AWS bill line item
DataTransfer-Regional-Bytes
You are being charged for data moving between Availability Zones within the same AWS region. It's billed on both the sending and the receiving side.
Cross-AZ (regional) data transfer
Verified against official AWS documentation last checked
Why it shows up
Same-AZ traffic over private IPs is free, but the moment two communicating resources land in different AZs, every byte is billed twice: once as the sender's outbound leg and once as the receiver's inbound leg.
Common triggers: an auto-scaled fleet or Kubernetes cluster spreading instances across AZs for high availability, a NAT gateway that exists in only one AZ while resources run in others, or chatty microservice/service-mesh traffic that isn't AZ-aware.
How to cut it
- Keep a NAT gateway (or gateway endpoint) in every AZ you use so traffic doesn't cross AZ boundaries just to reach the internet or an AWS service.
- Make AZ-aware routing/affinity a first-class concern in service mesh or load-balancer configuration so requests prefer same-AZ targets.
- For chatty replicated/clustered workloads, weigh the availability benefit of spreading across AZs against the steady-state cross-AZ transfer cost.
- Use flow logs alongside Cost Explorer's regional-transfer totals to identify which resource pairs are generating the volume. The billing data alone won't show you that.
$0.01 per GB, in each direction — us-east-1; billed as two separate $0.01/GB charges (in + out) for each transfer; as of 2026-07
How to read a usage type
DataTransfer-Regional-Bytes carries no region or direction token, so there's nothing to break down here.
Source
Source: AWS docs →Found DataTransfer-Regional-Bytes on your own bill?
Drop your Cost Explorer CSV into the AWS Bill Analyzer. It'll flag line items like this one on your own bill and link straight back to the fix.
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