UTF-8 vs ASCII: why UTF-8 won and when ASCII still matters
Default to UTF-8 for everything. ASCII is a historical subset that UTF-8 handles transparently. The only time you choose ASCII over UTF-8 is when you're interfacing with a system that genuinely cannot handle bytes above 0x7F.
ASCII maps 128 English characters to 7-bit values. UTF-8 is a variable-width encoding that is backwards-compatible with ASCII: every valid ASCII string is also a valid UTF-8 string. UTF-8 can represent every character in Unicode (over 140,000 and growing) using 1 to 4 bytes per character. ASCII-only systems are vanishingly rare in 2026.
By TechCompare · Updated
How this is calculated
UTF-8 is the dominant text encoding on the web and in file formats such as JSON, which requires UTF-8. Go and Rust use UTF-8 as their standard string encoding. Python text-file defaults depend on the version and environment: Python 3.15 enables UTF-8 Mode by default, while Python 3.14 and earlier generally use the locale encoding unless UTF-8 Mode is enabled. ASCII survives in legacy protocols and other systems that require 7-bit data. Use an explicit encoding when file compatibility matters.
Verdict
UTF-8 is widely used on the web, required by JSON, and used by default for strings in Go and Rust. Python's default text-file encoding depends on its version and environment. Python 3.15 enables UTF-8 Mode by default, while earlier releases generally use the locale encoding unless UTF-8 Mode is enabled. ASCII remains useful where a format or system explicitly requires 7-bit data.