Fixed Amount
Fixed Sum, Flat Rate
noun, の adjective
A determined amount is an amount that is fixed, aka a fixed amount, a fixed sum, or flat rate.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
定額のプランにしました。
I went with the flat rate plan.
ワニカニは月々定額で利用できますよ。
WaniKani is available for a fixed monthly amount.
あの〜、定額貯金の口座を開設したいんですが。
Excuse me, I’d like to open up a fixed deposit savings account.