Web Hosting Uptime Guarantees Explained: 99.9% vs 99.95% vs 100% — and How Hosts Define It
Each percentage is a specific number of allowable minutes per month. Here is the math, plus what the major hosts actually publish in their SLAs and how they define 'downtime' in the small print.
What each "nine" allows
Uptime percentages translate to specific allowable downtime per month: 99% = 7 hours 18 minutes; 99.9% ("three nines") = 43 minutes; 99.95% = 22 minutes; 99.99% ("four nines") = 4.3 minutes; 99.999% = 26 seconds. The marketing difference between 99.9% and 99.99% is one decimal place; the practical difference is 38 minutes per month, every month, of permitted downtime.
What the major hosts actually publish
Kinsta publishes a 99.9% uptime guarantee in its SLA at kinsta.com/legal/sla, with a documented service-credit policy if the metric is missed. DreamHost publishes a 100% uptime guarantee at dreamhost.com/legal/100-uptime-guarantee/, with a credit policy specifying the compensation per minute of unscheduled downtime. SiteGround, Bluehost, and DreamHost shared tiers reference uptime in their Terms of Service rather than a dedicated SLA page; the credit policies are typically a percentage of the affected month's bill.
How hosts define "downtime" — read the SLA exclusions
The number on the marketing page is the floor; the SLA's exclusions are what define the actual obligation. Common exclusions across hosting SLAs: scheduled maintenance windows (sometimes excluded entirely from the calculation, which is how a host can deliver "99.9%" while having multi-hour planned outages), customer-caused outages, third-party network failures (DDoS, ISP issues), and force majeure events. Read the linked SLA before treating the headline number as a commitment. DreamHost's 100% guarantee is unusually broad — it explicitly does not exclude scheduled maintenance from the calculation, per the policy page.
How to verify uptime independently
Self-reported uptime dashboards are not independent — providers exclude their own scheduled maintenance and can be slow to count incidents. For independent verification, run a tool like UptimeRobot, Pingdom, or Better Stack against your own URL with a 1- or 5-minute check interval. They report observed availability across geographically distributed monitors. Cross-checking against a vendor's SLA dashboard is the only way to know whether the published number reflects reality.
When the SLA matters and when it does not
For a personal blog or portfolio site, the SLA is mostly a marketing artifact — hours of downtime per year is annoying but not financially material. For an e-commerce site or a SaaS, the SLA matters because downtime translates directly to lost revenue. A site that does $100/day in revenue and is offline for 3 hours has lost $12.50 of margin plus an SEO impact if Google encountered errors during a crawl. For commercial sites, the relevant metric is not the SLA percentage but the credit-policy formula and the historical uptime on third-party monitors.
Sources
Kinsta SLA: kinsta.com/legal/sla. DreamHost 100% uptime: dreamhost.com/legal/100-uptime-guarantee. SiteGround terms: siteground.com/terms.htm. Bluehost terms: bluehost.com/terms-of-service. WordPress.org Hosting recommendations: wordpress.org/hosting. All URLs accessed 2026-04-30.