RST Signal Reporting System

See also: RSN for newer amateur radio signal reporting system which (also) includes the local noise level

RST stands for Readability, Strength, Tone quality. It was devised for amateur radio. It is useful and it works well. The R is straight forward:

R1 = Unreadable
R2 = Barely readable, occasional words distinguishable
R3 = Readable with considerable difficulty
R4 = Readable with practically no difficulty
R5 = Perfectly readable

S1 = Faint signal, barely perceptible
S2 = Very weak
S3 = Weak
S4 = Fair
S5 = Fairly good
S6 = Good
S7 = Moderately strong
S8 = Strong
S9 = Very strong signals

Ref: https://en.wikipedia.org/wiki/RST_code

RST Reporting wall chart pdf can be downloaded here