Both にしては and わりに（は） translate to “for…” or “considering…” and allow us to make statements about things in an objective manner when an outcome differs from what is generally expected.
彼は年の割に若く見えます。
He looks young for his age.
彼にしてはテストの点数が悪かった。
The test score was poor for him (considering it’s usually higher).
Subjective vs. Objective Standards
The difference between the two phrases is subtle, but it dictates why にしては cannot be used with adjectives. First, compare these example sentences:
安い割には、質がいい。
The quality is good given that it’s cheap.
安いにしては、質がいい。(✘)
The quality is good given that it’s cheap.
The difference is that にしては requires a mutual understanding or a specific, concrete standard, which adjectives by their very nature cannot provide. “Cheap” is subjective—what is considered cheap by you might not be cheap to me. 100 yen, however, is simply a fixed, objective value. Consequently, both structures are acceptable in the below examples.
１００円にしては、質がいい。
For 100 yen, the quality is good.
１００円の割には、質がいい。
For 100 yen, the quality is good.
JLPT Rule: Noun Connections
Notice the grammatical difference in how these two structures connect to nouns in the 100 yen examples above. This is heavily tested on Japanese exams:
わりに: Must use the の particle when attaching to a noun (Noun + の割に).
にしては: Attaches directly to the noun (Noun + にしては).
Nuance: Expressing Surprise
In terms of nuance, にしては strongly emphasises a feeling of surprise on behalf of the speaker at the outcome, whereas わりに is a slightly more neutral statement of fact.
お相撲さんにしては、体が小さい。
He’s pretty small for a sumo wrestler (wow, that’s surprising!).