How much downtime per year is 99.9, 99.99 and 99.999 percent availability?
Show the answer
Say this first
A year has 525,600 minutes. 99.9 percent allows about 8.76 hours of downtime a year, 99.99 percent about 52.6 minutes and 99.999 percent about 5.26 minutes. Each extra nine divides the allowed downtime by ten. Per 30-day month that is about 43 minutes, 4.3 minutes and 26 seconds.
Go deeper
| Availability | Downtime per year | Per 30-day month |
|---|---|---|
| 99% | 3.65 days (87.6 h) | 7.2 h |
| 99.9% | 8.76 h | 43.2 min |
| 99.95% | 4.38 h | 21.6 min |
| 99.99% | 52.6 min | 4.32 min |
| 99.999% | 5.26 min | 25.9 s |
Doing it in your head
Unavailability is 1 minus availability. 99.99 percent means 0.01 percent down, which is 1 in 10,000. 525,600 divided by 10,000 is about 52.6 minutes. Remember "about 9 hours, 53 minutes, 5 minutes" for three, four and five nines.
What to ask when someone quotes a number
- Measured on what: a device, a link, or the service the user sees?
- Does planned maintenance count? Many SLAs exclude scheduled windows, which changes everything.
- Over what period? A monthly SLA can be met while a single bad month ruins the yearly figure.
They will ask next
An ISP offers 99.5 percent on a circuit. Is that good?
It allows about 43.8 hours of downtime a year, nearly two days. Fine for a backup link, weak for a sole link to a site that matters. Ask what the credit is when they miss it; credits rarely cover the business loss.
Traps
- Mixing up 52 minutes and 5 minutes. Practice the table until it is automatic.