Redis HyperLogLog

Introduction to the Redis HyperLogLog data type

HyperLogLog is a data structure that estimates the cardinality of a set. As a probabilistic data structure, HyperLogLog trades perfect accuracy for efficient space utilization.

The Redis HyperLogLog implementation uses up to 12 KB and provides a standard error of 0.81%.


  • Add some items to the HyperLogLog:
> PFADD members 123
(integer) 1
> PFADD members 500
(integer) 1
> PFADD members 12
(integer) 1
  • Estimate the number of members in the set:
> PFCOUNT members
(integer) 3

Basic commands

  • PFADD adds an item to a HyperLogLog.
  • PFCOUNT returns an estimate of the number of items in the set.
  • PFMERGE combines two or more HyperLogLogs into one.

See the complete list of HyperLogLog commands.


Writing (PFADD) to and reading from (PFCOUNT) the HyperLogLog is done in constant time and space. Merging HLLs is O(n), where n is the number of sketches.


The HyperLogLog can estimate the cardinality of sets with up to 18,446,744,073,709,551,616 (2^64) members.

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