半斤八两 (bànjīnbāliǎng): six of one, half a dozen of the…

Under the old Chinese weight system, one 斤 (jīn, catty) was 16 两 (liǎng, tael) — so 'half a catty' equaled 'eight taels' exactly. The phrase appears in the Song-dynasty Chan Buddhist text 《五灯会元》 (Compendium of the Five Lamps): 秤头半斤，秤尾八两 — 'one end of the scale half a catty, the other end eight taels.' Modern China shifted to a 10-tael 斤 in 1959, but the idiom kept the older math.

usage

Usually carries a slight put-down: both sides are equal, but equally mediocre or equally at fault. Don't use it to praise two excellent things being on par — for that, prefer 不相上下 (bùxiāngshàngxià) or 旗鼓相当 (qígǔ-xiāngdāng).