Free tool
Uptime percentage calculator
Enter an uptime percentage to see how much downtime it actually allows. The full reference table is below, from 99% to five nines.
Allowed downtime at this level:
Uptime SLA table
Allowed downtime at each of the levels people actually write into contracts.
| Uptime | Per day | Per week | Per month (30d) | Per year (365d) |
|---|---|---|---|---|
| 99% | 14m 24s | 1h 40m 48s | 7h 12m | 3d 15h 36m |
| 99.5% | 7m 12s | 50m 24s | 3h 36m | 1d 19h 48m |
| 99.9% | 1m 26s | 10m 5s | 43m 12s | 8h 45m 36s |
| 99.95% | 43s | 5m 2s | 21m 36s | 4h 22m 48s |
| 99.99% | 9s | 1m | 4m 19s | 52m 34s |
| 99.999% | 864ms | 6s | 26s | 5m 15s |
How it is calculated
Allowed downtime is the share of the period you are permitted to be unavailable:
downtime = (100 - uptime%) / 100 x period length 99.9% over 30 days = 0.001 x 2,592,000s = 2,592s = 43m 12s
A month here is 30 days and a year is 365, which is the basis published SLA tables use. Your own contract may be written on calendar months, in which case a 31-day month allows about 3% more downtime at the same percentage. It is worth checking which one you are being held to.
Why the percentage is the easy part
Two providers quoting the same number can mean quite different things. Most SLAs exclude announced maintenance windows, so a provider can report 100% for a month you remember being offline. And the percentage says nothing about how the downtime arrives: 43 minutes as one outage during business hours is a different event from 43 minutes spread over ninety seconds at a time. Whatever you are promising, you need to be measuring it at an interval fine enough to notice.
Common questions
How much downtime does 99.9% uptime allow?
43 minutes 12 seconds per 30-day month, or 8 hours 45 minutes 36 seconds per year. Per week it is 10 minutes 5 seconds.
How is allowed downtime calculated?
Downtime = (100 - uptime percentage) / 100, multiplied by the length of the period. For 99.9% over a 30-day month: 0.001 x 2,592,000 seconds = 2,592 seconds, or 43m 12s.
What is the difference between 99.9% and 99.99%?
A factor of ten. 99.9% allows 43m 12s of downtime a month; 99.99% allows 4m 19s. In practice that is the difference between a deploy you can roll back by hand and one that has to fail over automatically.
Is a month 30 days or 31?
This calculator uses 30 days, which is the convention published SLA tables use, and 365 days for a year. A 31-day month allows about 3% more downtime at the same percentage, so check which basis your own SLA is written on.
Does downtime include planned maintenance?
That depends on the contract. Most vendor SLAs exclude scheduled maintenance windows announced in advance, which is why a provider can report 100% uptime in a month you remember being offline. Read the exclusions, not just the percentage.