Subscribe to The Chicago Manual of Style Online
Q. The abbreviation for “revolutions per minute” is rpm as stipulated in CMOS 10.49. A document I’m editing contains a picture with the following label: “RPM Gauge.” I don’t like RPM being all capitals, but I’m not sure if rpm or Rpm is any better. What should I use?
A. Although rpm is Chicago style, RPM is well accepted. (Rpm less so.) It may be difficult or expensive or time-consuming to get changes made in an illustration, so unless there’s a problem caused by the departure from Chicago style, you should consider looking the other way. (An example of a “problem” would be if the text specifically discusses how to abbreviate the term and specifies the use of rpm, so that the inconsistency is confusing.)