# Capacity planner

Work out how many instances you need for peak traffic, with room for growth and a failure.

## How it works

Peak load is average traffic, grown, times the peak factor. Each instance runs at your target utilisation, and spare instances cover a failure or a deploy.

```
instances = ⌈peak ÷ (capacity × target)⌉ + spare
```

## Questions

### Why not run at 100%?

Latency climbs sharply as utilisation nears 100%. Most services aim for 50 to 70%.

### How many spare instances?

At least one, so losing an instance or rolling a deploy doesn’t overload the rest.

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