SLA Uptime & Downtime Calculator
How much downtime does 99.9% really allow? Convert any SLA target into allowed downtime per day, week, month, and year — or work backwards from outage minutes to the uptime you actually achieved.
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Common SLA downtime reference
The allowable downtime for the SLA tiers teams target most often. A 30.44-day average month is used for the monthly column.
| SLA | Nines | Per day | Per month | Per year |
|---|---|---|---|---|
| 99% | Two nines | 14m 24s | 7h 18m | 3d 15h 36m |
| 99.5% | — | 7m 12s | 3h 39m | 1d 19h 48m |
| 99.9% | Three nines | 1m 26s | 43m 50s | 8h 45m 57s |
| 99.95% | — | 43s | 21m 55s | 4h 22m 58s |
| 99.99% | Four nines | 8.6s | 4m 23s | 52m 35s |
| 99.999% | Five nines | 0.9s | 26s | 5m 15s |
How the SLA calculation works
An availability SLA is just a percentage of a time window. To find the allowed downtime, take the fraction of time you are allowed to be down — 1 − (SLA ÷ 100) — and multiply it by the length of the period. For a 99.9% monthly SLA that is 0.001 × 30.44 days, which works out to about 43 minutes and 50 seconds. Each additional nine divides the allowable downtime by ten, which is why moving from three nines to four nines is such a large operational jump.
Knowing your budget is half the battle — the other half is catching it
A four-nines SLA leaves you just ~4 minutes of downtime per month. You cannot defend a budget that small with manual checks — by the time someone notices, you have already blown it. That is what uptime monitoring and instant alerting are for: detect the outage in seconds, page the on-call engineer, and start the clock on your response instead of your customers' patience. Pair it with an error budget calculator to turn the same number into a release-velocity decision.
Frequently asked questions
How much downtime does a 99.9% SLA allow?
A 99.9% (three-nines) SLA allows roughly 43 minutes and 50 seconds of downtime per month, about 1 minute 26 seconds per day, and 8 hours 45 minutes per year. Use the calculator above to convert any target percentage into allowed downtime per day, week, month, quarter, and year.
What is the difference between 99.9% and 99.99% uptime?
99.99% (four nines) allows only about 4 minutes 23 seconds of downtime per month, versus roughly 43 minutes 50 seconds for 99.9% (three nines). Adding each additional nine cuts your allowable downtime by 10x, which usually requires redundancy, automated failover, and faster incident response.
How do you calculate uptime percentage from downtime?
Uptime % = (total time − downtime) / total time × 100. For example, 43 minutes of downtime in a 30-day month is (43,200 − 43) / 43,200 × 100 = 99.90%. Switch the calculator to 'Downtime → achieved %' mode to do this automatically.
Does this SLA calculator send my data anywhere?
No. The calculator runs entirely in your browser using JavaScript. No SLA targets, downtime figures, or any other input is ever transmitted to a server.
Defend your SLA automatically
Alert24 monitors uptime from multiple regions, opens incidents, updates your status page, and pages on-call — so a tight SLA is something you defend, not something you discover you missed.