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File to Base64 Converter

Encode a local file up to 10 MB as canonical Base64 or decode strict Base64 into a downloadable file without uploading bytes.

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Choose File to Base64, select a file no larger than 10 MB, and copy the complete canonical padded output.
  2. 2.For the reverse direction, paste standard Base64 with no whitespace, set the intended filename and MIME type, and decode.
  3. 3.Download the temporary file and validate its real format and integrity; remember that the extension and MIME field do not inspect the bytes.

About File to Base64 Converter

File to Base64 Converter turns a local file into canonical RFC 4648 Base64 and reverses Base64 into a downloadable browser file. Choose a file to read its exact bytes and copy the encoded text, or switch direction, paste Base64, choose a filename and MIME type, and create a temporary download. Processing stays in the current browser tab.

Base64 represents each group of three bytes as four printable characters. When the last group contains one or two bytes, equals signs complete the final group. The encoder preserves every byte, including zeros and values that are not valid text. It does not inspect or rewrite the file format, normalize line endings, transcode images, or interpret metadata.

The decoder is strict by design. Input must use the standard alphabet with plus and slash, include required equals padding, contain no whitespace, and have zero-valued unused pad bits. It decodes the bytes and re-encodes them to reject alternate non-canonical spellings. Base64url, MIME line wrapping, data URL prefixes and omitted-padding profiles must be converted explicitly before use.

After decoding, the page creates a Blob URL that exists only in the browser session. The chosen filename controls the download suggestion and the MIME field controls the Blob media type. Neither field changes the bytes. A misleading extension or MIME value can confuse another application, so use values appropriate for the known file format rather than guessing from Base64 alone.

In file-to-Base64 mode, the browser File API reads an ArrayBuffer from the selected file. The application does not send the name, type or contents to the server. Browser extensions, device malware, clipboard managers, downloaded-file handlers and anything you paste the output into remain outside that local-processing boundary. Avoid sensitive files on an untrusted device.

Base64 is encoding, not encryption, hashing, signing, compression, sanitization or antivirus scanning. Anyone can decode it. Encoding increases text size by roughly one third and can expose recognizable content in logs, tickets, source code and analytics. A Base64 string can still contain malware, private data, credentials or executable bytes. Treat it with the same sensitivity as the source file.

The tool limits source and decoded files to 10,000,000 bytes. This bounds browser memory because the byte array, Base64 string and rendered output can coexist. Very large files belong in streaming command-line or application workflows. Output is not silently truncated, and malformed input never creates a partial download link.

The temporary Blob URL is revoked when a new conversion replaces it or the component closes. This prevents obsolete in-memory downloads from accumulating during normal use. The copied encoder output contains only Base64 characters and padding, without a data URL prefix, MIME header, line wrapping or filename. Add those containers only when the target application explicitly requires them.

Correctness is locked by the RFC 4648 empty, f, fo, foo, foob, fooba and foobar vectors plus a byte sequence that exercises alphabet characters plus and slash. Both directions are asserted against independent expected values. Negative cases cover whitespace, missing padding and nonzero pad bits so permissive browser behavior cannot hide malformed input.

Use the page by confirming the destination expects standard padded Base64. For encoding, select the file, verify its name and size, and copy the complete output. For decoding, remove any known container only according to its specification, paste canonical Base64, set an honest filename and MIME type, download, and validate the resulting format with an appropriate application or cryptographic hash when integrity matters.

Methodology & sources

The File API supplies exact bytes through arrayBuffer. Encoding uses RFC 4648 four-character quanta; decoding validates alphabet, padding and canonical pad bits before creating a revocable Blob URL with the user-selected media type.

Frequently asked questions

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