Amazon RDS vs Azure SQL Database: Why High Availability & Hybrid Licensing Create a 50% TCO Gap

At the base compute layer, a 4-vCPU AWS RDS db.m6i.xlarge running SQL Server Standard License Included costs $0.976/hr ($712.48/mo), compared to $1.008/hr ($735.84/mo) for 4 vCores on Azure SQL Database Gen5 General Purpose. However, AWS bills per provisioned instance, meaning enabling Multi-AZ high availability spins up a synchronous standby node and doubles compute spend to $1.952/hr ($1,424.96/mo). Azure SQL Database General Purpose includes built-in failover orchestration over shared remote storage at no additional vCore multiplier.

For intermittent or bursty workloads, the billing models diverge further. AWS RDS bills SQL Server License Included instances by the full clock hour with no native scale-to-zero capability for SQL Server. Azure SQL Serverless bills per second at $0.000145/vCore-second and automatically pauses during idle periods, paired with a permanent monthly free tier of 100,000 vCore-seconds and 32 GB storage per subscription.

Bringing existing Microsoft SQL Server licenses with active Software Assurance (SA) alters the TCO curve on both platforms. On AWS RDS, Bring Your Own Media (BYOM) lowers a 4-vCPU db.m6i.xlarge from $0.976/hr (Standard LI) or $2.054/hr (Enterprise LI) down to $0.600/hr—saving 38% to 71% on hourly rates. For teams buying new retail licenses plus SA, BYOM breaks even against On-Demand License Included at roughly 65% continuous utilization.

Microsoft tilts the licensing rules heavily in favor of its own cloud via Azure Hybrid Benefit (AHB). Applying AHB drops 4-vCore General Purpose compute by 40% to $0.608/hr ($0.152/vCore-hr) and Business Critical by 55% from $2.720/hr to $1.220/hr ($0.305/vCore-hr). Crucially, Azure grants a 1:4 virtualization multiplier allowing 1 on-premises SQL Server Enterprise core license to cover 4 General Purpose vCores in the cloud—an entitlement unavailable on AWS RDS.

At standard capacities (under 4 TB), both providers charge $0.115/GB-month for general-purpose SSD storage. However, Azure SQL General Purpose and Business Critical tiers enforce a strict 4 TB storage ceiling. Scaling past 4 TB—and up to 128 TB—requires migrating to the Azure SQL Hyperscale tier, which more than doubles the storage rate to $0.25/GB-month via elastic page servers and restricts Azure Hybrid Benefit discounts on newer deployments.

AWS RDS caps single EBS volumes at 64 TiB, but its Additional Storage Volumes feature allows SQL Server and Oracle instances to mount up to three extra volumes for a combined 256 TiB footprint. Database administrators can place cold archive filegroups on gp3 ($0.115/GB-mo) while isolating latency-sensitive transaction logs on io2 ($0.125/GB-mo + $0.10/IOPS-mo). Watch out for AWS RDS overage traps: scaling gp3 above its 3,000 IOPS / 125 MB/s baseline adds $0.02/IOPS-mo and $0.08/MB/s-mo, and running deprecated engine versions triggers Extended Support fees of $0.10 to $0.20 per vCPU-hour.