Glossary / Mining & consensus
Stale block
- Definition
- A stale block is a fully valid block that lost the race: another block of the same height won, so the network built on that one and abandoned this one.
Two miners solving within a second of each other is ordinary, and for a moment the network holds two candidate tips. Whichever branch gets extended first wins, and researchers measuring bitcoin in 2013 found roughly 1.7 percent of blocks ending up on the losing side. For you this is invisible: the payments inside a stale block return to the mempool and get mined again.
How it works
A stale block is produced by latency, not by any fault in the block itself.
Mining is a lottery with no coordination. When a machine finds a solution, its pool announces the block, and that announcement takes time to cross the planet. Until it arrives, every other miner is still grinding on the previous tip, and any of them can find a second valid solution at the same height. Now two versions of the truth exist, both correct, and the tie is broken by whoever publishes the next block on top of one of them. The other becomes stale.
The size of the window is measurable. Christian Decker and Roger Wattenhofer instrumented the network in 2013 and found a median of 6.5 seconds for a block to reach the majority of nodes, with about 1.7 percent of blocks ending up abandoned as a result. Relay has improved enormously since. Compact block relay, specified in BIP152 and shipped in Bitcoin Core 0.13.0 in August 2016, lets a peer reconstruct a block from transactions it already holds in its mempool, sending a few kilobytes instead of a megabyte or two, and dedicated relay networks push typical propagation below a second.
Losing costs the pool the entire block. A stale block's coinbase output is never part of the accepted history, so its subsidy and its fees are worth nothing at all. The protocol also assumes this can happen: coins created in a block are locked for 100 further blocks, roughly sixteen hours, precisely so that nobody can spend a reward that later turns out to have been on the wrong branch.
Where you see it
Explorers and pool dashboards are where a stale block becomes visible, usually under a different name.
Several explorers publish a list of dropped blocks and file it under orphaned blocks, which is where most of the terminology confusion in this corner of bitcoin comes from. Pool dashboards report a stale rate or reject rate for the shares their miners submit. Under a pay per share arrangement, individual miners are still credited for that work and the pool absorbs the loss, which is one of the things a pool fee buys.
Bitcoin Core exposes it plainly. Ask a node for its known chain tips and a recent stale block shows up as a separate tip marked valid fork: fully checked, correct in every respect, and simply not the chain with the most work behind it.
You will also see it as the reason confirmation counts exist at all. A payment with one confirmation is sitting in a block that could still turn out to be the loser of a race. A payment six blocks deep is not, which is why exchanges quote deposit policies in blocks rather than minutes.
Stale block vs chain reorganization
A stale block is an object; a reorganization is an action. The two describe the same event from different seats. If your node happened to receive the winning block first, the loser simply never becomes part of your chain and no reorganization occurs. If you received the loser first, your node has to disconnect it and switch, which is a one block reorganization. Single block events like this happen a handful of times a year and mean nothing is wrong. Reorganizations several blocks deep are a different story, and on bitcoin's main chain they have been vanishingly rare.