The Brennholt Institute of Applied Taxonomy lists twelve common rubber band numbers by flat length and width, from the No. 16 (2½ in by 1/16 in, or 63.5 mm by 1.6 mm) to the No. 117B (7 in by ⅛ in, or 177.8 mm by 3.2 mm). The chart below is the Institute's compilation of trade lists. It is not a published standard, because none exists.
Every length is the flat length, taken with the band laid on a table without stretch. Every width is the width of the cut, which in practice is the thickness of the rubber tube from which the band was sliced. The Institute explains the method on the page on how a rubber band is measured.
| No. | Length (in) | Length (mm) | Width (in) | Width (mm) | Institute grade |
|---|---|---|---|---|---|
| 16 | 2½ | 63.5 | 1/16 | 1.6 | A1 |
| 18 | 3 | 76.2 | 1/16 | 1.6 | B1 |
| 19 | 3½ | 88.9 | 1/16 | 1.6 | B1 |
| 30 | 2 | 50.8 | ⅛ | 3.2 | A2 |
| 31 | 2½ | 63.5 | ⅛ | 3.2 | A2 |
| 32 | 3 | 76.2 | ⅛ | 3.2 | B2 |
| 33 | 3½ | 88.9 | ⅛ | 3.2 | B2 |
| 34 | 4 | 101.6 | ⅛ | 3.2 | B2 |
| 62 | 2½ | 63.5 | ¼ | 6.4 | A3 |
| 64 | 3½ | 88.9 | ¼ | 6.4 | B3 |
| 105 | 5 | 127.0 | ⅛ | 3.2 | B2 |
| 117B | 7 | 177.8 | ⅛ | 3.2 | C2 |
The grade column belongs to the Institute and to nobody else. The letter gives the length band and the figure gives the width band, as set out on the page on the numbering system. A1 is a short, very fine band; C2 is a long band of middling width. The Institute added the column because the trade numbers do not let a reader see, at a glance, whether one band is longer than another.
The Institute compared nine supplier lists in 2025 and recorded the spread of the stated length for each number. For the No. 33, the stated flat length was 3½ in in eight lists and 88 mm in one, which is a difference of 0.9 mm and falls inside the rounding of the conversion. For the No. 117B, three lists gave 7 in, and one gave 180 mm, a difference of 2.2 mm. The Institute considers the differences small and recommends no correction.
Width is where lists disagree more. A band marketed as ⅛ in may be cut at 3 mm, 3.2 mm or 3.5 mm. The Institute found a range of 0.5 mm across suppliers for the same number and attributes it to the cutting blade and the roll of rubber tubing, not to any rule.
The chart has no thickness column, and that is deliberate. Wall thickness is set by the tube and differs from one batch to another. Trade lists give figures between 0.8 mm and 1.5 mm for office bands, and the Institute does not rank them. A reader who needs a thickness should measure the band, as described in the measurement page.
The Institute's review panel is split over the No. 30 band. Two members hold that it belongs to the thirty series, because its width is ⅛ in. The majority note that few suppliers list it and that its stated length of 2 in sits below the other thirties. The chart includes it, and the majority's view is recorded as a reservation. A reader who disagrees will find the minority argument in the Institute's file.
Where a standard does exist for a neighbouring object, the Institute says so. It does not, for rubber bands, and nothing in the chart should be read as claiming one.