S3 data transfer costs: egress, cross-region access, and common surprises

For many storage-heavy systems, S3 data transfer (egress) becomes the largest line item long before storage does. This guide helps you model transfer in a way that avoids double-counting and missed costs.

Step 1: Map the transfer boundary (draw the path)

Transfer estimates fail when you do not define what is "inside" vs "outside" the billed boundary. Start by drawing a simple path for a real request: client -> CDN (optional) -> origin (S3) -> compute -> other regions -> external systems.

  • Internet egress: S3 to the public internet (downloads to users, customers, partners)
  • CDN origin egress (cache fill): S3 to CDN on cache misses, revalidation, and edge fills
  • Cross-region reads: app in Region A reading a bucket in Region B
  • Replication/copy traffic: CRR/SRR, batch copies, migrations, backfills
  • Private networking paths: VPC endpoints, inter-AZ traffic, and "inside the cloud" flows that still show up as transfer usage types

Step 2: Estimate GB/month (3 practical methods)

Choose the method that matches what you can measure today. You can refine later.

  • Throughput method: if you have Mbps charts (APM, CDN analytics, load balancer metrics), convert Mbps to GB/month with Units Converter.
  • Request method: if you know requests/day and average bytes served, compute GB/month. Tool: API response transfer.
  • Billing/log method: prefer measured bytes from CUR/Cost Explorer, S3 access logs, or CDN metrics. This avoids unit mistakes.

Step 3: Price the right line item (egress vs cross-region vs CDN)

A common pattern is paying both: (1) CDN bandwidth (edge-to-user) and (2) origin egress (S3-to-CDN) on cache fill. They are not the same GB.

Worked estimate template (copy/paste)

  • User downloads = downloads/month * average object size (GB)
  • CDN cache fill = cache misses/month * average object size (GB) + revalidation traffic (if meaningful)
  • Cross-region reads = reads/month * average read size (GB) for each region boundary
  • Replication = changed data replicated (GB/month) + one-time backfill (if any)

Common pitfalls (transfer is easy to mis-price)

  • Separate CDN bandwidth (edge-to-user) from S3 origin egress (S3-to-CDN cache fill). Guide: origin egress vs CDN bandwidth.
  • Mixing replication traffic with user downloads. Replication is driven by writes/churn, not reads.
  • Forgetting the "inside cloud" boundary (cross-region reads, private networking, NAT/egress paths).
  • Using average object size without checking compression, partial responses, or range requests.
  • Unit errors: GB vs GiB and Mbps vs MB/s (use the Units Converter if you're unsure).

How to validate the estimate

  • In CUR/Cost Explorer, group by usage type and confirm your model maps to real transfer usage lines.
  • Compare your estimated GB/month to CDN analytics bytes served and to S3 access logs (if enabled).
  • Spot-check a top 1-3 endpoints/prefixes to confirm average response size assumptions.

Related calculators and guides

Data egress cost VPC transfer cost S3 cost calculator CDN bandwidth cost Replication cost guide

Sources

Related guides

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Related calculators

Data Egress Cost Calculator
Estimate monthly egress spend from GB transferred and $/GB pricing.
API Response Size Transfer Calculator
Estimate monthly transfer from request volume and average response size.
VPC Data Transfer Cost Calculator
Estimate data transfer spend from GB/month and $/GB assumptions.
Cross-region Transfer Cost Calculator
Estimate monthly cross-region transfer cost from GB transferred and $/GB pricing.

FAQ

Is S3 transfer usually more expensive than storage?
For many read-heavy systems, yes. Egress grows with downloads and deliveries, while stored GB grows slowly. It's common for transfer to dominate before storage does.
Does CloudFront bandwidth replace S3 egress?
Not automatically. CDN bandwidth is edge-to-user delivery. You may still pay S3 origin egress (cache fill) from S3 to CloudFront on cache misses and fills.
How do I estimate GB/month from requests?
Use average object/response size and request volume: requests/day x avg size -> GB/month. Validate with real bytes served from logs or metrics once available.
What are common double-counting mistakes?
Counting both CDN bandwidth and origin egress as the same GB, or mixing cross-region replication traffic with user download egress without separating boundaries.
Last updated: 2026-01-27