ずつ details

Examples

each; apiece — a fixed quantity is divided evenly per person or per unit

折おり紙がみ、みんなに三みっつずつ配くばるね。

origami, minna ni mittsu zutsu kubaru ne.

I'll hand out three pieces of origami paper to each of you. casual

参加者さんかしゃには資料しりょうを一いち部ぶずつお配くばりいたします。

sankasha ni wa shiryou o ichibu zutsu o kubari itashimasu.

We will distribute one copy of the materials to each participant. formal

little by little — a change progresses in small, steady increments

焦あせらないで、少すこしずつ慣なれていけばいいよ。

aseranaide, sukoshi zutsu narete ikeba ii yo.

No need to rush — it's fine to get used to it little by little. casual

リハビリを続つづけるうちに、少すこしずつ痛いたみが和やわらいできた。

rihabiri o tsuzukeru uchi ni, sukoshi zutsu itami ga yawaraide kita.

As I kept up with the rehab, the pain gradually eased little by little. neutral

Always rides a NUMBER or quantity word (一つ, 少し, 三人), never a bare noun
(×学生ずつ).

ごとに
Both distribute evenly, but ずつ names the SIZE of the share each recipient gets (お菓子[かし]を三[みっ]つずつ配[くば]る "hand out three sweets each"), while ごとに names ONE unit of the domain at a time, temporal or not (家[いえ]ごとに "for each house" — no interval or spacing involved); ずつ answers "how much each," ごとに answers "which one, taken singly."

Pattern	Attaches to
ごとに
おきに