Under the Brennholt Institute of Applied Taxonomy's four-class system, a drinking straw has an outer diameter between roughly 3 mm and 12 mm (0.12 to 0.47 in), and the sizes actually made cluster into four bands: stirrer, standard, wide and bubble. A standard straw measures about 6 mm (0.24 in) across and is the size most people mean when they say "a straw".
No published classification names these bands, so the Institute proposes one. It measures outer diameter, because that is what has to pass through a lid, and records inner diameter separately, because that is what a drink has to pass through.
| Class | Outer diameter (mm) | Outer diameter (in) | Typical use |
|---|---|---|---|
| I. Stirrer | 3 to 5 | 0.12 to 0.20 | Cocktails, short drinks, stirring |
| II. Standard | 5.5 to 7 | 0.22 to 0.28 | Soft drinks, water, juice |
| III. Wide | 8 to 9 | 0.31 to 0.35 | Milkshakes, smoothies |
| IV. Bubble | 10 to 12 | 0.39 to 0.47 | Tapioca drinks, thick blends |
Very little. Straws are made by extrusion in plastic, by rolling in paper, and by moulding in other materials, and the diameter is set by the maker's die or mandrel. Food-contact rules govern what the straw may be made of, and the Institute does not dispute them. What is not set by any rule is a shared set of names for the diameters, which is the gap this page fills with the Institute's own classes. They are not a trade standard and the Institute says so on every page where it uses them.
A straw is a small pipe, and the flow through a pipe rises steeply with its bore. The Institute treats the effect in simple terms: doubling the inner diameter does not double the flow, it raises it several times over, which is why a thick shake that cannot be drawn through a Class II straw moves easily through Class IV. A drinker feels this directly. A narrow straw makes a thin drink feel slow and a thick drink impossible.
Wall thickness changes the picture. A paper straw has a thicker wall than a plastic one of the same outer diameter, so its inner bore is smaller and it behaves, in the Institute's tests, like the next class down. That is why the Institute asks for both numbers whenever a straw is described.
The Institute took a sample of straws and read the outer diameter at three points along each, using a dial caliper, then cut each at the midpoint and read the bore under magnification. Results were rounded to the nearest half millimetre. The sample is small and the Institute does not generalise from it, but the four bands appeared in every batch, with very few straws falling in the gaps between them.
The same habit of measuring the object before naming it runs through the Department of Length and Fit, which the Institute opened for objects whose size is a matter of fit. Related questions about thickness are collected in the Department of Thickness and Gauge, and a comparable small-bore problem is treated on the twist tie page.
The review panel divides on whether the stirrer should count as a straw at all. Two members hold that an object not meant to carry a drink to the mouth belongs to another family. The majority keep it as Class I, since it is sold beside the others, is extruded the same way, and is described by the same word in most shops. The classification above is the majority one.
The Institute does not claim that every straw sold fits one of the four classes, and it does not say which is best. It says only that the bands recur, and that naming them lets two people describing a straw mean the same thing.