AWS bill line item

DataTransfer-Regional-Bytes

A charge for data that moves between Availability Zones in the same AWS region. AWS bills both the sending side and the receiving side.

Find this on your own bill →All AWS bill line items

Data transfer & networkingAmazon EC2

Cross-AZ (regional) data transfer

Verified against official AWS documentation last checked July 27, 2026

Why it shows up

Same-AZ traffic over private IPs is free. When two communicating resources sit in different AZs, AWS bills every byte twice. It bills once as the sender's outbound leg and once as the receiver's inbound leg.

Common triggers are an auto-scaled fleet or Kubernetes cluster that spreads instances across AZs for high availability. A NAT gateway in only one AZ, while resources run in others, is another. Chatty microservice or service-mesh traffic that is not AZ-aware is a third trigger.

How to cut it

Keep a NAT gateway (or gateway endpoint) in every AZ you use. Then traffic does not cross AZ boundaries 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