Paste the headers of any response and see, in plain English, how long a browser and a CDN may reuse it, when they must check back, and what's contradictory.
1 Response headers 2 processed on your device 3 How long it's cached
Cache-Control Explainer
Response headers
What caches will do
Browser
Result
CDN or proxy
Result
Directives and checks
Directive
Meaning
Build a header
Header
01
What it does
Reads response headers per RFC 9111 and shows how long browsers and CDNs may reuse a response, plus a header builder.
RFC 9111 freshness for private and shared caches
max-age, s-maxage, Expires and heuristic freshness
stale-while-revalidate and stale-if-error
Conflicts such as no-store with max-age
02
Private by design
Everything happens in your browser. Nothing you type, paste or upload is sent anywhere, and it keeps working offline once the page has loaded.
03
What you get
A caching timeline and a Cache-Control header, with a Copy and a Download button on every result.
HTTP
Caching
CDN
Headers
Why it matters
Caching is the biggest performance win on the web and the most confusing header. no-cache doesn't mean 'don't cache', s-maxage only applies to CDNs, and a missing header still lets browsers cache with a guessed lifetime.
This explainer applies the HTTP caching standard (RFC 9111) to the headers you paste and draws a timeline for a browser and a shared cache. The builder turns plain choices (who may store it, for how long, and what happens when it goes stale) into the right header.
How it works
InputResponse headers or a Cache-Control value
In your browserRFC 9111 rules for browsers and CDNs
OutputFreshness, revalidation and a fixed header
Where it fits in a designer's workflow
Plan
Design
Build
Test
Launch
Setting caching before launch and debugging stale pages.
By the numbers
Worked out by this tool's own RFC 9111 rules. With no Cache-Control at all, caches may guess: 10% of the time since Last-Modified, which is why the last example still gets cached for days. Source: RFC 9111: HTTP caching.Show the numbers as a table
Example
Browser
Shared cache or CDN
Hashed asset
1 year
1 year
HTML page
0 (revalidate)
0 (revalidate)
Account page
0 (revalidate)
not stored
No store
not stored
not stored
Risky CDN
0 (revalidate)
10 minutes
No header
2.1 days
2.1 days
How to use it
Paste the response headers (from DevTools or curl -I).
Tick the box if the request carried an Authorization header.
Read the timeline and the notes.
Or use the builder to write a new header.
FAQ
What's the difference between no-cache and no-store?
no-cache allows storing but needs a check with the server before each reuse. no-store means don't keep a copy at all.
Why does a CDN cache longer than the browser?
s-maxage applies only to shared caches such as CDNs, so you can keep the browser short and the edge long.
What's heuristic freshness?
With no max-age or Expires, caches may guess, commonly 10% of the time since Last-Modified. It's usually better to say what you want.
I build fast, accessible websites and web apps that pass these checks from day one. Tell me about your project and I'll reply personally with honest next steps.
