Uptime is the percentage of time your website is online and working. '99.9% uptime' means your site is almost always available - and the small gap is the downtime that costs you customers.
Also known as: Availability, downtime, site uptime
In plain English
Uptime measures how often your website is actually reachable. If your host promises 99.9% uptime, that allows about 8.7 hours of downtime per year. 99.99% allows about 52 minutes. The difference between those numbers is the difference between "rarely down" and "down for a full workday once a year."
Why it matters for your business
Every minute your site is down is a minute a customer can't reach you - and many just won't come back. For a store or a service business, downtime during business hours is lost revenue, and repeated outages damage trust and search rankings.
When you'll run into it
When comparing hosting plans, after a crash or outage, or when setting service-level expectations with a host or developer.
Run into this on your site?
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