Uptime SLA Calculator — What Does 99.9% Uptime Actually Allow?
SLA tiers are quoted in percentages that all sound nearly identical — 99%, 99.9%, 99.99% — but translate to wildly different amounts of real downtime, from hours per month to seconds. This tool converts between them, in both directions.
Allowed downtime at 99.9% uptime:
How this is calculated
Allowed downtime = (1 − uptime %) × period duration.
In reverse: uptime % = 1 − (downtime ÷ period duration).
Period durations: day = 1,440 minutes; week = 10,080; month = 43,200 (a 30-day approximation, the convention most SLA documents use); year = 525,600.
Uptime tiers at a glance
The common "nines" tiers and the downtime each one actually allows. Monthly figures use a 30-day month.
| Uptime | Downtime per day | Downtime per month | Downtime per year |
|---|---|---|---|
| 90% | 2 hours, 24 minutes | 3 days | 36 days, 12 hours |
| 95% | 1 hour, 12 minutes | 1 day, 12 hours | 18 days, 6 hours |
| 99%(two nines) | 14 minutes, 24 seconds | 7 hours, 12 minutes | 3 days, 16 hours |
| 99.5% | 7 minutes, 12 seconds | 3 hours, 36 minutes | 1 day, 20 hours |
| 99.9%(three nines) | 1 minute, 26 seconds | 43 minutes, 12 seconds | 8 hours, 46 minutes |
| 99.95%(three and a half nines) | 43 seconds | 21 minutes, 36 seconds | 4 hours, 23 minutes |
| 99.99%(four nines) | 9 seconds | 4 minutes, 19 seconds | 52 minutes, 34 seconds |
| 99.999%(five nines) | 1 second | 26 seconds | 5 minutes, 15 seconds |
Why these numbers matter
When you're negotiating an SLA with a client or choosing a hosting or monitoring provider, one extra digit hides a tenfold difference: 99.9% allows 43 minutes of downtime a month, 99.99% just over 4. The tier you commit to also sets the monitoring you need to verify it — 5-minute checks (Sentinel's Free plan) are fine for a 99.9% commitment, while the short outages a 99.99% SLA cares about only show up with checks every 30 seconds (Sentinel's Business plan).
Go deeper
An SLA is only as good as your ability to verify it
Sentinel tracks uptime for every site you manage and alerts you the moment one goes down, so the allowances above stay theoretical.
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