Skip to content
mathbehind

Uptime and SLA Downtime Calculator

Turn an uptime percentage like 99.9% into the downtime it allows per year, month, week and day.

Your numbers

%

For example 99.9 (three nines) or 99.99 (four nines).

Allowed downtime per year

8h 45m 36s

Per month
43m 48s
Per week
10m 5s
Per day
1m 26s
Minutes per year
525.6
The math behind it

  1. Allowed downtime share

    100% − 99.9%equals0.1%

  2. Per year

    0.1% × 365 d × 24 hequals8h 45m 36s

  3. Per day

    0.1% × 24 hequals1m 26s

Uptime targets are written as percentages, but what you actually need to know is how much downtime they allow. The difference between 99.9% and 99.99% sounds tiny, yet it is the difference between about 8 hours and about 53 minutes of downtime a year.

How it works

Subtract the uptime percentage from 100% to get the share of time you are allowed to be down. Multiply that by the length of the period.

The year is 365 days. A month is one twelfth of a year, about 30.4 days, and a week is 7 days. Some SLAs define a month as the actual calendar month, so check your contract if the exact minutes matter.

Each extra nine cuts allowed downtime by a factor of ten. 99% allows about 3.65 days a year, 99.9% about 8.8 hours, 99.99% about 53 minutes, and 99.999% about 5 minutes.

A worked example

A 99.9% uptime target allows 0.1% downtime: 8h 45m 36s over a year, 43m 48s in a month, 10m 5s in a week, or about 1m 26s a day.

Questions people ask

Does planned maintenance count as downtime?

It depends on the agreement. Many SLAs exclude scheduled maintenance announced in advance, which is why the fine print matters as much as the percentage.

How do dependencies affect my uptime?

If your service needs two systems that each have 99.9% uptime, and either one failing takes you down, your combined uptime is about 99.8% (0.999 × 0.999). Every hard dependency lowers the uptime you can realistically promise.

What is the difference between an SLA, SLO and SLI?

An SLI is the measurement, such as the share of successful requests. An SLO is your internal target for it. An SLA is the promise in a contract, usually with refunds or credits if you miss it, and is normally set a little looser than the SLO.

Guides