HostedSiteDemo99.9% vs 99.99% Uptime: What SLAs Really Mean
Each extra nine cuts allowed downtime by ten. 99.9% uptime permits about 8 hours 46 minutes of downtime a year; 99.99% permits about 53 minutes; 99.999% about 5 minutes. An SLA is the host’s promise to hit that figure, usually backed only by a small credit on your bill if it does not.
The nines in real time
| Uptime | Name | Downtime per year | Per month |
|---|---|---|---|
| 99% | Two nines | 3 days 15 hours | 7 hours 18 min |
| 99.9% | Three nines | 8 hours 46 min | 43 min 48 s |
| 99.95% | Three and a half | 4 hours 23 min | 21 min 54 s |
| 99.99% | Four nines | 52 min 34 s | 4 min 23 s |
| 99.999% | Five nines | 5 min 15 s | 26 s |
Work out any other figure with the uptime calculator.
What an SLA actually promises
A service level agreement sets a target and a remedy. The remedy is almost always a service credit: a percentage of that month’s fee, rising as uptime falls further below target. For a $10-a-month plan, a bad month might earn you a dollar or two back, and usually only if you claim it within a set period. It will not cover lost sales.
Read the small print for:
- Exclusions: scheduled maintenance, attacks, problems caused by your own code, and outages of third parties are often not counted.
- How downtime is measured: by the host’s own monitoring, often in 5-minute blocks, from inside their network.
- What is covered: the network may be covered but not the server, or the server but not your database.
Dependencies multiply
Your site is only up when everything it relies on is up. If your host is at 99.9% and your database service is at 99.9%, the combined uptime is 99.9% × 99.9% ≈ 99.8%, about twice the downtime. Add DNS, a payment provider and an API, and it drops further.
How to improve your own uptime
- Put a CDN in front of the site; many can keep serving cached pages while your server is down.
- Monitor from outside, with alerts, so you hear about problems before customers do.
- Keep automatic, tested backups somewhere other than your host.
- For critical sites, run in more than one region or with a hot standby.
Frequently asked questions
Does scheduled maintenance count against uptime?
Often not. Many hosting SLAs exclude announced maintenance windows from the calculation, so a host can be offline for several planned hours and still meet 99.9%. Read the exclusions section of the SLA.
How do I monitor my site’s uptime?
Use an external monitoring service that checks your site every minute or few minutes from several locations and alerts you by email or SMS. Many have free plans. Monitoring from outside your host is the only way to see what visitors see.
Is 100% uptime possible?
Not in practice. Any single server, network or data centre eventually fails. High availability is achieved by running copies in several places and switching between them, which is how large services reach 99.99% or better.