Two Azure VMs. Same vCPU count. Same RAM. Same region. Both running Windows. One costs around £112/month. The other is £223.
That's not an error. It's how Azure prices Windows licensing across VM families, and most teams don't notice until the bill arrives.
The quirk
A 4 vCPU, 16 GB VM in UK South:
| B4ms (burstable) | D4s_v5 (general purpose) | |
|---|---|---|
| Linux | ~£103/mo | ~£121/mo |
| Windows | ~£112/mo | ~£223/mo |
| Windows premium | ~£9 | ~£102 |
Same compute spec. Same Windows licence (Datacenter). £93/month of Windows charge on the D-series that doesn't exist on the B-series.
Why it happens
Microsoft prices Windows licensing per vCPU on the general-purpose families (D, E, F). Roughly £25 per vCPU per month. A 4 vCPU D-series VM has approximately £100/month of Windows baked into the list price.
B-series uses a different model. The per-vCPU charge drops sharply because B-series is positioned as an economy tier for burstable workloads. The standard Windows premium would make it uncompetitive against equivalent Linux VMs.
The difference doesn't appear anywhere obvious in the portal or the calculator. You only see it when you put identical specs side by side across families.
Hybrid Benefit shifts the maths
Azure Hybrid Benefit removes the Windows licensing premium when your organisation owns eligible Software Assurance entitlements.
On a D4s_v5, that's around £100/month per VM. Across 20 production Windows VMs, that's roughly £24,000 a year recovered from a checkbox.
On a B4ms, the same benefit is about £8/month per VM, because the Windows premium was already small. Same 20 VMs: roughly £1,900 a year.
If your organisation has Software Assurance, Hybrid Benefit should be applied. The hunt for unapplied Hybrid Benefit pays back very differently between families. In Windows-heavy estates I look at, D/E/F series VMs running without Hybrid Benefit are common. Each one is around £100/month at 4 vCPU scale.
The decision point
The D vs B choice isn't only about consistent versus burstable CPU. Windows licensing shifts the total cost of ownership materially. If a workload doesn't need the consistent CPU profile of a D-series, the B-series is often cheaper purely on the Windows pricing line.
Azure VM pricing reads like a simple catalogue. Different families apply different licensing models, and the calculator doesn't surface the comparison. If you've never looked at your Windows VMs by family with Hybrid Benefit status, there's a fair chance you're paying for licensing you already own.
Want an independent look at your Azure Windows licensing and VM sizing? Our free cost assessment includes a full breakdown of Hybrid Benefit coverage and VM family optimisation.