UTF-8 Validator
Validate if your text or file is valid UTF-8 with our free online UTF-8 Validator. The UTF-8 Validator performs byte-by-byte analysis against RFC 3629 encoding rules, detecting invalid start bytes, missing continuation bytes, overlong sequences, UTF-16 surrogates, and codepoints beyond U+10FFFF. Paste text directly or upload any file to get a comprehensive validation report with four views: an overall Summary with encoding suggestions, a detailed Error list with offsets and suggested fixes, a color-coded Byte View showing every byte type and validity status, and a Fixed Text view with invalid sequences replaced by U+FFFD replacement characters. The tool also analyzes byte patterns to suggest what encoding might have been intended when invalid UTF-8 is detected - Windows-1252, ISO-8859-1 (Latin-1), UTF-16, or Shift-JIS. All processing happens locally in your browser. No signup required.
Validate UTF-8 Encoding
Paste text or upload a file to validate its UTF-8 encoding. The tool checks every byte against RFC 3629, highlights invalid sequences, and suggests fixes.
Why Use Our UTF-8 Validator?
Byte-Level UTF-8 Validation
Our UTF-8 validator checks every byte against RFC 3629 encoding rules. It detects invalid start bytes, missing continuation bytes, overlong sequences, UTF-16 surrogates, and codepoints beyond U+10FFFF with precise offset reporting.
Comprehensive Error Detection
The UTF-8 validator identifies 6 distinct error types: invalid start bytes, missing continuation bytes, overlong encodings, surrogate codepoints, out-of-range values, and unexpected continuation bytes. Each error includes a detailed description and suggested fix.
100% Private Browser Processing
Your data never leaves your device. The UTF-8 validator processes everything locally using the Web Crypto API for encoding and pure JavaScript for byte analysis. No server uploads, no data storage, complete privacy.
Smart Encoding Suggestions
When invalid UTF-8 is detected, the validator analyzes byte patterns to suggest what encoding might have been intended - Windows-1252, ISO-8859-1 (Latin-1), UTF-16, or Shift-JIS. It also provides a fixed version with U+FFFD replacement characters.
Common Use Cases for UTF-8 Validator
Web Development & Markup Validation
Use the UTF-8 validator to ensure your HTML, CSS, and JavaScript files are correctly encoded. Invalid UTF-8 in web pages can cause display issues, validator warnings, and SEO penalties from search engines that expect valid UTF-8 content.
API & Data Feed Integration
Validate UTF-8 encoding in API responses, JSON payloads, RSS feeds, and webhook data. The UTF-8 validator helps identify encoding issues that can cause parsing failures when integrating with third-party services that expect properly encoded data.
Database Import & Migration
When migrating databases or importing CSV/SQL files, use the UTF-8 validator to check that text data is properly encoded. Invalid UTF-8 can cause data truncation, corruption, or silent data loss during ETL processes and database migrations.
Email Header & Content Validation
Email systems require proper UTF-8 encoding in headers (Subject, From, To) and body content. The UTF-8 validator checks that your email data is correctly encoded to prevent delivery issues and display problems across different email clients.
Text File & Document Processing
Validate UTF-8 encoding in plain text files, configuration files, log files, and source code. The UTF-8 validator is essential for CI/CD pipelines and data processing workflows that need to ensure all input files are properly encoded before processing.
Debugging Mojibake & Encoding Issues
When text displays as garbled characters (mojibake), use the UTF-8 validator to diagnose the root cause. The tool identifies exactly which bytes are invalid and suggests what the intended encoding might be, helping you fix corrupted text data.
Understanding UTF-8 Validation
What is UTF-8 Validation?
UTF-8 validation is the process of checking that a sequence of bytes conforms to the UTF-8 encoding standard (RFC 3629). UTF-8 encodes each Unicode codepoint as a sequence of 1 to 4 bytes, with specific byte patterns for each sequence length. A valid UTF-8 byte sequence must follow strict rules: ASCII characters (U+0000-U+007F) use a single byte (0x00-0x7F), while multi-byte sequences must start with specific lead byte patterns (0xC2-0xDF for 2-byte, 0xE0-0xEF for 3-byte, 0xF0-0xF4 for 4-byte) and each following byte must be a continuation byte (0x80-0xBF). Our UTF-8 Validator checks every byte against these rules and detects overlong sequences, UTF-16 surrogates, and codepoints beyond U+10FFFF - all of which are invalid in UTF-8.
How Our UTF-8 Validator Works
- Input text or upload a file - Paste text directly into the text area or upload a file. The UTF-8 validator reads the raw bytes using the browser FileReader API. If you paste text, it encodes it as UTF-8 bytes for analysis. All processing is local - nothing is uploaded to any server.
- Byte-by-byte sequence validation - The validator walks through every byte in the data, identifying start bytes and their expected continuation bytes. It validates each byte against its expected type (ASCII, start-2, start-3, start-4, continuation) and flags any violations. Each sequence is decoded to its codepoint and checked for overlong encodings, surrogate values (U+D800-U+DFFF), and out-of-range codepoints (above U+10FFFF).
- Review comprehensive results - Results are organized into four views: Summary (overall validity with percentage bar and encoding suggestions), Errors (detailed list of each invalid sequence with offset, error type, hex bytes, description, and suggested fix), Byte View (color-coded byte-by-byte table showing offset, hex, ASCII, type, and validity status), and Fixed Text (the original text with invalid sequences replaced by U+FFFD replacement characters).
