CDN Cache Hit Ratio Calculator: Origin Bandwidth and Egress Savings
Estimate how much CDN caching reduces origin response traffic, average and peak origin bandwidth, and origin egress cost. Compare your current cache hit ratio with a target scenario using measured delivered data or a requests-times-response-size estimate.
Traffic and cache inputs
Use response bytes delivered to viewers for the same period as the hit ratio.
Byte ratios are more reliable for bandwidth offload.
Use a byte share with a byte ratio or request share with a request ratio.
Estimated peak origin rate = period average × factor.
Leave blank to omit utilization.
Tip: press Ctrl/⌘ + Enter to recalculate.
Origin offload results
Enter valid inputs to calculate origin offload.
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Worked example: 25 TB at an 80% hit ratio
With no uncacheable or bypass traffic, 25 TB delivered over 30 days at an 80% byte hit ratio leaves 5 TB of estimated origin response data:
25 TB × (1 − 0.80) = 5 TB. At 0.09 per GB, the estimated origin egress cost is 450,
while 20 TB served from cache avoids 1,800 of origin egress.
Raising the byte hit ratio to 90% lowers estimated origin response data to 2.5 TB and avoids another 2.5 TB, worth 225 at that same egress rate. The 30-day average origin rate falls from about 15.43 Mbps to 7.72 Mbps; a 3× peak factor produces planning peaks of about 46.30 Mbps and 23.15 Mbps.
CDN cache offload formulas
Delivered bytes from requests: viewer requests × average response bytes
Cache-eligible bytes: delivered bytes × (1 − uncacheable share)
Bytes served from cache: cache-eligible bytes × hit ratio
Origin response bytes: delivered bytes − bytes served from cache
Average origin bandwidth: origin bytes × 8 ÷ period seconds
Estimated peak origin bandwidth: average origin bandwidth × peak factor
Origin egress cost: origin bytes ÷ 1,000,000,000 × price per GB
Origin egress savings: cache-served bytes ÷ 1,000,000,000 × price per GB
Data units and billing GB are decimal: 1 KB = 1,000 bytes, 1 GB = 1,000,000,000 bytes, and 1 TB = 1,000 GB. A ratio of 100% produces zero estimated response-body traffic from misses, but real origins can still receive revalidation, control, request-header, range-fill, purge, and logging traffic. Enter uncached or bypass response traffic in the uncacheable share rather than assuming the hit ratio applies to it.
Request hit ratio versus byte hit ratio
CDN dashboards often define cache hit ratio as the percentage of cacheable requests served from cache. That is useful for request load and latency, but it is not automatically the percentage of bytes offloaded. A site can cache many small files successfully while missing a few large downloads, producing a high request hit ratio and a lower byte offload ratio.
If you select Request hit ratio, this calculator uses a size-neutral approximation: hits and misses are assumed to have the same average response size. For egress planning, prefer measured bytes from cache misses, cache-fill bytes, or a provider byte hit ratio.
How to collect better CDN inputs
| Input | Preferred source | Common pitfall |
|---|---|---|
| Delivered response data | CDN viewer-response bytes for the selected distribution and period | Mixing request bytes, uploads, or a different time window into the total |
| Byte offload | Origin/cache-fill response bytes compared with total delivered response bytes | Substituting request hit ratio when response sizes differ materially |
| Peak factor | Peak origin bits per second divided by average origin bits per second | Using a monthly average to size a burst-sensitive origin path |
| Egress price | Effective marginal origin-to-CDN rate after tiers, credits, and commitments | Adding CDN delivery pricing when the field is for origin egress only |
| Origin capacity | The smallest relevant sustained limit across server, NIC, load balancer, and upstream path | Treating provisioned bandwidth as guaranteed application throughput |
Methodology and source notes
This is a deterministic planning model reviewed on July 28, 2026. Google Cloud documents cache hit ratio as the share of requests served from cache and distinguishes cache egress from cache fill. AWS similarly defines CloudFront cache hit rate by cacheable requests and notes that viewer bytes from misses are an approximation of origin-to-edge bytes that can exclude refresh hits.
CDN cache hit ratio FAQs
How does cache hit ratio affect origin bandwidth?
Apply the hit ratio to cache-eligible response bytes, then subtract cache-served bytes from total delivered bytes. With no bypass traffic, a 90% byte hit ratio attributes 10% of response bytes to misses at the origin.
What is the difference between request hit ratio and byte hit ratio?
Request hit ratio counts requests. Byte hit ratio weights each response by size. Use the latter for more accurate bandwidth and egress estimates.
Does the calculator include CDN charges?
No. It models only origin response-data egress at the price you enter. CDN delivery, cache-fill, request, invalidation, logging, tax, and other charges are outside the result.
Why can actual origin traffic be higher?
Revalidations, partial requests, range fills, multi-region cache warming, compression differences, request headers, errors, and provider-specific metric scopes can add traffic not represented by the simple response-byte model.
Can a target hit ratio be lower than the current ratio?
Yes. The calculator then shows additional data and savings as negative, which is useful for modeling a regression or a conservative scenario.
Are my inputs uploaded?
No. The calculations, copy output, and CSV creation run locally in your browser.
Limits and disclaimer
This calculator is an infrastructure planning aid, not a billing quote, performance guarantee, or capacity certification. It assumes the selected hit ratio applies uniformly to delivered response bytes unless the request-ratio approximation is selected. Validate production and financial decisions with CDN logs, origin telemetry, provider billing exports, contract pricing, cache behavior, and load tests.