Where the fifty six comes from
Take the thirty five bytes that an address encodes. Thirty five times eight is two hundred and eighty bits. Base32 packs five bits into every character, and two hundred and eighty divided by five is fifty six exactly. No remainder, so no padding.
The length is therefore a consequence rather than a decision. Nobody chose fifty six. It fell out of choosing an ed25519 key, a two byte checksum, a version byte and base32, and it will stay fifty six for as long as those four choices hold.
Older addresses were sixteen characters. Those stopped working in 2021 when the old format was switched off across the network. Any sixteen character address you find today is a historical artefact and will not resolve.
What fifty six buys you
The fastest sanity check available. Select a candidate address, count the characters, and if the answer is not fifty six then whatever you have is not a working mars market link. Most text editors and browsers will count a selection for you, so this takes a moment.
It catches truncation, which is the most common damage in practice. Addresses get cut off at the edge of a screenshot, at the width of a chat bubble, at a line break in a printed page. Truncated addresses look complete to the eye because nobody has memorised the ending.
It also catches padding. Somebody who reconstructs an address by guessing at missing characters will usually produce a string of the wrong length, because they have no way to know how many are missing.
What people get wrong about it
Counting the suffix. The address is fifty six characters, and with the six characters of .onion attached the whole hostname is sixty two. Both figures are correct and they measure different things. Arguments about this are usually two people who are both right.
The other error is treating the correct length as a good sign. It is a check that anything valid must pass and that anything hostile also passes trivially. Length filters out accidents, not adversaries.
Some people also expect a market to be able to shorten its address for convenience, the way a link shortener works. There is no such thing inside Tor. A shortener would be a third party in the middle of the connection, which is exactly what the address format is designed to remove.
What would move it
Only a new address format, and the last change took years of planning and a long transition. If it happens, the length will change together with the key size, not on its own.
Nothing about a particular market, a particular mirror or a particular moment changes the length. Every one of the three addresses published on this site is fifty six characters. So is every other onion address on the network.
Questions people ask
Does the count include the .onion part?
No. Fifty six is the address alone. Add the suffix and the full hostname is sixty two characters.
Are there shorter onion addresses that still work?
No. The sixteen character format was removed from the network in 2021 and nothing shorter than fifty six resolves today.
Why are onion addresses so much longer than they used to be?
Because the whole public key is now in the address rather than a truncated hash of it. The length is the cost of that improvement.