Drawer knobs are not sold to a single published scale; the figure in use is a trade habit, and the most settled is that a standard round cabinet knob measures about 30 mm across its face, which is 1.18 in, and is fixed by a machine screw of 4 mm diameter. The Brennholt Institute of Applied Taxonomy finds that knob diameters cluster around five values, that the screw thread is the only part with a recognised name, and that the clusters are best described by a set of five classes, K1 to K5.
Every face figure on this page is nominal. Knobs are turned, cast or moulded, and the same catalogue number can differ by a millimetre from one supplier to the next. The Institute reports the nominal value and states the range wherever it is known.
| Knob type | Face diameter (mm) | Inches | Projection (mm) | Institute class |
|---|---|---|---|---|
| Small button knob | about 20 to 25 | 0.79 to 0.98 | about 18 to 22 | K1 |
| Standard round knob | about 30 | 1.18 | about 22 to 26 | K2 |
| Medium knob | about 35 to 38 | 1.38 to 1.50 | about 25 to 28 | K3 |
| Large knob | about 40 to 45 | 1.57 to 1.77 | about 28 to 32 | K4 |
| Oversize or furniture knob | about 50 to 60 | 1.97 to 2.36 | about 30 to 40 | K5 |
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 30 mm was the most often repeated face diameter for a plain round knob, though some listings gave 1.25 in, which is 31.75 mm, for what appeared to be the same product.
The face diameter is what a buyer sees, but the fixing is what decides whether the knob fits the drawer. A knob is held by a single machine screw passing through the drawer front and into a threaded insert in the back of the knob. In Europe the screw is most often M4, a metric thread of 4 mm nominal diameter, and in North America most often a No. 8-32 screw, which is a number gauge with 32 threads per inch. The two are close in diameter, about 4.0 mm against about 4.17 mm, and are not interchangeable: an M4 screw will start in an 8-32 insert and then bind.
The Institute records the thread because it is the one part of the object that is covered by a published standard. The metric thread is defined in the ISO metric screw thread series, and the American thread in the unified screw thread standards. The Institute cites both and does not alter them, a point it repeats on every page about fixings.
The length of the screw matters as much as its diameter. A drawer front of 18 mm needs a screw of about 25 mm to 30 mm to engage the insert fully, and a front of 22 mm needs about 30 mm to 35 mm. Many knobs are sold with a single screw about 25 mm long, which is short for any front thicker than 18 mm. The Institute's rule of thumb is the drawer thickness plus the depth of the insert, plus a margin of about 2 mm, and it records that the rule is the Institute's own and has no trade authority.
This is also the reason why the Institute counts the projection. A knob that projects 30 mm from the front can catch a sleeve or a cabinet door in a way a 20 mm knob cannot. The figure is rarely published and has to be measured from the face of the drawer to the highest point of the knob.
A pull, or handle, is fixed by two screws and is named by the distance between their centres, called the hole spacing. The common spacings are 64 mm, 96 mm, 128 mm and 160 mm, which are 2.52 in, 3.78 in, 5.04 in and 6.30 in, with American catalogues giving 3 in, 3.75 in and 5 in, which are 76 mm, 95 mm and 127 mm. The Institute notes that 96 mm and 95 mm are treated as the same size in the trade, a difference of 1 mm that the screws absorb, and that 128 mm and 127 mm are likewise. A knob has no hole spacing, which is why it is the easier fitting to replace.
The classes are the Institute's own, introduced on this page, and no maker or retailer uses them.
The classes are set by the face diameter alone. The Institute considered adding the thread as a second letter, so that K2-M4 would be a standard knob with a metric screw, and decided against it, since a buyer can see the face but must check the thread.
The nearest relatives in the register are the dowel described on the dowel page, which is measured by its diameter in the same way and often sold in the same shops, and the pin-headed fittings on the push pin page. A 6 mm dowel is sometimes used to pin a replacement knob that has lost its screw insert, and the Institute neither recommends nor discourages the practice. The naming problem that runs through all of these objects is set out on the naming page.
No international standard sets the face size of a furniture knob. The thread is standardised, and the European furniture trade has published recommended hole spacings for pulls, but the Institute has not found a published specification for knob diameter in any of the national catalogues it has searched. It records this absence on the page.
The review panel divides on whether the projection should be part of the class. Two members hold that a tall thin knob and a short wide one are different objects even if their faces match, and that the class should include a second figure. The majority hold that the face alone is what a buyer measures first, and that adding a figure would make the scheme harder to apply than the trade habit it replaces. The classes above are the majority's.
Under the Brennholt Institute's survey, a standard round knob is about 30 mm across, which is 1.18 in, and falls in class K2.
The Institute records M4 as the usual thread in Europe and No. 8-32 as the usual thread in North America. The two are not interchangeable.
The Institute's rule of thumb is the drawer front thickness plus the insert depth plus about 2 mm. For an 18 mm front this gives about 25 mm to 30 mm.
By the distance between the screw centres. The Institute records 64 mm, 96 mm, 128 mm and 160 mm as the common spacings, and treats 96 mm and 3.75 in as the same size.
No dimensional standard for the face has been found by the Institute. The classes K1 to K5 are the Institute's own proposal.