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Decode Base64-encoded strings back to original text, binary data, or files with automatic format detection, MIME type identification, image preview, and direct file download for extracted content.
Note: AI can make mistakes, so please double-check it.
Decoded results will appear here
Common questions about this tool
Paste your Base64-encoded string into the decoder, and it instantly converts it back to the original text or binary data. The tool handles standard Base64, Base64URL, and validates the encoding format.
Yes, the Base64 decoder can decode Base64-encoded images, files, or any binary data. After decoding, you can download the original file or view images that were encoded in Base64 format.
Base64 uses + and / characters, while Base64URL uses - and _ for URL-safe encoding. The decoder handles both formats automatically, converting them back to the original data correctly.
Base64 strings contain only A-Z, a-z, 0-9, +, /, and = characters. The decoder validates the format and will show an error if the string isn't valid Base64, helping you identify encoded data.
Yes, the decoder converts Base64 back to the original data. For text, it displays as readable text. For binary data like images, you can download the file and open it with the appropriate application.
Verified content & sources
This tool's content and its supporting explanations have been created and reviewed by subject-matter experts. Calculations and logic are based on established research sources.
Scope: interactive tool, explanatory content, and related articles.
ToolGrid — Product & Engineering
Leads product strategy, technical architecture, and implementation of the core platform that powers ToolGrid calculators.
ToolGrid — Research & Content
Conducts research, designs calculation methodologies, and produces explanatory content to ensure accurate, practical, and trustworthy tool outputs.
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Learn what this tool does, when to use it, and how it fits into your workflow.
This tool converts Base64-encoded data back into its original form. It accepts Base64 strings, including URL-safe variants, and decodes them into text, JSON, images, PDFs, URLs, tokens, or raw binary bytes.
Base64 encoding is widely used to represent binary data as text. It appears in API responses, data URLs, JWTs, configuration files, and logs. While useful for machines, Base64 is hard for people to read or inspect directly.
The Base64 Decoder solves this problem by safely decoding input, detecting the underlying content type, and presenting meaningful previews. It also offers expert-level metadata and an optional AI insight layer to help you understand what the decoded data represents.
The tool is designed for developers, security analysts, testers, and technical users who regularly handle encoded data. A beginner can use it with basic guidance, while advanced users gain deeper visibility and control.
Base64 is a binary-to-text encoding scheme. It takes arbitrary bytes and maps them to a set of 64 safe characters: A–Z, a–z, 0–9, +, and /. It also uses = for padding. This makes binary data safe to send through systems that only handle text.
When you see long strings that look random but only contain these characters, they are often Base64. Examples include image data URLs in HTML, authorization headers like Basic or JWT payloads, and file attachments in emails.
Base64 does not compress or encrypt data. It simply changes the representation. In fact, it usually increases the size by about one third. Because of this, it is not suitable as a storage or security solution by itself. It is a transport and display format. A related operation involves encoding data in Base64 as part of a similar workflow.
URL-safe Base64 is a variant that replaces + with - and / with _. This keeps the encoded data safe for use inside URLs without additional encoding. Some systems also omit padding = characters for brevity.
Working with Base64 manually has challenges. You must strip whitespace, restore missing padding, convert URL-safe characters, and avoid decoding invalid sequences. If you get this wrong, you see errors like “Invalid character” or “Invalid string length.” It is also hard to tell what the decoded content actually is without tooling.
The Base64 Decoder automates these tasks. It normalizes the input, fixes common issues, and attempts to classify the decoded data. It can recognize JSON, multiple image formats, PDFs, JWT-like tokens, URLs, and raw binary streams. This classification helps you choose the correct next step, such as previewing, downloading, or further analysis.
Inspecting API responses: Many APIs return Base64 fields, such as file attachments or embedded JSON. Developers can paste these strings into the decoder to see actual content, check structures, and debug issues.
Analyzing data URLs: Frontend code often embeds images or small files as data URLs. By extracting the Base64 portion and decoding it, you can preview the image, verify MIME types, and save an actual file.
Reviewing email attachments: Email messages sometimes include Base64-encoded attachments or bodies. You can paste the encoded content to reconstruct text, JSON payloads, or binary files without relying on an email client. For adjacent tasks, base64 encoder operations addresses a complementary step.
Investigating JWT payloads: JSON Web Tokens contain segments encoded with Base64URL. While the decoder focuses on raw Base64 content, it can still decode JWT-like strings and classify them as JWT. Developers and security analysts can then inspect the decoded payload.
Checking configuration values: Some systems store configuration, secrets, or keys as Base64. Decoding them helps you verify values or ensure that your application is reading them correctly.
Debugging storage or log output: Logs and databases often capture Base64 blobs. The decoder lets you check if they are valid, what they represent, and whether they include sensitive data that should be protected.
