Azure Scale Set Calculator

Model a Virtual Machine Scale Set with a daily peak window and a Spot mix, and compare it to a fixed fleet.

About Azure Scale Set Calculator

A scale set bills every instance-hour, so the cost is the time-weighted average instance count rather than the peak. Mixing Spot capacity into the set multiplies part of that average by a much lower rate, but Spot instances can be evicted with 30 seconds' notice, so the Spot share should only cover work that tolerates interruption. The gap between the average and the fixed-fleet figure is what autoscaling is worth, and it is usually larger than any negotiated discount; Spot prices vary by region and size and change continuously, so treat the discount here as an assumption and check current rates in the Azure pricing calculator.

Formula

Azure Scale Set

average instances = (peak hours × peak count + (24 − peak hours) × baseline count) ÷ 24; monthly = on-demand share × rate × 730 + Spot share × discounted rate × 730.

average instances: Time-weighted instance count across a representative day
Spot rate: On-demand rate reduced by the current Spot discount
fixed peak fleet: What the same workload costs if you simply run peak capacity all month

Worked example

With the default values below, Azure Scale Set Calculator returns Monthly Cost of $239.67. Change any input above and every figure here is recalculated in your browser.

Inputs used

Baseline instancesaccepts 1–1,000 instances: 3 instances
Peak instancesaccepts 1–1,000 instances: 10 instances
Hours per day at peakaccepts 0–24 hours: 6 hours
On-demand instance rateaccepts 0–100 USD/hour: 0.096 USD/hour
Share of instances on Spotaccepts 0–100 %: 40 %
Spot discountaccepts 0–95 %: 70 %
Billed hours per monthaccepts 1–744 hours: 730 hours

Results

Monthly Cost: $239.67
Average Instances Running: 4.75 instances
On-demand Charge: $199.73
Spot Charge: $39.95
Cost of a Fixed Peak Fleet: $700.80
Saving vs a Fixed Peak Fleet: $461.13
Saving: 65.8%

Step by step

1Values used
2Azure Scale Set
3Monthly Cost= 239.67
4Average Instances Running= 4.75 instances
5On-demand Charge= 199.73
6Spot Charge= 39.95
7Cost of a Fixed Peak Fleet= 700.80
8Saving vs a Fixed Peak Fleet= 461.13

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, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 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 Scale Set calculated?

average instances = (peak hours × peak count + (24 − peak hours) × baseline count) ÷ 24; monthly = on-demand share × rate × 730 + Spot share × discounted rate × 730. A scale set bills every instance-hour, so the cost is the time-weighted average instance count rather than the peak. Mixing Spot capacity into the set multiplies part of that average by a much lower rate, but Spot instances can be evicted with 30 seconds' notice, so the Spot share should only cover work that tolerates interruption.

Why does Azure Scale Set matter?

The gap between the average and the fixed-fleet figure is what autoscaling is worth, and it is usually larger than any negotiated discount; Spot prices vary by region and size and change continuously, so treat the discount here as an assumption and check current rates in the Azure pricing calculator.

What values do I need to enter?

This calculator takes 7 inputs: Baseline instances, Peak instances, Hours per day at peak, On-demand instance rate, Share of instances on Spot, Spot discount, Billed hours per month. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.