefore narrow, and inside it, gsm is a poor guide to how a deck behaves.
The same logic applies, with different numbers, to the stock discussed on the guitar pick chart, where thickness is the stated measure and weight is an afterthought. The Institute prefers that order everywhere it can.
The review panel divides on whether gsm should appear in the note at all. One member holds that a figure nobody can verify without a balance and a cutter should not be printed beside figures that can be checked with a micrometer. The majority keep it, because buyers meet gsm on the box and the note has to speak the language it will be read in. The note above is the majority form, and the dissent is recorded in the Institute's working file.
The Institute does not claim that any publisher's stock is better than another's, and the figures here describe a sample, not a market. A reader with a particular deck in hand should measure it. The method on this page is fixed so that two readers measuring the same deck arrive at the same note, which is all a notation can promise.