How is a number turned into Base64?

The integer is first written in its minimal big-endian byte form, most-significant byte first, with no leading zero bytes. Those raw bytes are then encoded with standard Base64. For example 65535 becomes the bytes FF FF, which encode to "//8=".

Does it handle very large numbers?

Yes. The tool uses JavaScript BigInt, so integers far larger than 2^53 are processed exactly with no loss of precision. There is effectively no upper limit other than browser memory.

What encoding does it use — standard or URL-safe?

It uses standard Base64 with the + and / characters and = padding. If you need the URL-safe variant, replace + with - and / with _ after encoding.

Why does decoding sometimes give a different number?

Base64 encodes bytes, not numbers, so leading zero bytes are significant. "//8=" decodes to FF FF which is 65535, but "AP//" decodes to 00 FF FF which is also 65535 as an integer because leading zeros do not change an unsigned value.

Is the conversion done on a server?

No. Everything runs locally in your browser using the built-in btoa and atob functions plus BigInt, so your numbers never leave your machine.

Decimal Integer to Base64

What this tool does

It bridges two representations: a plain decimal integer and a Base64 string. Base64 is defined over bytes, so a number is first serialised to bytes and only then encoded.

How it works

To encode, the decimal value is parsed into a BigInt and repeatedly masked and shifted by 8 bits to produce its minimal big-endian byte sequence — the most-significant byte first, with no superfluous leading zero bytes (zero itself is a single 0x00 byte). Those bytes are then passed through standard Base64 encoding, which groups every 3 bytes into 4 printable characters and pads with =.

To decode, the Base64 is converted back to bytes, and the bytes are read big-endian into a BigInt: each byte shifts the accumulator left 8 bits and ORs in the next byte.

Example and notes

The number 65535 fits in two bytes, FF FF, which Base64-encode to //8=. A larger value like 16777216 becomes the bytes 01 00 00 00 and encodes accordingly. The tool always shows the intermediate hex bytes so the mapping is transparent.

Remember that Base64 carries byte-length information, while an integer does not: encoding-then-decoding a number always round-trips, but two different Base64 strings differing only by leading zero bytes can decode to the same integer. Use standard Base64 here; swap +/ for -_ if you need the URL-safe alphabet.