The quiet $0.01/GB each way that multi-AZ high availability costs you.
Spreading services across zones bills roughly $0.02/GB round trip — invisible until it is thousands of dollars. Keep chatter in-zone. See the wider-scope cost on the inter-region transfer calculator and Kafka impact on the managed Kafka calculator.
The Cross-AZ Data Transfer Cost Calculator estimates what you pay to move data between availability zones within a cloud region. You enter the volume of cross-AZ traffic per month (in GB) and the per-GB rate for each direction, and the tool multiplies volume by rate to produce a monthly cost. Because many cloud providers bill data leaving one AZ and data entering another, the calculator accounts for both directions, giving you the round-trip cost of traffic that crosses zone boundaries. The main drivers are simple but easy to overlook: the total gigabytes moved and the small per-gigabyte charge that quietly compounds as traffic grows.
This matters because multi-AZ high availability is not free. Spreading databases, application tiers, and replicas across zones improves resilience, but every replication stream, health check, and cross-zone request generates billable transfer. A rate that looks trivial per gigabyte can become a meaningful line item at scale. The key trade-off to watch is availability versus transfer cost: keeping chatty services co-located in one AZ reduces charges but weakens fault tolerance. Use the calculator to size that cost before committing to an architecture, so you can decide where cross-zone redundancy is worth paying for and where consolidating traffic makes more sense.
On AWS, traffic between availability zones is charged at about $0.01/GB for egress out of the source AZ and again for ingress into the destination — roughly $0.02/GB round trip. Chatty services split across zones pay this on every message.
Use zone-aware routing so services talk within their own AZ where possible, read from same-zone replicas (fetch-from-follower in Kafka), and weigh single-AZ deployments for workloads that do not need multi-AZ durability. HA is worth it, but design to minimize cross-zone chatter.
Cost at different volumes:
| Volume | Monthly | Yearly |
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