Library · Crypto, word by word · Inside Bitcoin
What is OP_RETURN?
OP_RETURN is a Bitcoin script command that marks a transaction output as unspendable, so it can carry a small piece of data instead of coins. It writes data such as a hash or a short message into the blockchain without adding an entry to the set of spendable coins every node keeps.
What it means for you
Any bitcoin assigned to an OP_RETURN output can never be spent, so value put there is gone for good. Data written this way is public and permanent once confirmed; it cannot be deleted later.
How it works
A null-data output has a script of OP_RETURN followed by data. Because it is provably unspendable, nodes need not keep it in the database of unspent outputs. The size limit is relay and mining policy, not a consensus rule: Bitcoin Core 0.9 and 0.10 relayed up to 40 bytes and later versions more, and Bitcoin Core 30.0 raised the default -datacarriersize to 100,000, which effectively removes the cap because the maximum transaction size is reached first, and allowed several OP_RETURN outputs per transaction. These outputs are exempt from the dust rule.
Sources: Bitcoin Developer Guide: Transactions (null data), Bitcoin Core 30.0 release notes (data carrier policy) · checked 4 October 2026
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