Origin egress vs CDN bandwidth: what's the difference?

If you are building a CDN cost calculator model, the easiest mistake is mixing up two similar-sounding line items: origin egress and CDN bandwidth. They are related, but they are not the same thing.

Quick definitions

• CDN bandwidth (edge egress): bytes delivered from the CDN edge to end users. This is the number most CDN pricing pages focus on (for example, $/GB).
• Origin egress: bytes transferred out of your origin (object storage, load balancer, VM, or service) to the CDN or to other destinations. This is billed by your origin provider, not by the CDN (unless the CDN is also your origin).

Why they differ

The CDN serves user traffic from cache when possible. When an object is not in cache (or is expired), the CDN pulls it from origin. That pull is often called cache fill. Cache fill contributes to origin egress, but not always in a 1:1 way with edge bandwidth.

• With a high cache hit rate, origin egress can be a small fraction of CDN bandwidth.
• With a low cache hit rate, origin egress can approach CDN bandwidth.
• Some workloads can have origin egress higher than edge bandwidth due to revalidation, multi-POP fills, or inefficient caching patterns.

A simple way to model costs (without double-counting)

Use separate line items and separate calculators:

1. Model CDN edge bandwidth with CDN Bandwidth Cost.
2. Model CDN request fees with CDN Request Cost.
3. If you pay for origin egress, estimate origin egress (cache fill) with CDN Origin Egress, then price it with Data Egress Cost using your origin's $/GB.

Then sum those line items to get a total. If you want a single page that bundles the two major CDN line items, start with CDN Cost Calculator.

Estimating origin egress from edge bandwidth

If you do not have origin egress data yet, you can approximate it from expected cache hit rate:

• Origin egress (GB/month) ≈ CDN bandwidth (GB/month) x (1 - cache hit rate)

This is a simplification. It's a good starting point for budgeting, and you can refine it later with real metrics.

Common pitfalls

• Using origin egress as CDN bandwidth: the origin egress line item may be much smaller (or larger) than the edge bandwidth line item.
• Forgetting cache fill: your CDN might look cheap until you realize your origin is paying large egress charges.
• Mixing units: provider dashboards might use GiB while pricing pages use GB. Use Units Converter to keep units consistent.

Related tools

CDN cost calculator CDN bandwidth cost CDN origin egress Origin egress cost

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