Free Tool
Uptime SLA Calculator
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.
How it works
Three steps, no signup.
Everything runs in your browser. Nothing you type is sent to a server or stored.
Pick a direction
Convert an uptime percentage into allowed downtime, or convert a downtime duration back into an uptime percentage. Toggle between the two modes.
Enter a value
Type a percentage or tap a preset tier, or enter a downtime duration and the period it falls in. Everything runs in your browser.
Read the allowance
See exactly how much downtime each tier permits per day, week, month and year, so a 99.9% promise turns into real minutes.
Reference
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 it matters
One digit hides a tenfold gap.
When you are 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. Five-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
What does 99.9% uptime actually allow?
99.9% uptime allows about 43 minutes of downtime a month, or roughly 8 hours and 45 minutes a year. Each extra nine cuts that by a factor of ten: 99.99% allows just over 4 minutes a month.
What are the "nines"?
"Nines" is shorthand for the number of nines in an uptime figure. Two nines is 99%, three nines is 99.9%, four nines is 99.99%, and five nines is 99.999%. Each nine is roughly ten times less downtime than the last.
Is a month 30 or 31 days in these figures?
This calculator uses a 30-day month (43,200 minutes), the approximation most SLA documents quote. Daily and yearly figures use 1,440 and 525,600 minutes.
What monitoring do I need to hold an SLA?
The tier sets the check frequency you need to verify it. Five-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).
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.