Definition
The sigop budget is Tapscript's mechanism (BIP 342) for limiting the computational cost of signature checking within a single spending input. Instead of counting signature operations against a block-wide ceiling, Taproot gives each script input its own budget: 50 plus the total serialized size in bytes of that input's witness (including its CompactSize length prefix).
How the budget is spent
Every time the script executes a signature opcode — OP_CHECKSIG, OP_CHECKSIGVERIFY, or OP_CHECKSIGADD — with a non-empty signature, the budget is decremented by 50. The moment the budget goes below zero, the script fails immediately. Signature opcodes called with an empty signature do not draw against it, so unused branches of a conditional are free.
Why tie it to witness size
Linking the allowance to witness weight ties signature-verification cost directly to the fees a spender already pays for that witness data. A transaction cannot pack in more expensive signature checks than its own size justifies, which neutralizes denial-of-service scripts without the rigid global cap legacy Bitcoin used. Crucially, Tapscript sigops do not count toward the legacy block-wide weighted limit of 80,000 — the per-input budget supersedes it.
The sigop budget is one of two forward-looking design choices in Tapscript, the other being the upgrade hooks described under OP_SUCCESS. Together they let Bitcoin's script layer grow without re-litigating resource limits each time.
In Simple Terms
The sigop budget is Tapscript’s mechanism (BIP 342) for limiting the computational cost of signature checking within a single spending input. Instead of counting signature…