Choose primary shards to divide data and work; choose replicas to provide additional copies and search capacity. Start from workload and recovery needs rather than a universal shard count.
Calculate a first estimate
Suppose primary index data is 120 GB and testing suggests approximately 30 GB per primary shard works for your workload:
120 GB / 30 GB = 4 primary shards
1 replica per primary = 4 additional shard copies
Total = 8 shard copies, roughly 240 GB of shard data
These are hypothetical sizing inputs. The storage estimate excludes growth and operating space for activities such as segment merges, which combine index files in the background.
