UTF-32

UTF-32 stands for Unicode Transformation Format in 32 bits. It is a protocol to encode Unicode code points that uses exactly 32 bits per Unicode code point (but a number of leading bits must be zero as there are fewer than 2²¹ Unicode code points). UTF-32 is a fixed-length encoding, in contrast to all other Unicode transformation formats, which are variable-length encodings. Each 32-bit value in UTF-32 represents one Unicode code point and is exactly equal to that code point's numerical value.

The main advantage of UTF-32 is that the Unicode code points are directly indexable. Finding the Nth code point in a sequence of code points is a constant time operation. In contrast, a variable-length code requires sequential access to find the Nth code point in a sequence. This makes UTF-32 a simple replacement in code that uses integers that are incremented by one to identify a character in a string, as was commonly done for ASCII.

The main disadvantage of UTF-32 is that it is space-inefficient, using four bytes per code point. Characters beyond the BMP are relatively rare in most texts, and can typically be ignored for sizing estimates. This makes UTF-32 close to twice the size of UTF-16. It can be up to four times the size of UTF-8 depending on how many of the characters are in the ASCII subset.

The original ISO 10646 standard defines a 32-bit encoding form called UCS-4, in which each encoded character in the Universal Character Set (UCS) is represented by a 31-bit value between 0 and 0x7FFFFFFF (the sign bit was unused and zero). In November 2003, Unicode was restricted by RFC 3629 to match the constraints of the UTF-16 character encoding: explicitly prohibiting code points greater than U+10FFFF (and also the high and low surrogates U+D800 through U+DFFF). Although the ISO standard had (as of 1998 in Unicode 2.1) "reserved for private use" 0xE00000 to 0xFFFFFF, and 0x60000000 to 0x7FFFFFFF these areas were removed in later versions. Because the Principles and Procedures document of ISO/IEC JTC 1/SC 2 Working Group 2 states that all future assignments of characters will be constrained to the Unicode range, UTF-32 will be able to represent all UCS characters and UTF-32 and UCS-4 are identical.

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