Reliability tools, free and without a signup

Which tool answers which question

Each tool answers one specific question. Start from the question you have, not the tool name.

• Is my website up right now?The Website Uptime Checker is a website uptime checker: it checks if a website is up from 3 probe regions at once and shows each region's status code and response time. If it fails in some regions only, the problem is usually a network, CDN or DNS path rather than the site itself.
• Is it down for everyone or just me?Is It Down? runs the same kind of request and explains what to try when the site is up everywhere except for you.
• Has my DNS change or certificate reached everyone?The DNS Checker compares answers from three public resolvers; the SSL Expiry Checker shows the certificate a host is actually serving and when it expires.
• What should we promise, and how often should we check?The SLA Uptime Calculator turns a percentage into allowed downtime, the Check Interval Planner weighs detection speed against check volume, and the Percentile Calculator turns raw response times into p50 to p99.
• What does downtime cost us?The Downtime Cost Calculator estimates lost revenue; the On-Call Cost Calculator compares what a responder rota costs on different tools.
• What does this error mean, and what will an alert look like?The HTTP Status Code Reference explains each code in monitoring terms; the WhatsApp Alert Test lets you request one sample downtime alert on your own number.
• How much reliability do we have left, and how fast do we recover?The Error Budget Calculator shows the budget remaining and when it runs out at the current burn rate; the MTTR and MTBF Calculator turns incident timings into recovery time, time between failures and availability.
• What do we tell customers during an incident?The Incident Message Generator drafts status page updates for each stage, from investigating to resolved.
• When does this job run, and what time was that?The Cron Expression Parser reads a cron schedule in plain English and lists its next runs; the Unix Timestamp Converter turns epoch values from logs into readable dates.

A one-off check tells you the state of a site at one moment. To test website uptime over days and weeks, and hear about an outage without having to look, set up a monitor that repeats the check on a schedule.