Base62 encode
Base62 encode. Runs on your device, nothing is uploaded.
Runs on your device. The file is never uploaded.
Base62 encode converts a file into 0 to 9, then A to Z, then a to z, with no padding and no punctuation. Leading zero bytes come back as leading 0 characters. Because the whole file becomes one big number, you cannot predict the output size from the input size, and one early change rewrites most of what follows.
Questions
Which alphabet does Base62 use here?
Digits first, then uppercase, then lowercase: 0 to 9, A to Z, a to z, in that order. There is no padding character and no punctuation at all, so the output is safe to drop into a URL, a filename or an identifier without escaping anything.
How is this different from Base64?
Base64 keeps plus, slash and equals, which need escaping in a URL. Base62 has none of them. The trade is speed and shape: Base64 encodes in fixed three-byte groups, while Base62 converts your whole input as one big number, so the output length is not a fixed ratio and one changed byte can rewrite most of the string.
How is it different from Base58?
Base62 keeps the four characters Base58 removes: the digit zero, capital O, capital I and lowercase l. That makes it four characters denser and slightly shorter, at the cost of being easier to mistype or misread when a human copies it by eye.
Why does my output start with a run of zeros?
Each leading zero byte in the input is written as one leading 0 character, because zero is the first symbol in the alphabet. Leading zeros do not affect the numeric value, so they are counted and re-emitted separately rather than being lost.
Can I use this to shorten a number?
Not directly. This tool encodes the bytes of a file, so a text file containing 12345 encodes those five characters, not the value. To convert a number itself, use the Number base converter, which handles bases 2 through 36 with exact big-integer arithmetic.
Does the file go anywhere?
No. It is read and converted inside this tab by a Web Worker. Nothing is uploaded, nothing is logged, and the page works offline once it has loaded. Because the conversion holds the whole file in memory as one big number, the practical limit is your own device rather than any server-side cap.