Validating file conversions: When converting files to and from Base64, you can use the tool to ensure round-trip accuracy. Decode, preview, and download to confirm that files remain intact.
The decoder first checks for empty input. If the string is blank, it returns a result marked as invalid with an “Empty input” error. It also records zero lengths in the metadata.
Next, it validates input size using a Blob object. This measures true byte size rather than simple character count. If the size exceeds 10 MB, the decoder returns an error result without attempting to decode. When working with related formats, decoding hexadecimal values can be a useful part of the process.
The tool then trims whitespace from both ends of the string and removes internal whitespace characters. This handles wrapped or formatted Base64, such as content from emails or logs that insert line breaks.
If the sanitized string contains - or _, the decoder assumes URL-safe Base64 and converts those characters to + and /. It records this as an auto-fix. Then it checks the length of the sanitized string. If it is not a multiple of four, it calculates how many = characters are needed for padding and adds them, recording another auto-fix.
The core decoding step uses atob to convert the sanitized string to a binary string. It then loops through the characters of that string, converting each one to its character code and storing it in a Uint8Array. This array is used for mime type detection and file downloads.
Content type detection happens in several stages. First, the decoder tries to parse the binary string as JSON. If parsing succeeds and the result is a non-null object, it marks the type as JSON and sets the MIME type to application/json. If not, it checks image signatures using known magic numbers for PNG, JPEG, GIF, and WebP formats.
After image checks, the decoder looks for the %PDF header at the start of the binary string. If present, it marks the type as PDF. Next, it tests the binary string and original input against a JWT-like regular expression that expects three Base64URL-like segments separated by dots. If that succeeds, it sets the type to JWT.
The decoder then attempts to interpret the binary string as a URL by passing it to the URL constructor. If it starts with http:// or https:// and parsing succeeds, it marks the type as URL. In some workflows, decoding Base64 images is a relevant follow-up operation.
Finally, if none of these checks mark the data as special, the tool inspects up to the first 1000 characters for non-printable characters. If it finds control characters other than tab, line feed, or carriage return, it assumes the content is binary and sets the type to Binary and MIME type to application/octet-stream. Otherwise, it treats the data as plain text.
If any error occurs during decoding, the catch block captures it. The decoder then returns a result with isValid set to false, a generic or specific error message, and any auto-fixes that were applied. The decoded bytes and text are set to null to prevent accidental use of partial data.
| Detected Type | Heuristic | Typical Use |
|---|---|---|
| JSON | Decodes to parsable JSON object | API payloads, configurations |
| Image | Matches PNG, JPEG, GIF, or WebP signatures | Embedded images in HTML or CSS |
| Text starts with %PDF | Documents, reports, exports | |
| JWT | Matches three-part token pattern | Authentication tokens |
| URL | Valid http/https URL string | Links, redirects, callbacks |
| Binary | Contains non-printable control characters | Executables, archives, custom formats |
| Text | Printable characters only | Plain text, logs, code snippets |
Do not treat Base64 as encryption: Remember that Base64 is only an encoding. Anyone can decode it. Do not rely on it to hide sensitive data. Use proper encryption and access controls instead.
Be careful with untrusted content: Decoded text can include scripts, URLs, or other potentially dangerous content. Avoid blindly executing or following decoded data from unknown sources.
Watch input size: Large inputs may indicate large files or combined bundles. Consider whether you really need to decode the entire content in the browser. For very large data, use local tools or server-side scripts as needed.
Use auto-fix information: If the decoder reports that it added padding or converted URL-safe characters, review your upstream systems. They may be emitting incomplete or non-standard Base64. For related processing needs, decoding HTML entities handles a complementary task.
Verify file types: While the tool detects common formats, it does not guarantee correctness. For critical workflows, open downloaded files with appropriate applications and verify contents manually.
Use expert mode when debugging: Length, byte counts, and MIME hints are valuable when debugging encoding pipelines or comparing outputs from different systems.
Limit AI insight on sensitive data: The AI insight feature sends a portion of decoded text to a backend service. Avoid using it with highly confidential or regulated information.
Keep original input: Always keep a copy of the original Base64 string, especially when debugging or sharing with teammates. It is the single source of truth for reproduced results.
Use truncation as a signal: If the tool truncates decoded content in the preview, consider downloading it as a file instead. Very long text blocks are easier to inspect in a dedicated editor.
Check validity before trusting output: Rely on the isValid flag and error messages. If decoding failed or the content is marked as Unknown, do not assume the output is correct.
Articles and guides to get more from this tool
1. Introduction: Decoding the Hidden Messages You receive an email with an attachment. Your email client automatically converts it back from…
Read full articleSummary: Decode Base64-encoded strings back to original text, binary data, or files with automatic format detection, MIME type identification, image preview, and direct file download for extracted content.