磅 borrows the English word 'pound'. Mainland China defaults to the metric 公斤 (kilogram); 磅 stays common in Hong Kong, Taiwan, and for body weight, paper thickness, and type size.
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磅 has a separate literary reading páng, used only in the binome 磅礴 'vast and majestic'. Different meaning, different headword - see the second entry on this page.
Stone radical - picks up weight/mass imagery for the 'pound' and 'scale' senses, where stone counterweights once anchored beam balances. Same family: 砖 brick, 碎 shatter, 碰 collide, 硬 hard.
Phonetic - 旁 supplies the sound. It survives unchanged in the literary reading páng, and shifts to bàng in the everyday 'pound/scale' sense. Same phonetic series: 膀 shoulder and 榜 ranked list.
