The formula
downtime = period × (1 − availability)
Where it comes from
An SLA is stated as a percentage of availability, but what matters is the downtime it permits, and that figure is surprisingly concrete. 99.9 % over a year is eight and three quarter hours: a whole afternoon. 99.99 % is fifty-two minutes. 99.999 % — the famous five nines — is five minutes and fifteen seconds, and that includes updates. Each nine added divides the downtime by ten.
How to work it out by hand
- Subtract the availability from a hundred: that is the downtime percentage
- Convert the period to minutes
- Multiply the minutes by that percentage
- Spread it across months, weeks or days
What is worth knowing
Going from one nine to the next is not a tweak, it is a different architecture and a different budget. Three nines needs a well-administered server and a maintenance window. Four needs real redundancy and zero-downtime deploys. Five means replicating across regions, automating failover and accepting that no human intervention fits inside the window. It is also worth reading the small print: most commercial SLAs exclude scheduled maintenance from the count, measure by month rather than by year, and compensate with service credit rather than actual damages. An SLA is not a guarantee that it will work: it is an agreement about what happens when it does not.