Free SSL Certificate Checker Online

SSL Certificate Checker: Inspect a Site's TLS Certificate, Expiry, and Chain

An SSL certificate checker connects to a host over TLS and reports back exactly what certificate it presented — who it was issued to, who issued it, the window of dates it's valid for, how many days remain before it expires, every alternative hostname it covers, its serial number, the signature algorithm used to sign it, and how many certificates deep the chain goes. This tool does that connection for you: enter a hostname, and the server opens a TLS connection to that host's TLS endpoint (port 443 by default, though a different port can be specified) and reads back the certificate the other end presents, then hands you a readable summary instead of raw certificate output.

That connection happens on the server side, not inside your browser tab — the hostname you enter is sent to FreeConvertingTools, which is the party that actually establishes the TLS handshake and inspects what comes back. Nothing about the hostname or the certificate details is retained once the response is returned to you. This is also why the check works even on certificates a normal browser would refuse to load a page over: an expired certificate, a self-signed one, or one issued for a different hostname than the one you connected to will all still be reported here, because reporting on exactly that kind of problem is the tool's job — it inspects whatever certificate is presented rather than declining to look at one that wouldn't pass ordinary trust validation.

How to Check a Certificate with This Tool

1. Enter the hostname you want to inspect — a bare domain like example.com works; a pasted URL has its protocol, path, and any port suffix stripped automatically, leaving just the hostname to send.
2. Click Check (or press Enter). The request goes to the server, which opens a TLS connection to that host on port 443 and retrieves the certificate it presents.
3. A status card appears first: a green lock and "Certificate Valid" for a certificate that's both currently within its validity window and not expired, or a warning icon with "Certificate Expired" or "Certificate Issue" otherwise — along with a countdown showing days until expiry (or days since it expired, if that's already happened).
4. Below that, a details card lists the issuer, the validity window's start and end dates, the serial number, the signature algorithm, and how many certificates were found in the chain presented by the server.
5. If the certificate covers additional hostnames beyond the one you checked, they appear as a list of Subject Alternative Names — useful for confirming a single certificate also covers a "www" prefix, a wildcard, or other related subdomains.
6. Run the check again against a different hostname or port any time — each check is an independent connection, so there's no state carried over between one lookup and the next.

Why People Check SSL Certificates

• Catching an expiring certificate before it lapses. A certificate that expires without being renewed breaks HTTPS for every visitor immediately — checking the days-until-expiry figure ahead of a renewal deadline is one of the simplest ways to avoid an unplanned outage.
• Verifying a renewal or reissue actually took effect. After renewing a certificate or switching certificate authorities, checking the issuer and validity dates directly confirms the new certificate is the one actually being served, rather than assuming a control panel change propagated correctly.
• Confirming which hostnames a certificate actually covers. The Subject Alternative Names list settles, definitively, whether a single certificate covers both the root domain and a "www" subdomain, or a wildcard covering every subdomain — information that explains a browser warning on one hostname but not another sharing the same certificate.
• Diagnosing a browser security warning. When a visitor reports a certificate warning, checking the certificate directly shows whether the problem is a genuinely expired certificate, a hostname mismatch, or something else entirely — without needing to reproduce the warning in your own browser first.
• Reviewing chain length and issuer for a third-party or vendor site. Before integrating with or trusting a partner's endpoint, seeing which certificate authority issued its certificate and how many intermediate certificates sit in the chain is a quick, independent sanity check.
• Checking a non-standard port. Plenty of internal tools, mail servers, and APIs serve TLS on a port other than 443 — this checker accepts a port number alongside the hostname, so those endpoints don't require a separate tool just because they're not on the default port.

What Each Field in the Report Means

Field	What it tells you
Subject	The identity (commonly the hostname, in the CN field) the certificate was issued for
Issuer	The certificate authority that signed the certificate
Valid From / Valid To	The window during which the certificate is considered current
Days Until Expiry	A countdown to the Valid To date, negative once that date has passed
Subject Alternative Names (SAN)	Every additional hostname (and wildcard) the certificate also covers
Serial Number	A unique identifier the issuing authority assigned to this specific certificate
Signature Algorithm	The cryptographic algorithm used to sign the certificate (e.g. RSA-SHA256, ECDSA-SHA256)
Chain Length	How many certificates deep the presented chain goes, from the leaf up toward a root

The signature algorithm field deserves a brief technical note: it's read directly out of the certificate's underlying binary structure rather than reported by a higher-level library shortcut, since the standard interface this tool relies on for the rest of the fields doesn't expose that particular value on its own. The chain length is walked defensively — following each certificate's issuer up the chain, but stopping automatically at a repeated or missing link rather than assuming a self-reported chain from an untrusted connection will always terminate cleanly.