Common UTF-8 Validation Errors Explained
- Invalid Start Byte: A byte that does not match any valid UTF-8 start byte pattern - for example, 0x80-0xBF without a preceding start byte, 0xC0-0xC1 (which would indicate a 2-byte overlong encoding), or 0xF5-0xFF (undefined in UTF-8). These bytes are often the result of corrupt data or a different encoding.
- Missing Continuation Byte: A multi-byte sequence that starts but is truncated before all required continuation bytes are found. This typically happens at the end of a file or when data is split at a multi-byte boundary. The fix is to complete the sequence or replace the incomplete sequence with U+FFFD.
- Overlong Encoding:Using more bytes than necessary to encode a codepoint - for example, encoding an ASCII character (U+0020) with 2 bytes instead of 1. Overlong sequences are invalid because they bypass security filters that only check ASCII ranges (like filtering "/" in path traversal checks).
- Surrogate Codepoints: Bytes that decode to the range U+D800-U+DFFF, which are reserved for UTF-16 surrogate pairs and are not valid Unicode codepoints in UTF-8. These appear when UTF-16 data is misinterpreted as UTF-8.
Privacy, Security & Limitations
100% Private: The UTF-8 Validator processes everything locally in your browser using the FileReader and TextEncoder/TextDecoder APIs. Your data never leaves your device - no server uploads, no data logging, no third-party requests.
Limitations: The UTF-8 validator validates byte sequences against RFC 3629 but does not check whether the decoded text makes semantic sense - a valid UTF-8 sequence can decode to control characters or unexpected codepoints. For files larger than 256 MB, consider validating a sample or segment. The tool provides encoding suggestions based on byte patterns, but these are heuristic and may not always identify the intended encoding, especially for short text snippets.
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Frequently Asked Questions About UTF-8 Validator
A UTF-8 validator checks whether a sequence of bytes conforms to the UTF-8 encoding standard (RFC 3629). You need a UTF-8 validator when you are processing text data from external sources, debugging mojibake (garbled text), validating file uploads, ensuring proper encoding in web applications, checking database exports for encoding issues, or troubleshooting API responses that display incorrectly. Invalid UTF-8 can cause application crashes, data corruption, security vulnerabilities (overlong sequences can bypass filters), and display problems across different platforms.
The UTF-8 validator can detect six distinct error types: Invalid Start Bytes (bytes that do not match any valid UTF-8 start byte pattern like 0xC0, 0xC1, or 0xF5-0xFF), Missing Continuation Bytes (truncated multi-byte sequences), Overlong Encodings (using more bytes than necessary to encode a codepoint, which is a security concern), UTF-16 Surrogates (codepoints in the range U+D800-U+DFFF that are reserved for UTF-16), Out-of-Range Codepoints (values above U+10FFFF), and Unexpected Continuation Bytes (continuation bytes without a preceding start byte).
An overlong encoding occurs when a codepoint is encoded with more bytes than necessary. For example, encoding a forward slash (U+002F, ASCII 0x2F) as the 2-byte sequence 0xC0 0xAF instead of 0x2F. This is a security concern because applications that check for dangerous characters (like path traversal sequences or SQL injection) typically only check the single-byte ASCII representation. An overlong encoding can bypass these filters while still being decoded by a lenient UTF-8 decoder, potentially enabling injection attacks.
The UTF-8 validator processes the entire byte stream and identifies invalid sequences wherever they occur. If a file is mostly UTF-8 with some bytes from another encoding (like Windows-1252 or ISO-8859-1), the validator will flag those non-UTF-8 bytes as errors and provide suggestions for what encoding might have been intended based on byte patterns. The validator also provides a "Fixed Text" view that replaces all invalid sequences with U+FFFD (replacement character), making the data usable while preserving the valid content.
Yes - you can either paste text directly into the text area or upload a file by clicking the Upload File button or dragging and dropping a file onto the text area. The UTF-8 validator reads the file as raw bytes using the browser FileReader API and performs all analysis locally. Text you paste is first converted to UTF-8 bytes using TextEncoder, then analyzed the same way as uploaded files. No data is ever sent to a server.
U+FFFD is the official Unicode Replacement Character (shown as a question mark in a diamond: �). It is used to replace invalid or unrepresentable byte sequences in text processing. When the UTF-8 validator generates the "Fixed Text" view, it replaces each invalid sequence with U+FFFD so that the resulting text is valid UTF-8. This allows downstream processing to continue without errors, though the replaced characters indicate data loss at those positions.
When invalid UTF-8 is detected, the validator analyzes byte patterns to suggest what encoding might have been intended. It checks for UTF-16 patterns (alternating null bytes and ASCII characters suggesting either LE or BE byte order), Windows-1252-specific bytes (0x80-0x9F which contain printable characters like smart quotes and the Euro sign), and high-byte patterns consistent with ISO-8859-1/Latin-1. These suggestions help you identify and convert the data from its original encoding to proper UTF-8.
A valid UTF-8 sequence is a byte sequence that follows the UTF-8 encoding rules - correct start byte, proper continuation bytes, no overlong sequences, no surrogates, and within range. However, a valid UTF-8 sequence may decode to a valid Unicode codepoint that is not a "character" in the normal sense - it could be a control character (U+0000-U+001F), a formatting character (U+200B zero-width space), or a private-use area codepoint. The validator checks byte validity, not semantic character appropriateness.
Yes - 100% free, forever. No signup, no account, no premium tier, no usage limits, and no ads. Validate as much text or as many files as you need. All processing is done locally in your browser with zero server uploads, ensuring complete privacy for sensitive data.