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EC2 Placement Group Calculator

Check a fleet against cluster, spread and partition placement group limits and see the blast radius each strategy gives.

About EC2 Placement Group Calculator

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.

Formula

EC2 Placement Group

spread capacity = 7 × Availability Zones per group; partition fault domains = partitions × Availability Zones; blast radius = ceil(instances ÷ fault domains) ÷ instances.

7
Hard limit on spread instances per AZ and partitions per AZ
fault domain
Rack or partition whose failure takes its instances with it
blast radius
Share of the fleet lost when one fault domain fails

Worked example

With the default values below, EC2 Placement Group Calculator returns Placement Groups Required of 2. Change any input above and every figure here is recalculated in your browser.

Inputs used

Placement strategyone of Cluster — one Availability Zone, lowest latency, Spread — 7 instances per Availability Zone per group, Partition — up to 7 partitions per Availability Zone
Spread — 7 instances per Availability Zone per group
Instances to placeaccepts 1–10,000 instances
30 instances
Availability Zones availableaccepts 1–6 zones
3 zones
Partitions per Availability Zoneaccepts 1–7 partitions
7 partitions

Results

Placement Groups Required
2
Instances One Group Holds
21
Instances per Availability Zone
10
Independent Fault Domains
30
Instances Lost per Fault Domain
3.3%
Feasibility
Exceeds one spread group — split across 2 groups

Step by step

  1. 1Values used
  2. 2EC2 Placement Group
  3. 3Placement Groups Required= 2
  4. 4Instances One Group Holds= 21
  5. 5Instances per Availability Zone= 10
  6. 6Independent Fault Domains= 30
  7. 7Instances Lost per Fault Domain= 3.3
  8. 8Feasibility= Exceeds one spread group — split across 2 groups

Assumptions and limits

  • The result is derived only from the 4 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 1 decimal place, 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 EC2 Placement Group calculated?

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.

Why does EC2 Placement Group matter?

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.

What values do I need to enter?

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.

Can I mix strategies?

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.

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EC2 Placement Group Calculator | Calcrivo