Several parts combine into one character.

算 combines 竹 (bamboo), 目 (eye), and 廾 (two hands) — bamboo (竹) above, two hands (廾) below working bamboo counting-rods watched by the eye (目). In antiquity people laid out bamboo rods (算木) to reckon numbers, and that is its core sense. The 竹 (bamboo) seen earlier sits on top.

Korean hanja-sound "san": 計算 (gyesan, calculation), 算術 (sansul, arithmetic), 豫算 (yesan, budget), 暗算 (amsan, mental arithmetic), 珠算 (jusan, abacus calculation), 算數 (sansu, arithmetic). It is the 算 of elementary-school 算數 (arithmetic).

Mandarin: suàn, 4th tone. 计算 (jìsuàn, to calculate), 打算 (dǎsuàn, to intend), 算了 (suàn le, never mind / forget it), 预算 (yùsuàn, budget), 算命 (suànmìng, to tell fortunes). 算了 (suàn le) and 打算 (dǎsuàn) are daily Mandarin essentials.

Japanese: on-reading サン (san) — essentially no kun-reading. 計算 (keisan, calculation), 算数 (sansū, arithmetic), 予算 (yosan, budget), 暗算 (anzan, mental arithmetic), 珠算 (shuzan, abacus). The 算 of the Japanese abacus (そろばん) is this.

Memory aid: bamboo (竹) counting-rods worked by two hands (廾) watched by the eye (目) — laying out rods to reckon numbers.