We present a fast local clustering service, FLOC, that partitions a multi-hop wireless network into nonoverlapping and approximately equal-sized clusters. Each cluster has a clusterhead such that all nodes within unit distance of the clusterhead belong to the cluster but no node beyond distance m from the clusterhead belongs to the cluster.By asserting m ≥ 2, FLOC achieves locality: effects of cluster formation and faults/changes at any part of the network are contained within at most m units. By taking unit distance to be the reliable communication radius and m to be the maximum communication radius, FLOC exploits the double-band nature of wireless radio-model and achieves clustering in constant time regardless of the network size. Through simulations and experiments with actual deployments, we analyze the tradeoffs between clustering time and the quality of clustering, and suggest suitable parameters for FLOC to achieve a fast completion time without compromising the quality of the resulting clustering.