KOI8-R is a single-byte character encoding for Russian Cyrillic defined in RFC 1489. It was the dominant encoding for Russian email and Usenet before UTF-8, and is still seen in legacy files, headers, and protocols.

Why is KOI8-R laid out the way it is?

Its clever design places each Cyrillic letter so that clearing the high bit yields a phonetically similar Latin letter. So if the 8th bit is stripped by an old 7-bit system, КОИ8 text degrades into a roughly readable Latin transliteration rather than gibberish.

How many bytes per character?

KOI8-R is a single-byte encoding, so each character is exactly one byte from 0 to 255. Bytes 0x00 to 0x7F match ASCII, and the upper half 0x80 to 0xFF holds Cyrillic letters and box-drawing characters.

What if my text has characters KOI8-R cannot store?

Only ASCII and Russian Cyrillic (plus some box-drawing glyphs) fit in KOI8-R. Ukrainian-specific letters, emoji, or other scripts have no byte and are marked with ?? when encoding so you can see what was dropped.

Is the conversion done locally?

Yes. The byte table is built in your browser from the native decoder and applied locally for both directions. Nothing is sent to a server.

What is the KOI8-R Encoder/Decoder?

Encode Russian Cyrillic text to KOI8-R byte values in hex, or decode KOI8-R hex bytes back to text. Implements the RFC 1489 single-byte code table, runs entirely in your browser, and flags any character outside the KOI8-R repertoire. It runs free in your browser on Gera Tools, with nothing uploaded.

KOI8-R Encoder/Decoder

Name: KOI8-R Encoder/Decoder
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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KOI8-R is the classic single-byte encoding for Russian Cyrillic, standardised in RFC 1489 and ubiquitous in pre-Unicode Russian email, news, and Unix systems. This tool converts Cyrillic text to KOI8-R hex bytes and decodes KOI8-R bytes back to text, running entirely in your browser.

How it works

KOI8-R maps every character to exactly one byte. The lower half 0x00–0x7F is plain ASCII; the upper half 0x80–0xFF carries Cyrillic letters and some box-drawing symbols. Its most famous property is the deliberate ordering of the Cyrillic block: each letter sits at the code point whose low 7 bits spell a phonetically related Latin letter, so stripping the high bit turns Cyrillic text into a rough Latin transliteration rather than noise.

The tool builds the exact byte-to-character table in your browser using the platform’s native KOI8-R decoder, then inverts it for encoding. Decoding feeds your hex bytes straight through that same decoder.

The clever 7-bit fallback design

The designers of KOI8-R made a deliberate choice: if the high bit of each byte is cleared — as would happen when Cyrillic text passes through an old 7-bit-only mail server or terminal — the surviving low-7-bit bytes happen to spell a Latin transliteration of the Russian. The Russian word РЫНОК (market) encoded in KOI8-R, with the high bit stripped, becomes something like rynok in Latin letters, preserving readability. This was a practical solution to the reality that much 1990s internet infrastructure was built for 7-bit ASCII and silently stripped the 8th bit from non-ASCII bytes.

Example encoding

"Привет, мир" (Privet, mir — Hello, world) encodes to the KOI8-R bytes:

П  р  и  в  е  т  ,     м  и  р
f0 d2 c9 d7 c5 d4 2c 20 cd c9 d2

Each Cyrillic letter becomes one byte in the 0x80–0xFF range; the comma (0x2C) and space (0x20) stay as plain ASCII.

When you encounter KOI8-R today

KOI8-R is most often encountered in:

Legacy email archives. Email from Russian-speaking users before the mid-2000s was frequently encoded in KOI8-R or Windows-1251. If a .eml file shows charset=koi8-r in the Content-Type header, decode its body bytes with this tool.
Old Usenet posts. Russian-language Usenet groups were predominantly KOI8-R; archived posts from the 1990s and early 2000s may still arrive in this encoding.
Unix configuration files. Some older Russian Linux distributions and configs stored comments and strings in KOI8-R.
IRC and chat logs. Pre-Unicode IRC channels for Russian speakers often used KOI8-R.

KOI8-R covers Russian but not all Cyrillic; Ukrainian-specific letters such as Ї, Є, and І live in the related KOI8-U variant (RFC 2319) and will be flagged as unmapped here. Windows-1251 is the other common single-byte Russian encoding and has different byte values for the same letters.