The Brennholt Institute of Applied Taxonomy measures a rubber band in three dimensions: flat length, taken with the band laid unstretched on a table and measured along one side (a No. 33 gives 88.9 mm, or 3½ in); width, taken across the cut (3.2 mm, or ⅛ in); and thickness, taken through the wall of the band (usually 0.8 mm to 1.5 mm). The Institute uses a ruler for the first, and callipers for the other two.
Only the flat length and the width appear in the trade numbering described on the numbering page. Thickness does not, which is why two bands with the same number can behave differently in the hand.
A rubber band is a closed loop cut from a tube. Its flat length is half its circumference when the loop is laid without stretch, since the loop folds into two layers. The Institute therefore reports flat length, and the unfolded circumference is twice that figure. A No. 33 band, with a flat length of 88.9 mm, has an unstretched circumference of 177.8 mm. Several suppliers give the circumference, not the flat length, and the Institute's first rule for reading a list is to check which one is meant.
The Institute's procedure is its own. It is intended to make measurements by different people comparable, and it is not a published standard.
| Dimension | Instrument | Typical office band | In the trade number? |
|---|---|---|---|
| Flat length | Steel rule | 63.5 mm to 177.8 mm | Yes |
| Width | Callipers | 1.6 mm to 6.4 mm | Yes |
| Thickness | Callipers | 0.8 mm to 1.5 mm | No |
The Institute's measurements of 120 bands in 2025 found that the width of a nominal ⅛ in band ranged from 2.9 mm to 3.6 mm. The spread comes from the cutting. The tube is slit by a rotating blade, and blade wear widens or narrows the cut over a production run. A single band is also not uniform, and the Institute's rule of averaging three readings follows from this.
Thickness governs the force a band applies. Of the three dimensions it matters most to the user, and it is the one the trade number leaves out. The Institute's sample suggested that a thicker band of the same number holds a bundle more firmly and perishes at about the same rate, but the sample was 31 bands and the Institute does not generalise from it.
Rubber is measured in several recognised ways in industry. The ISO standards for vulcanised rubber describe tensile testing and hardness measurement, and the Institute makes no claim to those. It claims only the three-dimension procedure above, for office bands, where no standard describes how a flat length should be taken.
The panel disagrees about whether the zero of the rule should sit at the outer or the inner edge of the loop's end. The inner-edge method gives a shorter figure, typically by 3 mm to 4 mm, because the inner edge of a rounded end is closer to the centre. The majority prefer the outer edge, on the ground that an outer measurement is what a user compares with an object. The dissenters hold that the inner measurement is closer to the loop's true geometry. The published figures use the outer-edge method.
The Institute proposes the procedure above because a measurement that two people can repeat is the first requirement of a classification, and the second is that it be written down. Rubber bands have long had the first without the second.