Base64 Decode
Runs 100% in your browserDecode standard or URL-safe Base64 back to UTF-8 text locally. Handles Data URI prefixes, whitespace, missing padding, strict validation, malformed input errors, and readable output statistics.
Developer encoding utility
Encode and decode Base64 locally
Work with UTF-8 text, API payloads, data URIs, and local files without sending data to a server. Base64 is an encoding—not encryption.
Step 1
Input
Choose a local file. The file is read in your browser and is never uploaded.
Step 2
Encoding settings
Step 3
Output
At a glance
Encoding statistics
Private by architecture
Text and local files are processed with browser-native APIs. No fetch, upload, server-side conversion, or persistent history is used. Base64 only changes representation; it does not protect secrets like encryption.
Session only
Recent conversions
Recent conversions disappear when this page closes.
Frequently asked questions
A Base64 decoder maps Base64 characters back to the original byte sequence and interprets those bytes as UTF-8 text. It is useful for reading encoded API values, JWT payload segments, Data URI content, configuration strings, and test fixtures.
Paste a Base64 string into the decoder input. The tool strips optional whitespace, restores missing padding when safe, converts the bytes to UTF-8 text, and reports a clear error if the input cannot be decoded correctly.
Yes. Choose URL-safe Base64 when the input uses - and _ instead of + and /. The decoder converts the URL-safe alphabet back to standard characters before decoding so both variants work correctly.
Yes. The decoder recognizes a prefix such as data:image/png;base64, and automatically strips everything up to the comma before decoding. The payload is decoded to text for text-compatible MIME types.
MIME email, PEM certificates, and some APIs wrap Base64 at 64 or 76 characters per line. Enable Ignore whitespace while decoding to remove spaces, tabs, and line breaks before conversion so the full payload decodes correctly.
The decoder can restore trailing = padding when the string length allows it. For strict API interoperability keep the canonical padding your API, token format, or file specification expects.
Common causes include characters outside the Base64 alphabet, malformed or misplaced padding, a string length that cannot form valid four-character groups, a Data URI prefix that was not stripped, or binary data being interpreted as UTF-8 text.
You can paste a single Base64URL segment from a JWT to inspect the raw JSON. For structured header, payload, expiry, and claim inspection use the dedicated JWT Decoder tool. Decoding a segment does not verify the JWT signature.
No. Base64 does not encrypt or protect information. Anyone who can read the Base64 string can decode it instantly. Never treat Base64 as a password-hiding or security mechanism — use proper authenticated encryption for sensitive data.
Decoding runs entirely in your browser using native JavaScript byte and atob functions. The page does not upload input, call a remote decoding API, or persist conversion history in localStorage. Use care with secrets on shared devices.
Paste the Base64 string and the decoded text appears instantly. The decoder detects the standard and URL-safe alphabets automatically.
Yes. If the input is an encoded file or a data URI, the decoder rebuilds the binary locally so you can preview or download the original file.
The decoder tolerates missing = padding and strips whitespace and line breaks before decoding, which fixes most copy-paste problems.
Standard Base64 uses + and / characters, which clash with URLs. The URL-safe variant (RFC 4648) replaces them with - and _ so the encoded value can travel in links, filenames, and JWT segments. The tool supports both.
The bytes were probably not UTF-8 text — they may be binary data or text in another encoding. Try file mode, or check where the value came from.
No. Base64 is an encoding, not encryption — anyone can decode it. Use it for transport and embedding, never for protecting secrets.
A JWT is three Base64url segments joined by dots. You can decode a single segment here, but the dedicated JWT Decoder splits all three parts and explains the claims for you.
Everything runs locally in your browser with no upload, no account, and no logging. You can confirm in your browser DevTools Network tab that no request carries your data.
What is Base64 Decode?
A Base64 decoder reverses the encoding process: it maps each group of four Base64 characters back to three bytes, then interprets those bytes as UTF-8 text when the payload is text-compatible. Decoding is necessary whenever you receive a value that was Base64-encoded for transport and need to inspect or use the original content.
