Free Tools
Free monitoring tools, no signup
Fast, genuinely useful tools for anyone who runs a website, backed by the same monitoring network that powers Sentinel. Use them free, no account required.
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Website Health Check
One report card for your whole site: HTTPS, security headers, HTTP/2, DNS, DNSSEC, domain expiry, robots and sitemap, all graded A to F.
For the agent web
AI Readiness Check
See whether AI agents can reach, read, and use your site: AI-crawler rules, JavaScript-free content, llms.txt, structured data, and MCP, all graded.
Health & availability
Live URL Checker
Check any site's status live from our monitoring network in one click, with the DNS, TCP, TLS and time-to-first-byte breakdown.
Check a URLIs It Down?
Check whether any site is down right now, verified from a real monitoring network instead of a single location.
Check a siteWebsite Speed Test
Test response times from multiple monitoring regions at once, each with its DNS, TCP, TLS and time-to-first-byte phases.
Run speed testSSL, DNS & domains
SSL & TLS Checker
Run a real TLS handshake against any domain. Certificate expiry, issuer, protocol, cipher, and an A-to-F security grade.
Check SSLDNS Lookup
Every DNS record for a domain in one query: A, AAAA, CNAME, MX, NS, TXT and SOA, each with its TTL.
Look up DNSDNS Propagation Checker
Query several public DNS resolvers at once and see how far a record change has spread. When they disagree, it is still propagating.
Check propagationDomain Expiry Checker
WHOIS lookup for any domain: registrar, creation and expiry dates, transfer-lock status, and nameservers.
Check domainHTTP & requests
HTTP Header Checker
See exactly what a server returns: status code, redirect count, full response headers, and which security headers are missing.
Check headersRedirect Checker
Follow the full redirect chain for any URL: every hop, its status code, and the final destination. Spot loops and slow chains.
Trace redirectsHTTP Status Codes
A searchable, plain-English reference to every HTTP status code: what each one means, what causes it, and how to fix it.
Browse codesCalculators & builders
Downtime Cost Calculator
Put a real dollar figure on an outage. Enter your traffic and revenue to see what an hour, or a day, of downtime actually costs.
Calculate costUptime SLA Calculator
Translate a "99.9%" promise into real minutes. See how much downtime each SLA tier allows per day, week, month and year.
Open calculatorCron Expression Builder
Turn a cron expression into plain English and preview its next 5 run times. Start from a preset or write your own.
Build a scheduleWatch your sites around the clock.
These tools take a snapshot. Sentinel keeps checking from multiple regions and tells you the moment something breaks.