What Is Uptime? Definition, Percentages, and Monitoring

Uptime is the amount of time a system, server, or service stays operational and accessible. It's usually expressed as a percentage over a given period — 99.9% uptime over a month means roughly 43 minutes of total downtime. The inverse is downtime: the minutes or hours when the service was unreachable or returning errors.

In practice, uptime matters because it's what users experience directly. A website with 99% monthly uptime sounds high, but that's over 7 hours of downtime — enough for customers to notice, transactions to fail, and trust to erode. Production SLAs (service level agreements) typically target 99.9% or higher, where every additional nine cuts the allowable downtime from hours to minutes to seconds.

How uptime is measured

Uptime monitoring works by sending periodic requests to a URL or endpoint and recording whether the response comes back successfully. A monitor might check every 60 seconds and expect a 200 HTTP status code. If the check fails — a timeout, a 500 error, or no response at all — the system marks the service as down and starts counting downtime. The uptime percentage is then calculated as the ratio of successful checks to total checks over the measurement period.

The check interval affects accuracy. A monitor checking every 5 minutes can miss a 3-minute outage entirely, while 1-minute or 30-second checks catch shorter incidents. Multi-region monitoring adds another dimension: a service might be reachable from Europe but down from the US, and single-location monitors won't detect regional outages.

How WatchCron tracks uptime

WatchCron's uptime monitoring checks your endpoints at regular intervals and alerts through email, Slack, Telegram, or any connected channel when a service goes down. Every HTTPS check automatically includes SSL certificate monitoring — expiry tracking and chain validation at no extra cost. A public status page shows real-time uptime data to your users.

Related terms: cron job, SLA, SLO, MTTR, health check