Glossary · Reliability metrics
Nines of availability
The nines describe an availability target by its number of nines: two nines is 99%, three nines 99.9%, four nines 99.99% and five nines 99.999%. Each extra nine allows ten times less downtime.
Percentages close to 100 look alike, but the downtime they allow does not. 99.9% allows 43 minutes and 49 seconds in an average month; 99.999% allows 26 seconds, less than it takes most teams to read an alert.
Higher targets need different engineering, not just more effort: redundancy across machines or regions, deploys without downtime, and detection measured in seconds. A service also can’t be more available than what it depends on: two dependencies at 99.9% each, both needed for every request, leave at most about 99.8%.
The table below uses the same definitions as the SLA and uptime calculator: a year is 365.25 days and a month a year ÷ 12 (30.44 days). Allowed downtime is rounded down to the second.
| Uptime | Per day | Per month | Per year |
|---|---|---|---|
| 99% Two nines | 14m 24s | 7h 18m 18s | 3d 15h 39m 36s |
| 99.5% Two and a half nines | 7m 12s | 3h 39m 9s | 1d 19h 49m 48s |
| 99.9% Three nines | 1m 26s | 43m 49s | 8h 45m 57s |
| 99.95% Three and a half nines | 43s | 21m 54s | 4h 22m 58s |
| 99.99% Four nines | 8s | 4m 22s | 52m 35s |
| 99.999% Five nines | 864ms | 26s | 5m 15s |
Uptimecheck shows uptime with three decimals close to 100%, so 99.95% and 99.99% never both round to 100%. Reports mark the monitors below an SLA target you pick: 99%, 99.5%, 99.9%, 99.95% or 99.99%.
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Guides 6 min read
99.9% uptime allows 43 minutes and 49 seconds of downtime in an average month. What each level of nines allows per day, month and year, how SLAs measure uptime, how service credits work and how to choose a target.
ReadGuides 5 min read
The uptime formula, worked examples with incidents and maintenance, how to combine the uptime of several components, and how uptime, availability and reliability differ.
Read99.999% allows 5 minutes and 15 seconds a year, or 26 seconds in an average month.
Not as a promise. Hardware fails, networks drop packets and every change carries risk. A service can have a month without measured downtime, but SLAs stop at a number of nines because perfect availability can’t be guaranteed.
No. Five nines allows 26 seconds a month, less than the time between two checks. Checks every minute suit targets from 99.9% to 99.99%; proving more needs far more frequent measurements or data from real traffic.
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