ElastiCache Memory Calculator
Size ElastiCache nodes from dataset size plus key overhead, fragmentation, replicas and reserved-memory headroom.
About ElastiCache Memory Calculator
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
Worked example
With the default values below, ElastiCache Memory Calculator returns Memory Needed per Node of 25.56. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Raw dataset sizeaccepts 0–100,000 GB
- 40 GB
- Key and structure overheadaccepts 0–200 %
- 15 %
- Memory fragmentation ratioaccepts 1–3 x
- 1.25 x
- Reserved memory percentaccepts 0–80 %
- 25 %
- Shards (node groups)accepts 1–500 shards
- 3 shards
- Replicas per shardaccepts 0–5 replicas
- 1 replicas
- Memory per nodeaccepts 0.5–1,000 GB
- 26.32 GB
- Node priceminimum 0 $/hour
- 0.411 $/hour
- Billed hours per monthaccepts 0–744 hours
- 730 hours
Results
- 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
Step by step
- 1Values used
- 2ElastiCache Memory
- 3Memory Needed per Node= 25.56 GB
- 4Nodes in the Cluster= 6 nodes
- 5Total Cluster Memory= 157.92 GB
- 6Usable Cache Memory= 59.22 GB
- 7Headroom on the Chosen Node= 2.9
- 8Monthly Node Cost= 1,800.18
Assumptions and limits
- The result is derived only from the 9 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.