Uptime / SLA Calculator
Turn an uptime percentage into the downtime it actually permits.
What you provide
Result
99.9% allows 43.2 min of downtime a month
At 99.9% you may be down 8.76 h in a year and still meet the target. Two things decide whether that number means anything: the window it is measured over — a monthly target resets the budget twelve times a year, an annual one does not — and what counts as downtime, since most agreements exclude scheduled maintenance, which is where the difference between a stated and an experienced availability usually lives.
- Per day
- 1.4 min
- Per week
- 10.1 min
- Per month (30 days)
- 43.2 min
- Per quarter
- 2.19 h
- Per year
- 8.76 h
- Commonly called
- three nines
What this cannot tell you
- Calculated in your browser. Nothing you enter is uploaded or logged.
- Uses a 30-day month and a 365-day year, which is the convention almost every agreement uses. A calendar-month agreement gives a slightly different budget in February and in a 31-day month.
- Says nothing about what a specific agreement counts as downtime. Scheduled maintenance is excluded from most, which is where the gap between a stated and an experienced availability usually lives.
- Service credits are not compensation for the loss. They are typically a small percentage of the monthly fee, and they have to be claimed.
Take this with you
How this calculation works
The arithmetic is simple: the allowed downtime is the complement of the percentage applied to the period. What that number means depends on two things the percentage does not say. First, the window it is measured over — a monthly target resets the budget twelve times a year, so a single bad day is absorbed, while an annual target means one long outage can consume the year. Second, what counts: most agreements exclude scheduled maintenance, and some exclude anything caused by a third-party network, which can be most of the internet.
What the results mean
- Three nines (99.9%)
- About 43 minutes a month. Achievable with a single well-run system and some luck.
- Four nines (99.99%)
- About 4 minutes a month — less than one unplanned restart. Needs redundancy and automated failover, not effort.
- Five nines (99.999%)
- About 26 seconds a month. Beyond the point where a human can be involved in recovery at all.
- The measurement window
- Whether the budget resets monthly or annually changes what the same percentage permits in practice more than a decimal place does.
Common problems and fixes
- We met the SLA but customers were angry
- Availability measured as a percentage of time hides when the downtime happened. Ten minutes at 3am and ten minutes at midday on a Monday score identically and are not remotely the same.
- The provider says they met it and we disagree
- Read the exclusions and the measurement method. Uptime measured from inside the provider's network, with scheduled maintenance excluded, can be genuinely higher than what you experienced.
- We need four nines
- Check the arithmetic first: four minutes a month means every failure has to be detected and recovered automatically. If a person has to be paged, the number is not reachable.
Frequently asked questions
How much downtime does 99.9% allow?
About 43 minutes and 12 seconds in a 30-day month, or roughly 8 hours 45 minutes across a year. Enough for a couple of unplanned restarts a month, and not enough for one bad afternoon.
What is the difference between 99.9% and 99.99%?
One decimal place and an order of magnitude in engineering. Three nines allows about 43 minutes a month; four nines allows about 4. The first can be met by a careful team with a single system; the second requires automatic failover, because no human process detects and fixes anything within four minutes.
Do SLA credits actually compensate for an outage?
Rarely. Credits are usually a percentage of the monthly fee for the affected service, capped, and they have to be claimed within a window. For most businesses the cost of an outage exceeds the entire monthly bill, so the credit is a signal rather than a remedy.
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<iframe src="https://runthetests.com/embed/sla-uptime-calculator/" width="100%" height="560" style="border:1px solid #e5e5e5;border-radius:8px" title="Uptime / SLA Calculator" loading="lazy"></iframe>
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