Read bicycle rim dimensions and distinguish them from tire width

The short answer: rim width is a specific measurement for a specific model

There is no single mountain bike rim width. Each rim has its own documented width, and manufacturers usually give two numbers. Internal width is the distance between the inside walls of the rim, where the tire beads sit. External width is the distance across the outside faces of the rim. For example, DT Swiss lists its F572 rim (part number RDF057BDPB28SA2880, a 27.5-inch/650B model) with a 30 mm internal width, a 35 mm external width and a 20 mm rim height. To find the width of your rim, look up the exact model and check which of these two figures a given number refers to. Tire-fit guidance relies on the internal width.

Tire width is a separate measurement of the tire, not the rim. A tire marked 37 mm wide does not mean the rim is 37 mm wide. Schwalbe pairs a 37 mm tire with a rim whose internal width is 19 mm.

The five dimensions and where each is measured

Dimension	Measured between	Documented example
Internal rim width	The inside faces of the two rim walls	30 mm (DT Swiss F572); 19 mm (Schwalbe's 622 x 19C rim example)
External rim width	The outside faces of the two rim walls	35 mm (DT Swiss F572)
Bead seat diameter	The bead-seat reference points on opposite sides of the wheel, not the outside of the tire tread	622 mm (Schwalbe's 622 x 19C rim and 37-622 tire)
ERD (effective rim diameter)	A separately labeled rim specification	564 mm (DT Swiss F572)
Tire width	The widest points of the tire, including the outside of the tread lugs	37 mm (Schwalbe 37-622)

ERD and bead seat diameter are different specifications. When a rim spec sheet lists an ERD, do not read it as the diameter the tire mounts to. The F572's 564 mm ERD is not its bead seat diameter.

Decoding tire and rim markings

Metric tire markings put width first and diameter second. Schwalbe explains that a tire marked 37-622 is 37 mm wide and has a 622 mm inner diameter. In Schwalbe's technical booklet, the matching rim is written 622 x 19C, which means a 622 mm bead seat diameter and a 19 mm internal width. The rim notation reverses the order and puts diameter first.

Tire:  37-622     ->  37 mm tire width  |  622 mm inner diameter
Rim:   622 x 19C  ->  622 mm bead seat diameter  |  19 mm internal rim width

The shared number, 622, is the link between the two parts. It means both are documented for the same mounting diameter. Inch sizes such as 27.5 inch describe only the approximate outside diameter and width. Two products with the same inch label do not necessarily have the same inner diameter, so use the metric diameter figure when one is printed.

Why a tire's width is not a fixed number

Schwalbe measures tire width across the tire's widest points, including the outside of the lugs on knobby tires. Inflation pressure and the time since mounting can both change the width you measure. This is another reason the tire figure cannot stand in for the rim's internal or external width.

How to read a rim or tire specification

Copy the complete marking or part number exactly as printed, for example RDF057BDPB28SA2880 or 37-622.
Identify the notation. In tire markings like 37-622, width comes first. In rim markings like 622 x 19C, diameter comes first. Inch sizes are only approximate.
Match each number to its measurement points: inside walls for internal width, outside faces for external width, bead seats for diameter, and widest points for tire width.
Keep ERD in its own field and do not use it as the bead seat diameter.
Record the document and version each value came from, so figures from different sources do not get mixed.

Here is an illustrative way to set up those records. The Schwalbe tire and rim are documented as a linked pair, so they share a row group. The DT Swiss rim is a separate example. Combining the two would create a wheel specification that no manufacturer documents.

Component	Exact model / marking	Dimension	Value	Measured between
Tire (Schwalbe example)	37-622	Tire width	37 mm	Widest points, including lugs
Tire (Schwalbe example)	37-622	Inner diameter	622 mm	Bead seat reference points
Rim (Schwalbe example)	622 x 19C	Bead seat diameter	622 mm	Opposite bead seats
Rim (Schwalbe example)	622 x 19C	Internal width	19 mm	Inside rim walls
Rim (separate example)	DT Swiss F572 RDF057BDPB28SA2880	Internal width	30 mm	Inside wall faces
Rim (separate example)	DT Swiss F572 RDF057BDPB28SA2880	External width	35 mm	Outside wall faces
Rim (separate example)	DT Swiss F572 RDF057BDPB28SA2880	ERD	564 mm	Separately labeled; not bead seat diameter

Checking whether a tire suits a rim

Matching numbers are only a starting point. Schwalbe's fit guidance has two parts. The tire's width must suit the rim's internal width, and the rim maker's limits on permitted tire types, tire widths and maximum pressure must be followed. Before mounting a tire, check the instructions for that exact rim and that exact tire. If you use a rim-and-tire width chart, note its edition and which rim constructions, tire types and models it covers before relying on any entry.