What does this test?
Big CDNs have hundreds of locations (points of presence, POPs). Which one answers you decides how fast every site on that CDN loads for you, and it isn't always the nearest one.
The page asks one small file on each CDN from your browser, the way a site would, and reads which location answered. Then it shows how far that location is from you and the round trip to it, next to the least that distance allows.
Nothing is sent to this site: the requests go from your browser to the CDNs.
How is the location read?
Each CDN writes it into its answer, in its own way:
Cloudflare: the colo field of /cdn-cgi/trace (HKG).
Amazon CloudFront: the cdn-pop entry of Server-Timing, or X-Amz-Cf-Pop (NRT57-P11).
Akamai: Akamai-Request-BC, which names the city (JP_13_TOKYO).
Fastly: X-Served-By (cache-nrt-…-NRT).
Azure Front Door: X-Azure-Ref (…C1TYO…).
Bunny: the Server header (BunnyCDN-JP1-…).
Wikimedia: X-Cache, whose cache host number names the data center.
A browser may only read these when the answer allows it, so each CDN is tested through a public file whose answer does. Google, Alibaba Cloud, Tencent EdgeOne, Wangsu, Gcore and CDN77 allow none, so they are timed only.
Why am I sent to a far location?
CDNs don't route by distance. Cloudflare, Akamai and the others announce their addresses from many places at once (anycast) or answer DNS by your resolver's location, and your network picks the path.
In mainland China this is the usual case: Cloudflare's free and lower plans are reached through the US west coast or other far locations on many networks, because those routes cost less. Paid plans and the China Network (with JD Cloud) are served from Hong Kong or from inside China.
A proxy or VPN moves you too: the CDN sees the proxy's address and serves its location.
What is the least round trip?
Light in fiber covers about 200 km per millisecond, so a round trip over a distance d can't be faster than d ÷ 100 ms. Real routes are 1.5 to 2 times longer than a straight line. A round trip much longer than twice that (plus 40 ms) is marked as a long way round: the route detours, or waits in a congested link.
What does "not the nearest" mean?
For the CDNs that publish all their locations (Cloudflare, Bunny, Wikimedia), the page finds the one nearest to you. When the location that answered is more than 1,000 km and twice as far as that one, it says so. It is not a fault: the CDN or your network chose another route.
How does the website check work?
Enter a domain. This site's Worker looks up its DNS with Cloudflare's resolver (so a local or censored answer can't mislead it) to find the CDN its name points to, asks /cdn-cgi/trace on the site for Cloudflare, and reads the location headers of its favicon when the site lets pages read them. Then it times the site.
The domain goes to this site's Worker and Cloudflare's DNS; neither keeps it.
Where does my location come from?
From your IP address, as Cloudflare places it (this site runs on Cloudflare). It's usually right to the city, but can be hundreds of km off, and with a proxy it's the proxy's. "Use precise location" asks your browser instead; it stays in the page.
Does it collect my data?
No. The requests go from your browser straight to each CDN, which sees your IP address like any site does. This site stores nothing. A share link holds the locations and round trips; your city, its coordinates (to about 10 km) and your network only when you tick the box, and never your IP address. The code is open source on GitHub.
