Availability Numbers Every Programmer Should Know

Availability refers to the amount of continuous uptime of a service. This is measured in a percentage, where 100% means a service has zero downtime and 0% means the service is never operational.

A service level agreement (SLA) is a commitment from a provider to a customer. When SLAs are mentioned for availability, providers will be stating their commitment to the uptime of the system, after which you are usually entitled to some form of compensation. Most services aim to be 99% available or higher and large cloud providers set their SLAs at 99.9% or above. Providers often refer to the number of 9s in the overall percentage as the SLA. For example, an SLA of 99.99% is referred to as 4 9s of availability. Usually, periods of expected downtime (i.e., system upgrades) do not apply to an availability SLA, but it varies per service - be sure to read their documentation.

High availability SLAs

High availability (HA) systems typically guarantee 99.99% or greater of availability. The easiest way to remember a 99.99% SLA (4 9s) is to remember that you have roughly 4 minutes of downtime each month. 4 9s, minutes of downtime per month is the mantra.

Once you have this memorized, you can quickly memorize all the other monthly availability numbers. For example in 99.9%, since you are less available, you gain a power of ten so simply shift the decimal to be 40 minutes rather than 4 minutes. For 99.999% you are more available so you go from 4 minutes to about 40 seconds. For 99.9999% you get about 4 seconds of monthly downtime.

Below is a brief table which lists the daily, weekly, monthly, and yearly acceptable downtime to meet a particular availability SLA.

Roughly acceptable downtime to meet SLA (easier to remember)

Availability SLA	Daily	Weekly	Monthly	Yearly
95%	1h	8h	1d	18d
99%	15m	2h	7h	3d
99.9%	1m	10m	40m	9h
99.99%	10s	1m 0.48s	4m	50m
99.999%	1s	6s	40s	5m
99.9999%	0.1s	0.6s	4s	30s

Exact acceptable downtime to meet SLA

Availability SLA	Daily	Weekly	Monthly	Yearly
95%	1h 12s	8h 24m	1d 12h 13m 27s	18d 2h 41m 28s
99%	14m 24s	1h 40m 48s	7h 14m 41s	3d 14h 56m 18s
99.9%	1m 26s	10m 4.8s	43m 28s	8h 41m 38s
99.99%	8.6s	1m 0.48s	4m 21s	52m 9.8s
99.999%	0.86s	6s	26s	5m 13s
99.9999%	0.086s	0.6s	2.6s	31s

Thanks for reading this far. If you want to calculate other availability numbers that are not in this table, check out https://uptime.is/ for a calculator.

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