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.

Inputs

Baseline instancesinstances

Peak instancesinstances

Hours per day at peakhours

On-demand instance rateUSD/hour

Share of instances on Spot%

Spot discount%

Spot prices float with regional capacity and can be evicted with 30 seconds' notice.

Billed hours per monthhours

Monthly Cost

$239.67

Average Instances Running

4.75instances

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

Values used
Baseline instances = 3 instances; Peak instances = 10 instances; Hours per day at peak = 6 hours; On-demand instance rate = 0.0960 USD/hour; Share of instances on Spot = 40 %; Spot discount = 70 %; Billed hours per month = 730 hours
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.
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

How it works

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

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.

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