合計
ごうけい · goukei · JLPT N3 · noun
sum
🧮 合計 = combine + count: add everything together and count it
合 means to join or fit together, as in 合う (to match). 計 means to measure or count, as in 計算 (calculation). Join everything, then count it, and you get the grand total.
合 — 乛 (lid/roof) + 口 (mouth)
Originally a pictograph of a lidded vessel — a cover (亼) closing over an opening (口) — giving the core sense of 'come together, join, fit.' From 'fit/match' it extended to 'combine, unite, agree' and to measure-word uses. The reading hé reflects this 'joining' meaning.