How the Azure Functions calculator works

01 Fixed instance sizes

Azure Functions doesn't take a free-form memory value. Flex Consumption, the recommended serverless plan, offers three instance sizes: Azure docs

Python apps default to an HTTP concurrency of 1 at every size. Azure docs Each instance also gets 272 MB the platform keeps for system and host processes, which isn't billed. Your function code and the Functions host share the instance's resources. Azure docs

02 From peak usage to an instance size

memory = peak × (1 + limit margin)
size   = the smallest Flex Consumption size ≥ max(memory, 2,048 MB), else the smallest Elastic Premium SKU ≥ memory

Running out of memory recycles the instance, so, as with Lambda and Cloud Run, only the peak sets the size. The limit margin comes from the sensitivity, workload type and environment you choose; the sizing model explains it.

Microsoft writes the sizes in MB but measures memory in MB and MiB interchangeably, so the calculator compares MiB with those MB as equal; the sizes being powers of two point the same way. Assumption

Worked example

Peak 630 MiB, generic workload, medium sensitivity, production:

A small function peaking at 50 MiB would fit 512 MB, but gets Microsoft's 2,048 MB default:

Lambda and Cloud Run also size from the peak alone: see the Lambda guide and the Cloud Run guide.

03 Microsoft's guidance: 2,048 MB and concurrency

Microsoft suggests 2,048 MB as the default for most apps, and 512 MB or 4,096 MB when concurrency or processing power call for it. Larger instances handle more concurrent executions, and CPU and network bandwidth grow with the size. Azure docs

So the calculator never goes below 2,048 MB: it picks the smallest size the measured peak fits in, from 2,048 MB up. 512 MB stays a choice you make by hand for a small, low-concurrency app, which then gets 0.25 cores and a default HTTP concurrency of 4; the note under the result says so. Assumption

Executions on an instance share its memory, so the measured peak includes the concurrency the app ran at. Raising concurrency, or moving to a size with a higher default, can need more memory per instance. Azure docs Assumption

04 Above 4,096 MB: Elastic Premium

Premium bills for the cores and memory provisioned, keeps at least one instance running, and every function app in the plan shares its instances. Azure docs EP1 never comes up, because Flex Consumption's 4,096 MB already covers it, and the calculator assumes your app is the only one on the plan. Assumption

The legacy Consumption plan runs every instance with up to 1.5 GB and has nothing to size. Azure docs

05 What the calculator writes

az functionapp scale config set --resource-group <resource-group> --name <app> --instance-memory 2048

functionAppConfig: {
  scaleAndConcurrency: {
    instanceMemoryMB: 2048
  }
}

• az functionapp scale config set --instance-memory changes a Flex Consumption app's size at any time. Azure docs
• In Bicep or ARM, functionAppConfig.scaleAndConcurrency.instanceMemoryMB sets the memory of each instance. Azure docs
• For Premium, az functionapp plan update --sku EP2 changes the plan's SKU. Azure docs

06 Every warning, and why

07 Where the numbers come from

Azure Monitor's Memory working set (MemoryWorkingSet) is the memory the app uses now, and Average memory working set its average; both can be split by instance. They aren't available for Linux apps on the legacy Consumption plan. Azure docs Take the highest per-instance working set over a representative period as the peak.

08 All assumptions in one place

Besides the margins of the shared sizing model:

Our default: a value we chose. Our reading: how we read documentation that isn't explicit. About your setup: something we take for granted about your data or cluster.

Values you can change

The calculator’s Advanced: margins and defaults section, closed by default, lets you replace these values when your setup differs. Each field’s “?” says how it moves the result; a value outside the range is used at its nearest end, and “Reset to defaults” puts them all back.

09 References

• Flex Consumption plan — instance sizes, cores, the 272 MB buffer and Microsoft's default.
• Concurrency in Azure Functions — default HTTP concurrency per size.
• Premium plan — EP1–EP3 and billing.
• Scale and hosting — memory limits per plan, Consumption's 1.5 GB.
• Manage Flex Consumption apps — az functionapp scale config set --instance-memory.
• Microsoft.Web/sites template reference — instanceMemoryMB.
• az functionapp plan — update --sku.
• Monitoring data reference — MemoryWorkingSet and AverageMemoryWorkingSet.

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