The Dust Limit: Why You Can't Send 999 Satoshi

Blythex

Every now and then someone tries to send a few hundred satoshi and the wallet refuses. Nothing is broken – you've hit consensus rule #13, the dust limit. Here is exactly what it does and why it exists.

TL;DR

The floor is 1,000 satoshi (0.00001 AQA). An output below that makes the whole transaction invalid.
It's rejected, not collected. A dust output doesn't quietly go to the miner – the transaction never enters the mempool.
Tiny change becomes tip. Leftover under 1,000 satoshi is added to the miner's tip instead of creating a coin.
Mining rewards are the one exception – coinbase outputs may be smaller.
The reason is storage, not economics: every output lives in the UTXO set of every full node, forever.

The rule

Any output below 1,000 satoshi (0.00001 AQA) is invalid.

That's it. A fixed floor in satoshi – not a formula, not a percentage, not something your node calculates from current traffic. Every node on the network uses the same number, so every node reaches the same verdict on the same transaction.

Note that this is not how Bitcoin does it. There, the dust threshold depends on the size of the output and the current fee rate, so it moves. On AntiQua it is one constant.

Why 1,000 – the real reason

It isn't about "too small to be worth anything". It's about who has to remember it.

Every unspent output goes into the UTXO set – the list of all spendable coins – and every full node keeps that list in memory and on disk for as long as the coin remains unspent. That list is the part of a blockchain that never shrinks on its own.

Without a floor, anyone could create millions of one-satoshi outputs for almost nothing and force every node on the planet to carry them indefinitely. That's not a theoretical worry: it's one of the cheapest denial-of-service attacks against a UTXO chain, and it has been used against others.

AntiQua has a second reason on top. Post-quantum signatures are big – several kilobytes per input. A coin worth 500 satoshi would cost far more in fees to spend than it is worth, so it would simply never be spent. It would sit in every node's UTXO set forever as pure dead weight.

There's a neat coincidence here: the minimum fee for a typical post-quantum payment (about 10 KiB at 100 satoshi per KiB) lands at roughly 1,000 satoshi – the same number as the dust limit. That's not the same rule, and the two are not linked in the code. But it does show the floor sits about where a coin stops being economically spendable.

What your wallet does instead

You will rarely meet this rule head-on, because the wallet handles it before you see it:

Leftover change below 1,000 satoshi: no change output is created at all. The remainder is added to the tip. The miner gets it, you don't get a coin that would cost more to spend than it holds.
Leftover up to 50,000 satoshi may optionally be folded into the tip as well – same reasoning, your choice.
Anything above that comes back to you as a normal change output.

So if you ever wonder why the fee on a transaction is a little higher than the lane you picked: that's your own leftover, not a surcharge. (More in the fee post)

The one exception

Block validation allows coinbase outputs – the ones paying the miner – to fall below the limit. Everything else is held to the floor, including:

ordinary payments
contract transfers and contract change
the identity output created when you deploy a contract
relay lottery payouts (their minimum is the dust limit)

The exception exists for a plumbing reason, not a privilege: the coinbase is created by the miner inside the block and never passes through the mempool, where the check normally happens.

Not to be confused with

What you see	Which rule
"Fee too low"	The fee floor, a different rule entirely
An output of exactly 0	Rejected as an invalid amount, before dust is even checked
A mining reward under 1,000 sat in a block	Allowed – the coinbase exception
An old coin of 1,200 sat you still hold	Perfectly spendable. The rule applies when an output is created, never retroactively

Frequently asked

Can I collect lots of dust and spend it together?
There is no dust in your wallet to collect – the transaction that would have created it was rejected. You can hold many small-but-valid coins, and those you can consolidate, but consolidating costs the input factor. Do it once when the chain is quiet.

Will the limit ever change?
The code notes it could be adjusted later by soft fork. Today it's a constant, and nothing in the node reads it from a config file. Any change would go through a network upgrade and be announced in the Core Changelog first.

I really want to send someone 500 satoshi.
Send 1,000. It is 0.00001 AQA.

Questions below.

— Blythex