# 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%.

## Examples

- 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.

## Performance

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.

## Limits

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

## Learn more

- Redis new data structure: the HyperLogLog has a lot of details about the data structure and its implementation in Redis.
- Redis HyperLogLog Explained shows you how to use Redis HyperLogLog data structures to build a traffic heat map.