A netstring is a self-delimiting encoding invented by Daniel J. Bernstein. It is the byte length of the data in ASCII decimal, a colon, the raw data, then a trailing comma. For example the string hello becomes 5:hello, which is unambiguous to parse.

Is the length in bytes or characters?

The length is the number of bytes in the UTF-8 encoding of the data, not the number of characters. A single emoji can be four bytes, so this tool counts bytes with TextEncoder to match the spec exactly.

Why does the format end with a comma?

The trailing comma is a sanity check. A correct parser reads exactly length bytes after the colon and must then find a comma; if it does not, the netstring is malformed. This makes streaming many netstrings together safe.

Can netstrings contain colons, commas, or binary data?

Yes. Because the length prefix tells the parser exactly how many bytes to read, the payload can contain any bytes at all, including colons, commas, and null bytes, without escaping.

Is my data sent anywhere?

No. All encoding and decoding happens in JavaScript in your browser, so secrets and private payloads never leave your device.

Netstring Encoder/Decoder

A netstring is a tiny, self-describing way to wrap a string so a parser always knows exactly where it ends. Invented by Daniel J. Bernstein, it is widely used in protocols like SCGI and in qmail. This tool encodes any text into a netstring and decodes netstrings back to their payload, all in your browser.

How it works

A netstring has the shape [byte-length]:[data],:

Encoding measures the data in UTF-8 bytes (not characters) using TextEncoder, then writes that count in ASCII decimal.
It appends a colon :, the raw data, and a trailing comma ,.
Decoding reads digits up to the colon to learn the declared length, slices exactly that many bytes of payload, and then requires a comma immediately after — otherwise the netstring is rejected as malformed.

Because the length is known up front, the payload may contain any bytes, including :, ,, and null, with no escaping needed.

Tips and examples

The string hello encodes to 5:hello,. An empty string is the perfectly valid 0:,. The 4-byte emoji sequence 😀 encodes to 4:😀, because the length counts bytes, not the single visible character. When decoding, a prefix length that does not match the real payload length is flagged as an error so you can spot truncated or corrupted data.