Why This Tool Reports Problems Instead of Blocking Them

A typical web browser refuses to load a page at all if its certificate is expired, self-signed, or issued for the wrong hostname — that's the correct behavior for browsing, where trust matters. This tool is built for a different purpose: diagnosing exactly those situations, so it deliberately does not perform the same trust validation a browser does. It connects, retrieves whatever certificate the host presents, and reports on it factually — expired, self-signed, and hostname-mismatched certificates are all read and shown rather than causing the connection to be refused. That's what makes it useful for troubleshooting a certificate problem in the first place: a tool that refused to look at a broken certificate would be useless for figuring out exactly how it's broken.

What the tool does still guard against is a different kind of risk entirely: being pointed at an internal or private target rather than a genuine public host. Before opening any connection, the hostname you provide is checked against a public-address requirement — a target that resolves to a private, loopback, internal, or link-local address is refused outright, so this tool can only ever be used to inspect certificates on public hosts, never to reach into a private network or probe internal infrastructure. That check happens before the TLS handshake is even attempted, and it's separate from (and unaffected by) the fact that the handshake itself doesn't validate certificate trust — one is about where the connection is allowed to go, the other is about what's reported once it gets there.

Common Certificate-Checking Pitfalls

The most frequent mix-up is treating "isValidNow: true" as a statement about trust rather than timing — this tool's validity flag reflects whether the current date falls within the certificate's stated validity window, not whether a browser would trust the issuing authority or accept the hostname match. A certificate can be self-signed, entirely untrusted by any browser, and still report as "currently within its validity dates," because those are two separate questions. A second pitfall is checking the wrong hostname: a certificate covering example.com and www.example.com via its Subject Alternative Names might not cover api.example.com unless that name is separately listed or a wildcard covers it — always check the exact hostname you actually care about, not a related one. A third is forgetting that many sites terminate TLS at a load balancer or CDN in front of the actual application server, so the certificate reported here is whatever that front-facing layer presents, which may differ from a certificate configured deeper in the infrastructure that never gets checked from the outside.

Manual Certificate Checking vs. Using This Tool

• A browser's padlock and certificate viewer. Clicking the padlock icon in most browsers shows basic certificate details for a page you're currently visiting, but it only works for a site you can actually load in the browser — an expired or clearly broken certificate often blocks the page (and the details) behind a warning interstitial first.
• OpenSSL's command line. The openssl s_client command retrieves and prints full certificate details for any host and port, but requires OpenSSL installed, a fairly involved command with several flags, and manual parsing of its plain-text output.
• Browser DevTools' Security panel. Chrome and Firefox both expose a certificate viewer through their developer tools, useful when a page has already loaded, but again unavailable for a certificate you can't get the browser to connect to at all.
• Third-party SSL diagnostic services. Several dedicated services run a broader battery of TLS configuration checks (protocol versions, cipher suites, known vulnerabilities) — a heavier and slower option if all you need is the certificate's own subject, issuer, and expiry.
• This tool's API. Teams that need expiry watched across many hosts on a schedule, wired into a deployment pipeline or an alerting system instead of opened in a browser, can call this same check as a metered API endpoint — submit a hostname and optional port and get the certificate fields back as JSON; the API documentation lays out the request shape and credit pricing.

Can this tool check a certificate that's self-signed or already expired?

Yes. Because the connection doesn't perform trust validation, a self-signed, expired, or hostname-mismatched certificate is retrieved and reported the same as any other — the status card will flag it clearly as expired or otherwise problematic, rather than refusing to connect.

Can this tool be used to check certificates on internal or private servers?

No. Every hostname is validated before any connection is attempted, and a target that resolves to a private, loopback, internal, or link-local address is refused. This tool only ever reaches public hosts on the open internet — it cannot be used to inspect certificates on internal infrastructure or a private network.

What does "days until expiry" mean if the certificate has already expired?

The count goes negative — a certificate that expired five days ago reports minus five, shown as "Expired 5 days ago" rather than a raw negative number, so an already-lapsed certificate is unmistakable at a glance rather than requiring you to do the date math yourself.

Does checking a certificate on a non-default port work the same way?

Yes — provide the port alongside the hostname and the tool connects there instead of port 443. Plenty of services (custom APIs, mail servers, admin panels) serve TLS on a non-standard port, and this checker handles those exactly the same way it handles a standard HTTPS endpoint.

Related Network Tools

A certificate is only one part of how a host presents itself. Pair this checker with the HTTP headers viewer to see the full response headers the same host returns, or the website status checker to confirm it's actually up and responding. Use the DNS lookup tool to see what address the hostname resolves to in the first place, or the WHOIS lookup to check who registered the domain the certificate was issued for.