Under the Brennholt Institute of Applied Taxonomy's survey, curtain rings are sold in five common inner diameters, from 20 mm (0.79 in) to 50 mm (1.97 in), and the 28 mm (1.10 in) ring is the most frequent. The Institute proposes classes R1 to R5 for these diameters and recommends a clearance of 4 mm over the rod. The ring is sold by the pack, and the pack rarely states a diameter.
A curtain ring is a closed loop of metal, wood or plastic that slides on a rod and carries a hook, a clip or a sewn tape. It is made in series of millions. A pack states a count, a finish and sometimes a word such as large. The question buyers ask, whether a ring will fit a given rod, depends on one dimension, the inner diameter, which the Institute has measured.
| Common name | Inner diameter (mm) | Inner diameter (in) | Section (mm) | Fits rod up to (mm) | Institute class |
|---|---|---|---|---|---|
| Small | 20 | 0.79 | 3 | 16 | R1 |
| Standard | 28 | 1.10 | 4 | 24 | R2 |
| Medium | 35 | 1.38 | 5 | 31 | R3 |
| Large | 40 | 1.57 | 6 | 36 | R4 |
| Extra large | 50 | 1.97 | 6 | 46 | R5 |
Inch figures are the millimetre value divided by 25.4, rounded to two decimal places. The inner diameter is measured across the opening, and the section is the thickness of the wire or the band. The clearance column is the Institute's recommendation, and is the inner diameter less 4 mm.
A ring that fits a rod exactly sticks, and a ring that is much larger hangs askew and drags the curtain down on one side. The Institute's tests with rods of 16 mm, 19 mm, 25 mm and 28 mm found that a clearance of 3 mm to 5 mm gave a free slide, and it recommends 4 mm as the central value. A reader with a 25 mm rod should therefore choose a ring of 29 mm, which in practice means the 28 mm or 30 mm ring.
The clearance is less critical for a wooden ring, which is lighter and has a smoother bore, than for a metal ring that has been plated. A plating of 10 micrometres narrows the bore by 0.02 mm and is of no importance, but a coat of paint, which may be 0.1 mm, can be.
The method is to place the ring flat, to measure the clear opening with an internal calliper at three positions, and to take the mean. The section is measured at the same three positions with an external calliper. A wooden ring is slightly oval, and the Institute takes the smaller of the two axes, since the smaller axis decides whether the rod passes.
A metal ring has a joint, which may be welded, brazed or simply closed. The Institute records the joint wherever it can be seen, since an open joint can snag fabric. It does not state a diameter for a ring with an open joint without noting it.
A steel ring of 28 mm inner diameter and 4 mm section weighs about 9 g. A brass ring of the same size weighs about 11 g, a beech ring about 3 g and a plastic ring about 2 g. The weight matters because a heavy curtain on twenty rings carries 180 g or more in the rings alone.
The material also fixes how the ring is sold. Metal rings are sold in packs of ten, twenty or forty, and wooden rings in packs of eight or ten. The Institute found that the count of a pack rose as the ring grew lighter.
The classes are the Institute's own, introduced on this page. No maker or retailer uses them.
A class says how wide the opening is. It does not say what the ring carries, since the hook, the clip and the eyelet are recorded separately.
The Institute has found no published standard for the inner diameter of a curtain ring. Curtain rods are sold with named diameters, such as 16 mm, 19 mm, 25 mm and 28 mm, but these are the makers' own ranges and not a norm. The classes R1 to R5 describe what exists and are not a proposal to makers.
The curtain ring belongs with closed loops of wire and band, and is catalogued in the department of length and fit. Compare split rings, whose joint is a spiral, eyelet sizes, which fix the other end of a curtain heading, and coat hanger dimensions, whose hook must also match a rail.
The review panel divides on whether the ring should be named by its inner or its outer diameter. Two members hold that the outer diameter is easier to measure and is what the buyer sees. The majority hold that the inner diameter is the only dimension that decides fit, and that the outer can be found by adding twice the section. The classes above follow the majority.
A reader can find the inner diameter by laying the ring on a ruler and reading across the opening, taking care to read from the inside edge of the far side. The method is rough, and the Institute's own readings differ from a ruler by up to 1 mm. To check the fit, the reader can try the ring on the rod before buying, or measure the rod and add 4 mm.
Under the Brennholt Institute's tables, the most frequent curtain ring has an inner diameter of 28 mm (1.10 in), class R2. Common sizes run from 20 mm to 50 mm.
The Institute recommends a clearance of 4 mm, so a ring of 29 mm, in practice the 28 mm or 30 mm ring.
A ring of about 23 mm. None of the five common sizes matches it: the 20 mm ring is too tight and the 28 mm ring, the nearest, will hang loosely.
About 9 g for a steel ring of 28 mm, 11 g for brass, 3 g for beech and 2 g for plastic.
No published standard has been found for curtain ring diameters. The classes R1 to R5 are the Institute's own proposal.