Definition
The activation threshold is the proportion of blocks that must signal support for a proposed soft fork before it locks in and becomes enforced. It is the central tuning parameter of miner-activated deployments: set it high and you demand near-universal readiness before changing the rules; set it lower and you risk activating a change that a meaningful minority has not adopted.
The Classic 95% Figure
Under BIP9 version bits, signaling is measured over each 2,016-block difficulty period. On mainnet the threshold has historically been 1,916 blocks out of 2,016, which is 95%. If that many blocks set the deployment's version bit within a single retarget window, the soft fork moves from "started" to "locked in," then becomes active after the next full period. Testnet uses a relaxed 1,512 of 2,016 (75%) to make experimentation easier. If the timeout passes without the threshold being met, the deployment fails.
Why the Number Is Political, Not Just Technical
A high threshold protects against activating a contentious change, but it also hands a small slice of hashpower an effective veto—precisely the dynamic that stalled SegWit and motivated user-activated alternatives. Later deployments experimented with different thresholds and timelines, including the "Speedy Trial" approach used for Taproot, which paired a 90% threshold with a short signaling window to get a quick yes-or-no answer. The choice of threshold encodes a judgment about how much consensus is "enough."
The activation threshold is the linchpin of a miner-activated soft fork; when miners stall below it, the network may turn to a user-activated soft fork to force the question.
In Simple Terms
The activation threshold is the proportion of blocks that must signal support for a proposed soft fork before it locks in and becomes enforced. It…