Azure Availability Set Planner

Plan availability set fault and update domains so a rack failure or maintenance wave still leaves enough capacity.

About Azure Availability Set Planner

An availability set spreads VMs across two or three fault domains and up to twenty update domains, so the worst-case simultaneous loss is one full fault domain — roughly a third of the set with three domains. Sizing for that means provisioning enough VMs that the survivors still carry peak load, which is where the fault-domain fraction in the formula comes from. The 99.95% availability set target only covers connectivity to at least one instance, not your application's capacity, so a set sized exactly for peak becomes an outage the moment a fault domain drops; update domains additionally cap how fast Azure can patch your fleet.

Formula

Azure Availability Set

438: Minutes that one percent of a 730-hour month represents
fault domain: Group of racks sharing power and network, lost together in a hardware failure
update domain: Group rebooted together during platform maintenance, one at a time

Worked example

With the default values below, Azure Availability Set Planner returns VMs Recommended of 9. Change any input above and every figure here is recalculated in your browser.

Inputs used

VMs in the availability setaccepts 1–200 VMs: 9 VMs
Fault domainsone of 2 fault domains, 3 fault domains (maximum in most regions): 3 fault domains (maximum in most regions)
Update domainsaccepts 1–20 domains: 5 domains
VMs needed to carry peak loadaccepts 1–200 VMs: 6 VMs
Availability targetaccepts 90–99.999 %: 99.95 %

Results

VMs Recommended: 9 VMs
VMs per Fault Domain: 3 VMs
VMs Surviving a Fault Domain Loss: 6 VMs
VMs per Update Domain: 2 VMs
Capacity Lost in the Worst Case: 33.3%
Allowed Downtime per Month: 21.9 minutes
Design Verdict: Survives a fault domain loss with 0 VMs spare

Step by step

1Values used
2Azure Availability Set
3VMs Recommended= 9 VMs
4VMs per Fault Domain= 3 VMs
5VMs Surviving a Fault Domain Loss= 6 VMs
6VMs per Update Domain= 2 VMs
7Capacity Lost in the Worst Case= 33.3
8Allowed Downtime per Month= 21.9 minutes

Assumptions and limits

The result is derived only from the 5 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 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 Azure Availability Set calculated?

VMs per fault domain = ceil(VMs ÷ fault domains); to keep peak capacity through a fault-domain loss you need ceil(required × fault domains ÷ (fault domains − 1)) VMs; downtime = (100 − SLA%) × 438 minutes per month. An availability set spreads VMs across two or three fault domains and up to twenty update domains, so the worst-case simultaneous loss is one full fault domain — roughly a third of the set with three domains. Sizing for that means provisioning enough VMs that the survivors still carry peak load, which is where the fault-domain fraction in the formula comes from.

Why does Azure Availability Set matter?

The 99.95% availability set target only covers connectivity to at least one instance, not your application's capacity, so a set sized exactly for peak becomes an outage the moment a fault domain drops; update domains additionally cap how fast Azure can patch your fleet.

What values do I need to enter?

This calculator takes 5 inputs: VMs in the availability set, Fault domains, Update domains, VMs needed to carry peak load, Availability target. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.