⚡ HTTP/2 & HTTP/3 Test

📚 About HTTP Protocol Versions

Modern HTTP protocol versions significantly improve website performance. Here's what each version offers:

Performance Improvements

How to Check Which HTTP Version a Site Uses

There are three easy ways to tell whether a site is on HTTP/1.1, HTTP/2, or HTTP/3:

• This tool — enter a URL above. It opens a connection, reads the negotiated ALPN protocol, and checks the Alt-Svc header that advertises HTTP/3.
• Browser DevTools — open the Network tab, right-click the column header, enable the Protocol column, and reload. You'll see h2 (HTTP/2) or h3 (HTTP/3) next to each request.
• Command line — curl -I --http2 https://example.com shows if HTTP/2 negotiates; curl -I --http3 https://example.com (on a build with HTTP/3 support) tests QUIC.

HTTP/3 is advertised, not served on the first hit: a browser connects over HTTP/2, sees the Alt-Svc: h3=... header, and upgrades to HTTP/3 (QUIC over UDP) on the next connection. That's why this tool reports HTTP/3 as "advertised via Alt-Svc."

Why It Matters for Performance and SEO

HTTP/2 and HTTP/3 both remove head-of-line blocking and multiplex many requests over one connection, so pages with lots of assets load faster — especially on high-latency mobile networks. HTTP/3 goes further by running over QUIC (UDP) with 0-RTT connection setup, which shines when the network is lossy or connections keep changing (a phone moving between cell towers and Wi-Fi). Faster loads improve Core Web Vitals, which feed into Google's page-experience signals — so protocol version is an indirect but real SEO lever.

Frequently Asked Questions

How do I check what HTTP version a website uses?

Enter the URL in the tool above, or open your browser's DevTools Network tab and enable the Protocol column (h2 = HTTP/2, h3 = HTTP/3). On the command line, curl -I --http2 https://example.com confirms HTTP/2.

What is HTTP/3 and QUIC?

HTTP/3 is the latest version of HTTP, running over QUIC — a transport protocol built on UDP instead of TCP. QUIC removes connection-setup round-trips (0-RTT), eliminates TCP head-of-line blocking, and handles network changes gracefully, which makes it noticeably faster on mobile and lossy connections.

Is HTTP/3 better than HTTP/2?

For most sites, yes — especially on mobile. HTTP/3's UDP/QUIC transport avoids the TCP-level head-of-line blocking that can still affect HTTP/2 on unreliable networks. On a fast, stable connection the difference is smaller. Serving both (with HTTP/3 advertised via Alt-Svc) gives clients the best of each.

Does HTTP/2 or HTTP/3 improve SEO?

Indirectly. Neither is a direct ranking factor, but both speed up page loads and improve Core Web Vitals, which Google uses as a page-experience signal. Faster pages also reduce bounce and improve conversions.

Why does the tool show HTTP/3 as "advertised" rather than "supported"?

HTTP/3 is discovered through the Alt-Svc header sent over an HTTP/2 connection. A server advertises h3 there, and clients upgrade on a later connection. Detecting that header is the standard way to confirm HTTP/3 availability without a full QUIC handshake.