Uptime calculator

Type in an uptime percentage and see the allowed downtime, or type in the minutes and see the percentage.

Uptime target

Allowed downtime:

per day: 1m 26s
per week: 10m 5s
per month: 43m 50s
per year: 8h 45m

Downtime

min

Uptime that leaves:

in a day: 97.9167%
in a week: 99.7024%
in a month: 99.9316%
in a year: 99.9943%

The nines

Allowed downtime at each common target. A month here is a twelfth of a year, 30.44 days, which is how most SLAs count.

Uptime	Per day	Per week	Per month	Per year
90%	2h 24m	16h 48m	3d 1h 3m	36d 12h 36m
95%	1h 12m	8h 24m	1d 12h 31m	18d 6h 18m
99%	14m 24s	1h 40m	7h 18m	3d 15h 39m
99.5%	7m 12s	50m 24s	3h 39m	1d 19h 49m
99.9%	1m 26s	10m 5s	43m 50s	8h 45m
99.95%	43s	5m 2s	21m 55s	4h 22m
99.99%	9s	1m	4m 23s	52m 36s
99.995%	4s	30s	2m 11s	26m 18s
99.999%	1s	6s	26s	5m 16s

Reading the numbers

Two nines is about a quarter of an hour a day. Three nines, the figure most SLAs promise, is under nine hours a year and three quarters of an hour a month. Four nines is under an hour a year, too little for an outage that a person has to notice and fix. Five nines is five minutes a year, which needs automatic failover.

The check interval eats into the budget before anyone knows there is an outage. At a check every five minutes, an outage is found up to five minutes in; at 99.9% a month that is a ninth of the budget spent on noticing. A check a minute, with two failures in a row before it counts, finds the outage within two or three minutes. The uptime monitoring guide goes through the rest of the arithmetic.

The error budget on a page

A StatOSS page with an uptime target draws the month's error budget under the headline: how much downtime the target allows, how much has been spent, and how much is left, from the checks themselves rather than from a figure typed in. Maintenance windows are kept out of it, and the page docs explain how a partial outage counts.