Beads are not sold to a single published scale; the figures in use are trade habits, and the most settled is that a round bead is named by its diameter in millimetres, with 6 mm, 8 mm and 10 mm as the sizes most often stocked. The Brennholt Institute of Applied Taxonomy finds that bead diameters cluster around five bands, that seed beads use a separate numbering that does not translate cleanly into millimetres, and that the clusters are best described by a set of five classes, B1 to B5.
Every figure on this page is nominal. Beads are drawn, pressed, moulded or turned, and the same size can vary by a few tenths of a millimetre within one bag. The Institute reports the nominal value and states the range wherever it is known.
| Bead type | Diameter (mm) | Inches | Typical hole (mm) | Institute class |
|---|---|---|---|---|
| Seed bead, size 11/0 | about 2.0 to 2.4 | 0.08 to 0.09 | about 0.8 to 1.0 | B1 |
| Seed bead, size 6/0 | about 3.5 to 4.5 | 0.14 to 0.18 | about 1.0 to 1.5 | B1 |
| Round bead, 4 mm | 4 | 0.16 | about 1.0 | B2 |
| Round bead, 6 mm | 6 | 0.24 | about 1.0 to 2.0 | B2 |
| Round bead, 8 mm | 8 | 0.31 | about 1.0 to 2.0 | B3 |
| Round bead, 10 mm | 10 | 0.39 | about 1.5 to 2.5 | B3 |
| Round bead, 12 mm | 12 | 0.47 | about 2.0 to 3.0 | B4 |
| Large bead, 20 mm | 20 | 0.79 | about 3.0 to 5.0 | B5 |
Inch figures are the millimetre value divided by 25.4, rounded to two decimal places. Where a range is given, the inch range follows from the endpoints. The Institute's survey of retail listings found that 6 mm, 8 mm and 10 mm made up the large majority of the round sizes offered for jewellery, and that sizes between them, such as 7 mm and 9 mm, were rare outside specialist suppliers.
Seed beads are named by an aught number, written 6/0, 8/0, 11/0 and so on, where a larger number means a smaller bead. The number is a trade code that originally related to how many beads of that size fitted along a given length of strand, and it is not a diameter. Different glassmakers draw their beads to different tolerances, so a size 11/0 from one maker is not the same as a size 11/0 from another. The Czech and Japanese products are the two best known, and the Japanese cylinder beads are deliberately more regular than the rounder Czech product.
The ranges in the table are therefore the Institute's own summary of what retail listings state, and not a conversion formula. Anyone who needs an exact fit should measure the beads in hand, a point the Institute makes repeatedly on its measurement pages. The same caution applies to the wire and needle sizes discussed on the knitting needle page, where the numbering also fails to translate into a single diameter.
The bead diameter is what a buyer sees, but the hole decides what it can be threaded on. A bead with a 1 mm hole takes a 0.8 mm cord or wire, with a little room for a knot, and a bead with a 2 mm hole takes leather cord of 1.5 mm. The hole is rarely stated and varies more than the diameter, sometimes by half a millimetre in one bag. Pandora-style large-hole beads, with holes of about 4.5 mm to 5 mm, are a separate trade with its own fittings.
The Institute recommends choosing the thread first and the bead second. A thread that fills the hole will stiffen the piece and abrade against the glass, while a thread that is too thin lets the beads slide and knot badly. The numbering of threads and wire is described on the thread spool page and on the wire gauge page.
The practical question is how many beads make a given length. A strand of 6 mm round beads holds about 166 beads to a metre, since a metre is 1000 mm and each bead occupies 6 mm. A strand of 8 mm beads holds 125 to a metre and a strand of 10 mm beads holds 100. The Institute calls this the packing count and notes that it assumes beads touch end to end, with no knots between them. A necklace of 45 cm in 8 mm beads therefore needs about 56 beads, and the Institute adds one or two to allow for the clasp.
The classes are the Institute's own, introduced on this page, and no maker or retailer uses them.
The classes are set by diameter alone. Shape, whether round, tube, bicone or faceted, is recorded separately, since a faceted bead of 8 mm is measured across the widest point and looks smaller than a smooth round bead of the same size.
Beads share a problem with buttons, which are named in a unit that does not match the metric system, as described on the button page. They also share the problem of the zip, whose numbering is covered on the zip page: a code that began as a count and became a name. The Institute groups the three under the same heading in the department of thickness and gauge, because in each case the label and the measurement have separated.
No international standard sets the diameter of a decorative bead. Some national standards govern the safety of small parts in children's products, and these set limits on the size of an object that can be swallowed, not on bead dimensions. The Institute has not found a published dimensional specification for jewellery beads in any of the national catalogues it has searched, and it records this absence on the page.
The review panel divides on whether seed beads belong in the same scheme as round beads at all. Two members hold that an aught number is a different kind of quantity and that B1 should be a separate scale. The majority place both in one scheme, since the buyer's question is the same, namely how large the bead is, and the table already warns readers not to convert. The classes above are the majority's.
Under the Brennholt Institute's tables, a 6 mm bead is 0.24 in across and falls in class B2.
The Institute records a size 11/0 seed bead as about 2.0 mm to 2.4 mm across, but warns that makers differ and the aught number is a trade code and not a diameter.
The Institute's packing count is 125 beads to a metre, so a 45 cm necklace needs about 56 beads, plus one or two for the clasp.
The Institute records holes of about 1.0 mm to 2.0 mm for a 6 mm bead, and advises choosing the thread before the bead.
No dimensional standard has been found by the Institute. The classes B1 to B5 are the Institute's own proposal.