Commonly stocked marbles run from about 6 mm to 50 mm across, and the Brennholt Institute of Applied Taxonomy records the standard marble at about 16 mm, which is 5/8 in. Under the Institute's proposed nomenclature this places it in class M3, which runs from 14 mm to under 18 mm.
This page gives the measurements. The names that go with them are on marble sizes and names.
| Diameter (mm) | Diameter (in) | Fraction (in) | Institute class | Usual use |
|---|---|---|---|---|
| 6 | 0.24 | about 1/4 | M1 | Decorative, counting |
| 9.5 | 0.37 | 3/8 | M1 | Small play, craft |
| 12.7 | 0.50 | 1/2 | M2 | Small play |
| 14 | 0.55 | about 9/16 | M3 | Lower edge of common |
| 15.9 | 0.63 | 5/8 | M3 | The common marble |
| 19 | 0.75 | 3/4 | M4 | Shooter |
| 22.2 | 0.88 | 7/8 | M5 | Large shooter |
| 25.4 | 1.00 | 1 | M5 | Large shooter, display |
| 31.8 | 1.25 | 1 1/4 | M6 | Giant |
| 38.1 | 1.50 | 1 1/2 | M7 | Display |
| 50 | 1.97 | about 2 | M7 | Display |
The millimetre figures for the fractional sizes were calculated by multiplying the inch value by 25.4, since the inch is defined as exactly that many millimetres. The remaining figures are the Institute's records of sizes commonly stocked.
The Institute has found no published standard fixing marble diameters. What exists is trade usage: a 'standard' marble at about 5/8 in, a 'shooter' at 3/4 in or more. Both are consistent with the table. The inch and the millimetre are themselves exactly related, and the Institute does not contradict that relation.
The classes are the Institute's own and are no part of any trade usage.
Not very. The Institute measured 24 marbles sold under a single nominal size and found a spread of up to 0.6 mm in diameter, with 17 of the 24 within 0.3 mm of the nominal. A glass marble is formed by rolling, and perfect roundness is not a property of the process. A marble sold as 16 mm may measure anywhere from about 15.7 mm to 16.3 mm. The sample is small and the Institute offers it as an observation.
The practical consequence is that a marble should be measured, not trusted, if it must pass through a gap or sit in a cup of given size. The method is the same as for other small objects and follows the sheet used by the Department of Thickness and Gauge.
To convert inches to millimetres, multiply by 25.4. To convert millimetres to inches, divide by 25.4. A marble of 20 mm is 0.79 in; a marble of 7/8 in is 22.2 mm. Where a source gives a marble size as a fraction, the Institute converts it with the figures above and records the result to one decimal place.
For a comparable conversion on another small object, see the guitar pick thickness chart, and for the department that holds this page, the Department of Nomenclature.
The review panel divides on whether a marble's nominal size should be its largest or its mean diameter. One member argues for the mean, since it is closer to what a gauge ring would report. The majority use the largest reading, which is the one that decides whether a marble passes through a gap. The table uses the majority position.