EC2 Placement Group Calculator

spread capacity = 7 × Availability Zones per group; partition fault domains = partitions × Availability Zones; blast radius = ceil(instances ÷ fault domains) ÷ instances. The three strategies trade latency against correlated failure. Cluster packs everything into one AZ on the same network spine for the lowest latency and the worst blast radius; spread puts every instance on distinct underlying hardware but caps at 7 per AZ per group, so larger fleets need several groups; partition groups sit in between, giving up to 7 named fault domains per AZ that a partition-aware application like HDFS or Cassandra can map replicas onto.

The spread limit of 7 per AZ is the constraint people discover at launch time with an InsufficientCapacity-style error, and blast radius is the number that decides whether one rack failure is an incident or an outage.

This calculator takes 4 inputs: Placement strategy, Instances to place, Availability Zones available, Partitions per Availability Zone. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Not inside one group — a placement group has a single strategy and an instance belongs to at most one group. The usual pattern is a partition group for the stateful tier and no placement group at all for stateless instances, which AWS already spreads across the AZs you launch into.