Kalium Music, formerly known as Circle K Strings, is a Southern California–based manufacturer of premium guitar and bass strings with a particular focus on extended-range, multiscale, and alternate-tuning instruments. The company built its reputation on offering balanced-tension sets at gauges that were difficult to source elsewhere, catering to drop-tuned bass players, 7- and 8-string guitarists, and players using fanned-fret or extended-scale instruments who need single strings or custom configurations rather than off-the-shelf packs.

The catalog covers electric bass, electric guitar, and acoustic guitar and bass strings, available as full sets or as individual singles in a wide range of gauges. Beyond strings, Kalium offers the Quake Bass, a custom electric bass built in-house, as well as the TruCube line of subwoofers. The brand is most often associated with the bass and extended-range guitar communities, where players seeking tightly specified tension across non-standard tunings have driven much of its following.

Kalium Strings has 3 products cataloged on Equipboard, including Strings. Their gear is featured by 3 artists, with the strongest followings in Rock, Electronic, and Stage & Screen. Notable users include Josh Gilbert, Hank Shermann, and Connor Carbon.