-는 김에: While You're at It — Taking the Chance

-는 김에 (and -은 김에) means on the opportunity of A, also B — while you’re at it.

Gloss matches. 우리말샘 김 「008」: dependent noun used in -은/는 김에 for the chance or occasion of some act. Not “while” as pure overlap in time.

What it does — hitchhike an errand

The first action is already happening (or already done). You piggyback a second useful act.

마트에 가는 김에 우유도 사요.
서울에 온 김에 친구를 만나요.
청소하는 김에 창문도 닦아요.
조명을 바꾸는 김에 커튼도 갈아요.

Interior Unit 4 loves this: once the room is torn open for one fix, other fixes ride along.

Forms

Shape	Feel
-는 김에	opportunity of an ongoing / present-oriented act
-은/ㄴ 김에	opportunity after a completed occasion

Dictionary lists both -은/는 김에. Do not invent a third attachment on bare dictionary-form verbs (가다 김에).

Near neighbours

Form	Job
-는 김에	take the chance to add B
-으면서	while doing A (concurrent)
-을 때	when / at the time
-자마자	as soon as

청소하면서 음악을 들어요.     simultaneous
청소하는 김에 쓰레기도 버려요. opportunity add-on
청소할 때 창문을 열어요.       when-time habit

When to use — and when not

Use it when A creates a natural window for B (already going out, already renovating, already open).

Do not use it for pure simultaneous description with no “extra errand” reading — that is -으면서.

Do not use it as a generic when — that is -을 때.

Practice rule: if English “while you’re at it / since you’re already…” fits, 김에 is the noun you want.