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Server Count For Load Calculator

Work out server count for load instantly with clear inputs, formula shown and shareable results.

About Server Count For Load Calculator

Capacity alone needs peak divided by per-server capacity at the target utilisation. Surviving the loss of one availability zone requires the remaining zones to carry the whole load, which multiplies the count by z/(z-1): 50 percent overhead across two zones, but only 33 percent across three and 25 percent across four. That is the main capacity argument for three zones over two.

Formula

Server count

base = ceil(peak / (per-server capacity x utilisation)); with zone loss = ceil(base x zones / (zones - 1)); per zone = ceil(total / zones)

Worked example

With the default values below, Server Count For Load Calculator returns Servers to provision of 18. Change any input above and every figure here is recalculated in your browser.

Inputs used

Peak request rateminimum 0.01 req/s
4,200 req/s
Capacity per serverminimum 0.01 req/s
600 req/s
Target utilisationaccepts 1–100 %
65 %
Availability zonesaccepts 1–10
3

Results

Servers to provision
18
Servers for capacity alone
11
Servers per availability zone
6
Overhead for zone redundancy
63.6%

Step by step

  1. 1Values used
  2. 2Server count
  3. 3Servers to provision= 18
  4. 4Servers for capacity alone= 11
  5. 5Servers per availability zone= 6
  6. 6Overhead for zone redundancy= 63.6

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

Why do three zones cost less overhead than two?

Losing one of three zones removes a third of capacity, so the survivors need 50 percent more each. Losing one of two removes half, so survivors need to double.

Should autoscaling replace this calculation?

Autoscaling handles growth but not instantaneous zone loss, since scaling takes minutes. The redundant capacity must already be running.

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