AWS · Free tool
AWS data transfer cost calculator
Plug in monthly volumes for each transfer type. See the line items that show up under DataTransfer-* in your CUR, broken down by route. The cross-AZ and inter-region lines are usually the surprise; this calculator makes them legible before the invoice does.
Estimated monthly data transfer cost
$0
$0 per year
| Line item | Volume | Rate | Monthly |
|---|
If a line is bigger than expected, check these first
- Cross-AZ: chatty service-to-service traffic spread across AZs. Audit caller-callee graphs and pin to single AZ where HA is not required.
- Internet egress: object reads served directly from S3 instead of cached behind CloudFront, or external API responses without compression.
- Inter-region: a multi-region active-active workload that should be active-passive, or a misconfigured replication target.
- Transit Gateway: large attachment count with low traffic per attachment. The hourly fee dominates and a VPC peering mesh would be cheaper.
Why data transfer is the line every CFO eventually asks about
The single largest data transfer line we have ever audited was $84,000 a month, all of it cross-AZ. A microservices migration had spread services across three AZs for HA, then the chattiest pair (auth and orders) ended up in different AZs, calling each other synchronously on every request. No traffic left the region. The AWS bill grew $84K in a month and nobody knew where to look.
The fix was 30 lines of Terraform to colocate the pair, then a gradual move back to multi-AZ once the calling pattern was batched. Total project: two weeks. Annualized saving: just over $1M. The audit found it in 90 minutes.
Run this on your real account
Free 14-day audit, read-only IAM role, one-page CFO summary.
We pull your CUR, classify every data transfer line item by usage type and resource, and identify the routes generating the bill. The report includes the Terraform diff for the top three fixes. Free, no card, the report is yours.
Frequently asked
What is cross-AZ data transfer and why does it cost money?
Traffic between EC2 instances in different Availability Zones within the same region is charged $0.01 per GB in each direction, so $0.02 per GB round-trip. The fee applies regardless of whether you use private IPs or public IPs. Two chatty services in different AZs can rack up several thousand dollars per month with no traffic ever leaving the region.
Is data into AWS really free?
Inbound data from the internet to AWS is free. Outbound data is what costs money: $0.09 per GB for the first 10 TB per month in most US regions, sliding down to $0.05 above 150 TB. Inbound from another AWS region or another AZ is generally not charged on the receiving side, only on the sending side.
How do I find which workload is generating the data transfer?
Cost and Usage Report broken down by usage type. Look for line items containing DataTransfer-Regional-Bytes (cross-AZ), DataTransfer-Out-Bytes (internet egress), and the inter-region equivalents. Filter by resource ID where available, then map back to which service or workload that resource belongs to. VPC Flow Logs are the second-line tool when CUR resource IDs are not granular enough.
What is the cheapest fix for cross-AZ data transfer?
Run chatty workloads in a single AZ when high availability is not a requirement (development, batch jobs, internal tools). For production HA workloads, the fix is at the application layer: use connection pooling, batch requests, and avoid the synchronous service-to-service call pattern that fans out across AZs. VPC Endpoints help for AWS service traffic but do not help for service-to-service.
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