ElastiCache Memory Calculator
Size ElastiCache nodes from dataset size plus key overhead, fragmentation, replicas and reserved-memory headroom.
Inputs
Redis stores per-key metadata, pointers and expiry data alongside the value.
Watch mem_fragmentation_ratio in INFO — 1.0 to 1.5 is normal.
Headroom for replication buffers, failover, backups and copy-on-write forks.
cache.r6g.xlarge offers about 26.32 GiB.
Memory Needed per Node
25.56GB
Nodes in the Cluster
6nodes
Total Cluster Memory
157.92GB
Usable Cache Memory
59.22GB
Headroom on the Chosen Node
2.9%
Monthly Node Cost
$1,800.18
Step by step
Values used
Raw dataset size = 40 GB; Key and structure overhead = 15 %; Memory fragmentation ratio = 1.25 x; Reserved memory percent = 25 %; Shards (node groups) = 3 shards; Replicas per shard = 1 replicas; Memory per node = 26.32 GB; Node price = 0.4110 $/hour; Billed hours per month = 730 hours
ElastiCache Memory
per-node memory = dataset × (1 + overhead) × fragmentation ÷ shards ÷ (1 − reserved-memory percent); nodes = shards × (1 + replicas per shard).
Memory Needed per Node
= 25.56 GB
Nodes in the Cluster
= 6 nodes
Total Cluster Memory
= 157.92 GB
Usable Cache Memory
= 59.22 GB
Headroom on the Chosen Node
= 2.9
Monthly Node Cost
= 1,800.18
How it works
Four multipliers stack between raw data and node size: per-key overhead, allocator fragmentation, division across shards, and the reserved-memory percentage that ElastiCache holds back so a backup fork or replication buffer cannot trigger an out-of-memory eviction. Replicas add nodes and cost but no usable capacity, because each replica holds a full copy of its shard. The node price is an editable input with a realistic us-east-1 default — confirm current ElastiCache pricing with AWS for your region and node family. A Redis node that runs out of memory starts evicting or, worse, fails a failover mid-fork, so the reserved-memory headroom is not optional padding — it is what makes backups and replica promotion survivable.
Formula
ElastiCache Memory
per-node memory = dataset × (1 + overhead) × fragmentation ÷ shards ÷ (1 − reserved-memory percent); nodes = shards × (1 + replicas per shard).
- overhead
- Redis per-key metadata on top of the value bytes
- fragmentation
- Ratio of resident memory to logical data, from mem_fragmentation_ratio
- reserved-memory-percent
- Share of node memory held back for replication and forks
- shard
- Node group holding one slice of the keyspace
Frequently Asked Questions
How is ElastiCache Memory calculated?
per-node memory = dataset × (1 + overhead) × fragmentation ÷ shards ÷ (1 − reserved-memory percent); nodes = shards × (1 + replicas per shard). Four multipliers stack between raw data and node size: per-key overhead, allocator fragmentation, division across shards, and the reserved-memory percentage that ElastiCache holds back so a backup fork or replication buffer cannot trigger an out-of-memory eviction. Replicas add nodes and cost but no usable capacity, because each replica holds a full copy of its shard. The node price is an editable input with a realistic us-east-1 default — confirm current ElastiCache pricing with AWS for your region and node family.
Why does ElastiCache Memory matter?
A Redis node that runs out of memory starts evicting or, worse, fails a failover mid-fork, so the reserved-memory headroom is not optional padding — it is what makes backups and replica promotion survivable.
What values do I need to enter?
This calculator takes 9 inputs: Raw dataset size, Key and structure overhead, Memory fragmentation ratio, Reserved memory percent, Shards (node groups), Replicas per shard, Memory per node, Node price, Billed hours per month. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why does usable capacity not include the replicas?
A replica holds an identical copy of its primary's keyspace, so it adds read throughput and availability but zero extra room for data. Only the shard count multiplies usable memory.