AWS Cross-AZ Data Transfer Cost Calculator
Cross-AZ traffic is charged $0.01/GB in each direction — a round trip is $0.02/GB. The 10K-rps three-tier app below burns ~$5,260/month on east-west traffic alone. Model your boundary, then measure what AZ-affinity routing saves.
💡 Payload = request + response bytes crossing the AZ boundary. Chatty microservices often move 5-20 KB per call even for tiny payloads.
The east-west bill nobody budgets for
Compute and internet egress get all the attention, but cross-AZ data transfer is a billable boundary inside your own region. AWS charges $0.01/GB in each direction between Availability Zones for EC2, RDS, Redshift, ElastiCache, DAX and Elastic Network Interfaces. Every request that crosses an AZ boundary is a metered event.
At production scale this is not noise. A three-tier application at 10,000 req/s with a 10 KB average payload moves about 100 MB/s across the boundary — roughly 263 TB/month, or ~$5,260/month at $0.01/GB per direction (round-trip $0.02/GB). That is a real line item that architecture choices control.
The fix: AZ-affinity routing
For stateless workloads, the single fastest reduction is disabling cross-zone load balancing on Application and Network Load Balancers — AWS now lets you do this independently per load balancer, routing each request to instances in the AZ it arrived from. Stateful pairs should co-locate: put RDS read replicas and ElastiCache nodes in the same AZ as the consumers that query them. Traffic between S3, DynamoDB, SQS, SNS and EC2 in the same region stays free — the billable boundary is between EC2-class services across AZs.
See also: AWS ALB vs NLB Cost Calculator · AWS Egress Bandwidth Cost Estimator · Concurrent Users → Bandwidth · Network Economics · 📰 The AZ Tax (Full Briefing)