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About this tool
Decode Base64 encoded strings into readable ASCII text instantly.
The Base64 to ASCII Converter turns a Base64 string back into readable text using the browser's built-in atob() function and a UTF-8 TextDecoder — no library, no server round-trip. It normalises the input before decoding: whitespace and line breaks are stripped, URL-safe characters (- and _) are folded back to + and /, and missing = padding is restored, so soft-wrapped MIME blocks and unpadded URL tokens both work. If the whole input isn't one valid Base64 blob, it automatically retries line by line, so a list of separately encoded strings decodes to one result per line.
Open Base64 to ASCII on AltFTool — it loads instantly in your browser.
Paste your Base64 into the input box, or click Paste to read the clipboard or Upload File to load a plain-text .txt file (10,000 character limit).
The decoded text appears in the output terminal as you type — whitespace, URL-safe characters, and missing = padding are all handled for you.
Copy the result, download it as decoded-ascii.txt, share it, or open the output fullscreen to read long text with line numbers.
The output updates on every keystroke — the Convert button is only a visual cue that also scrolls the result into view on small screens. There is no request to wait on, because atob() and TextDecoder run locally.
Standard and URL-safe alphabets, stray newlines and tabs, and missing = padding are all handled automatically. Only a genuinely impossible length (a character count that leaves remainder 1 when divided by 4) or a character outside the Base64 alphabet is rejected, with a message saying which.
Decoded bytes pass through TextDecoder("utf-8"), so accented letters, symbols like π, and emoji come out intact instead of turning into mojibake. Output is shown in a line-numbered terminal you can expand fullscreen.
The tool makes no network request at all. Decoding, file reading, copy, and download as decoded-ascii.txt all happen inside the browser tab — no account, no usage cap.
Paste the Base64 string into the input box — the decoded text appears in the output panel immediately, with no button press needed. You can also click Paste to pull from your clipboard, or Upload File to load a plain-text .txt file, then copy the result or download it as decoded-ascii.txt.
Yes. Before decoding, every - is converted to + and every _ to /, which is exactly the base64url alphabet used in JWTs and URL query parameters. Unpadded URL-safe tokens decode fine because the missing = padding is added automatically.
No. If the string length isn't a multiple of 4, the tool appends the right number of = characters for you. The only length it can't fix is one that leaves a remainder of 1 when divided by 4 — that's mathematically impossible Base64, and you'll get "Incomplete Base64 — the string length is invalid."
Because the input contains a character outside the Base64 alphabet (A–Z, a–z, 0–9, +, /, -, _, and trailing =). Common causes are quotation marks copied along with the string, a data: URI prefix such as data:text/plain;base64,, or the dots in a full JWT — remove those and paste just the encoded portion. Spaces, tabs, and newlines are fine; they're stripped automatically.
Yes — put one per line. The tool first tries the entire input as a single blob, which is what handles MIME and PEM text that's been soft-wrapped across lines. If that fails and the input has multiple lines, it decodes each line as its own Base64 string and joins the results, so a list of separately padded values comes back one decoded result per line.
No. Decoding uses the browser's native atob() and TextDecoder, and uploaded .txt files are read locally with the FileReader API. The tool sends no network request and stores nothing — closing the tab discards everything.
10,000 characters. The input box caps at that length and shows a live counter, and uploaded files are trimmed to the same limit. 10,000 Base64 characters is roughly 7,500 bytes of decoded data, since Base64 encodes 3 bytes as 4 characters.
Not usefully — this tool is for text. It decodes the bytes and then interprets them as UTF-8, so binary formats like PNG or PDF come out as a stream of replacement characters rather than a usable file. Use a Base64-to-file converter for binary data, and this one for text, JSON, JWT segments, and config values.
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