Developers decode Base64 constantly: reading API response fields that contain binary-encoded data, inspecting the header and payload segments of a JWT (JSON Web Token) before signature verification, extracting the content of a Data URI to see what image or file it contains, checking MIME email attachment content, reading PEM certificate content, and diagnosing encoding bugs when an encoded value does not match expectations.
Decoding URL-safe Base64
Tokens issued by OAuth providers, OpenID Connect systems, Firebase, Supabase, Auth0, and most modern authentication services use Base64url — the URL-safe variant with - and _ instead of + and /. Standard decoders fail on these characters. ToolsSonic's decoder detects the URL-safe alphabet and converts it back to standard before decoding, so both variants work without extra manual steps.
Data URI decoding
A Data URI starts with data:MIME-type;base64, followed by the encoded payload. When you paste a Data URI into the decoder it automatically strips the prefix and decodes only the Base64 portion. This is useful for extracting the content of an embedded image, reading a base64-encoded SVG from a CSS stylesheet, or inspecting a data URI that came from a third-party tool.
Whitespace and line break handling
Base64 strings from MIME email, PEM certificates, SSH keys, and wrapped API responses contain newlines and sometimes spaces. The Ignore whitespace option strips all whitespace before decoding, which prevents the false "malformed input" errors that most online decoders produce when they receive wrapped Base64. This is the most common reason developers find decoding fails silently on wrapped content.
Missing padding restoration
Many systems strip trailing = padding to shorten tokens. Without padding, a naive decoder throws an error. ToolsSonic's decoder automatically restores the correct number of padding characters when the string length implies they were removed, making it compatible with unpadded JWT segments, URL-safe tokens, and stripped API values.
Strict validation mode
When you need to verify that a value is valid Base64 — for input sanitisation, test assertions, or debugging — enable strict validation. The decoder checks the character set, padding position, and group length and surfaces a precise error message describing exactly what is wrong rather than silently producing garbage output.
Why ToolsSonic Base64 Decoder leads competitors
Most online Base64 decoders accept text and produce output with no options. ToolsSonic's decoder adds URL-safe alphabet support, automatic Data URI prefix stripping, whitespace removal, padding restoration, strict validation with actionable error messages, UTF-8 byte statistics, clipboard paste, one-click copy, download, and session history — all without a server round-trip. It is the most complete free Base64 decoder available in a browser.
Common use cases
- Reading a Base64 API response or configuration value
- Inspecting a JWT header or payload segment before signature verification
- Decoding a Data URI to read its embedded text or SVG content
- Handling MIME or PEM strings that contain line breaks
- Diagnosing invalid alphabet, padding, and length errors in Base64 tokens
- Decoding OAuth Base64url tokens that use - and _ instead of + and /
Why use ToolsSonic's Base64 Decode?
ToolsSonic's Base64 Decoder supports standard and URL-safe alphabets, automatic Data URI prefix stripping, whitespace removal, padding restoration, strict validation with clear error messages, UTF-8 byte statistics, one-click copy, download, and session history. Everything runs locally — no upload, no server call, no storage.
Related tools
Base64 Encode
Editor's choiceEncode UTF-8 text and local files to standard or URL-safe Base64 with optional padding, line wrapping, Data URI prefixes, byte statistics, and browser-only processing.
URL Decoder
Decode percent-encoded URLs and query strings back to readable UTF-8 text — handles %20, +, Unicode escapes, mixed encoding, malformed escape checks, and local URL analysis.
JWT Decoder
Editor's choiceDecode and inspect JWT header, payload, signature, registered claims, custom claims, algorithm, timestamps, and expiry state locally without verifying or uploading the token.
Hex Converter
Editor's choiceConvert text to UTF-8 or Latin-1 hexadecimal bytes and decode hex back to text — configurable separators, 0x prefix, uppercase or lowercase, byte preview, strict validation, round-trip check, local file input, and private browser-only processing.
JSON Formatter
Editor's choiceFormat, beautify, minify, and validate JSON locally. Choose indentation, sort keys, preserve Unicode, inspect statistics, and copy or download clean output.