Server Capacity Planning Calculator
Size a fleet for peak traffic with headroom and redundancy.
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Server Capacity Planning Calculator by TheCalcTools
How the Server Capacity Planning Calculator works
Enter the peak traffic you must handle, how much one instance can serve at saturation, and the utilisation you want at peak.
Running below 100% leaves room for spikes and failures.
Add spare instances so the loss of one does not push the rest over the target.
The formula
Worked example
Using the default values shown in the calculator:
| Peak requests per second | 2400 |
|---|---|
| Capacity per instance (req/s) | 300 |
| Target utilisation (%) | 65 |
| Extra instances for redundancy | 1 |
- At 100% utilisation you would need 2,400 ÷ 300 = 8 instances.
- At 65% target utilisation: ⌈2,400 ÷ 195⌉ = 13 instances.
- With 1 spare the fleet is 14 instances, at 57.1% utilisation.
Result:
| Instances at 100% utilisation | 8.00 |
|---|---|
| Instances at target utilisation | 13 |
| Total with redundancy | 14 |
| Utilisation with all instances | 57.1% |
Frequently asked questions
What utilisation should I target?
50–70% is typical for latency-sensitive services. Batch workloads can run hotter.
What does N+1 mean?
The fleet has one more instance than the load needs, so it can lose any one instance and still meet the target.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.