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Azure Availability Zone Planner

Size a zone-redundant deployment for N+1 zone loss and compare the composite SLA against a single-zone design.

About Azure Availability Zone Planner

Zone-redundant sizing works backwards from the failure you tolerate: if one of three zones can go, each surviving pair must carry the whole peak, so each zone holds half the load and you deploy 1.5× the capacity. Because zones are independent failure domains, spreading across them multiplies the failure probabilities, which is how the published target rises from 99.9% for a single VM to 99.99% across zones. The overhead figure is the real price of a 99.99% target, and it is paid every hour whether a zone fails or not; cross-zone traffic is metered too, so chatty services that gossip between zones add a data charge on top of the compute overhead.

Formula

Azure Availability Zone

VMs per zone = ceil(required ÷ (zones − tolerated losses)), total = per-zone × zones; composite availability = 1 − (1 − single-zone availability)^(tolerated losses + 1).

surviving zones
Zones you assume remain after the failure you design for
composite availability
Combined figure when independent zones must all fail to cause an outage
overhead
Extra VMs bought purely to absorb a zone failure

Worked example

With the default values below, Azure Availability Zone Planner returns Total VMs to Deploy of 18. Change any input above and every figure here is recalculated in your browser.

Inputs used

VMs needed to carry peak loadaccepts 1–2,000 VMs
12 VMs
Zones usedaccepts 1–3 zones
3 zones
Zones that may fail simultaneouslyaccepts 0–2 zones
1 zones
Availability of one zone deploymentaccepts 90–99.999 %
99.9 %
VM rateaccepts 0–100 USD/hour
0.192 USD/hour
Cross-zone trafficaccepts 0–10,000,000 GB/month
3,000 GB/month
Cross-zone data price per GBaccepts 0–5 USD
0.01 USD

Results

Total VMs to Deploy
18 VMs
VMs per Zone
6 VMs
Redundancy Overhead
50.0%
Composite Availability
100.000%
Allowed Downtime per Month
0.0 minutes
Monthly Cost
$2,552.88
Cost of the Redundancy
$870.96

Step by step

  1. 1Values used
  2. 2Azure Availability Zone
  3. 3Total VMs to Deploy= 18 VMs
  4. 4VMs per Zone= 6 VMs
  5. 5Redundancy Overhead= 50.0
  6. 6Composite Availability= 100.000
  7. 7Allowed Downtime per Month= 0.0 minutes
  8. 8Monthly Cost= 2,552.88

Assumptions and limits

  • The result is derived only from the 7 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, 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 Azure Availability Zone calculated?

VMs per zone = ceil(required ÷ (zones − tolerated losses)), total = per-zone × zones; composite availability = 1 − (1 − single-zone availability)^(tolerated losses + 1). Zone-redundant sizing works backwards from the failure you tolerate: if one of three zones can go, each surviving pair must carry the whole peak, so each zone holds half the load and you deploy 1.5× the capacity. Because zones are independent failure domains, spreading across them multiplies the failure probabilities, which is how the published target rises from 99.9% for a single VM to 99.99% across zones.

Why does Azure Availability Zone matter?

The overhead figure is the real price of a 99.99% target, and it is paid every hour whether a zone fails or not; cross-zone traffic is metered too, so chatty services that gossip between zones add a data charge on top of the compute overhead.

What values do I need to enter?

This calculator takes 7 inputs: VMs needed to carry peak load, Zones used, Zones that may fail simultaneously, Availability of one zone deployment, VM rate, Cross-zone traffic, Cross-zone data price per GB. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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Azure Availability Zone Planner | Calcrivo