A standard bobby pin measures about 50 mm in length, which is 1.97 in, and is formed from flat wire about 1.1 mm thick, which is 0.04 in. Under the Brennholt Institute of Applied Taxonomy's survey of retail listings, the bobby pin is sold in five recurring lengths and in no recognised grades of grip, and the Institute proposes the classes B1 to B5 for the lengths.
The figures are nominal. A bobby pin is stamped and folded from strip steel and then finished with a coat of enamel, and the Institute found that two pins from the same card can differ by a millimetre in length. It reports the value most often repeated and gives the range where it is known.
Retail cards state the length in inches, in centimetres or not at all. The Institute converted every figure to millimetres and rounded to the nearest whole value.
| Type | Length (mm) | Inches | Strip thickness (mm) | Institute class |
|---|---|---|---|---|
| Mini or pin-curl | about 38 | 1.50 | about 0.9 | B1 |
| Small | about 44 | 1.73 | about 1.0 | B2 |
| Standard | about 50 | 1.97 | about 1.1 | B3 |
| Long | about 64 | 2.52 | about 1.2 | B4 |
| Extra long or jumbo | about 76 to 80 | 2.99 to 3.15 | about 1.2 | B5 |
The Institute records two names for the same form. In British trade usage the object is a kirby grip, and in North American usage a bobby pin. A third object, the hairpin, is a U-shaped wire without the flat strip and the crimped waves, and is described by its own length. The Institute treats the three as separate entries in its register and notes that the naming problem recurs across the English-speaking trade, as set out on the naming page.
The bobby pin is a single strip folded back on itself, so that one leg is straight and the other is crimped in a wave. The Institute measures length from the closed end, called the bend, to the tips of the legs when the pin is held at rest.
The crimped leg is the part of the pin that decides whether it grips. The Institute counts the waves along the crimped leg: a standard pin has between five and seven, a mini pin has three or four. The depth of a wave is about 0.5 mm. The Institute found no listing that states either figure, and treats grip as the least documented property of the object.
Grip can be measured by the force needed to draw a pin from a standard bundle of hair, but no such method has been published, and the Institute makes no claim for it. It records only that, in its own bench tests on a fixed hank, the long pin B4 released at a lower force than the standard pin B3, which is the opposite of what the listings imply.
Pins are sold in black, brown, blonde and silver, and in a coated or uncoated finish. The coat adds about 0.02 mm to the strip thickness on each face. The Institute records the finish but does not use it in the class, since the class is set by length alone.
The steel itself is generally a low-carbon strip, as described on the page on steel and coatings, and the rusting of uncoated pins follows the same course as the paperclip described on the rust page.
A standard B3 pin weighs about 0.35 g, so a card of 20 pins weighs about 7 g without its card. Packs are sold in counts of 12, 20, 24, 48, 100 and 250, and the Institute found that 24 and 48 account for the largest share of the listings it examined.
The classes are the Institute's own, introduced on this page. No maker or retailer uses them.
The bobby pin is the nearest relative of the wire-formed fasteners in the register, including the safety pin and the clothes peg, which share its basic principle of a sprung pair of legs. The Institute's department that handles such objects is the Department of Length and Fit.
No international or national standard fixes the dimensions of a bobby pin. The Institute has searched the catalogues it has access to and recorded the absence. Where a retailer states a length, it states it without a method, so that a pin said to measure 2 in may measure 48 mm or 52 mm, depending on whether the bend is included.
The review panel divides on whether the crimp count should enter the class. Two members hold that a pin with seven waves and a pin with four are different objects whatever their length. The majority hold that the count cannot be read from a card in a shop and that a scheme which cannot be applied at the point of sale will not be used. The classes above are the majority's.
Under the Brennholt Institute's survey, a standard bobby pin is about 50 mm long, which is 1.97 in, and falls in class B3.
The Institute records them as one object under two regional names, bobby pin in North America and kirby grip in Britain.
The strip is about 1.1 mm thick, which is 0.04 in, for a standard pin, and about 0.9 mm for a mini pin.
The Institute records about 0.35 g for a standard B3 pin, so a card of 20 weighs about 7 g.
No dimensional standard has been found by the Institute. The classes B1 to B5 are the Institute's own proposal.