Uptime percentage: what does 99.9% mean?
Updated 29 October 2026 · Læs på dansk
Uptime is given as a percentage of time. The difference between 99% and 99.99% sounds small, but it is the difference between almost four days of downtime a year and less than an hour.
What does 99.9% uptime mean?
99.9% uptime means the service may be down at most 43 minutes and 12 seconds in a 30-day month, or about 8 hours and 46 minutes in a whole year. It is often called "three nines".
Table: allowed downtime
| Uptime | Per day | Per month (30 days) | Per year (365 days) |
|---|---|---|---|
| 99% | 14 min 24 s | 7 h 12 min | 3 d 15 h 36 min |
| 99.5% | 7 min 12 s | 3 h 36 min | 1 d 19 h 48 min |
| 99.9% | 1 min 26 s | 43 min 12 s | 8 h 45 min 36 s |
| 99.95% | 43 s | 21 min 36 s | 4 h 22 min 48 s |
| 99.99% | 9 s | 4 min 19 s | 52 min 34 s |
| 99.999% | 1 s | 26 s | 5 min 15 s |
The formula
Uptime = 1 − downtime ÷ monitored time. If a service was down for 30 minutes in a 30-day month (43,200 minutes), uptime is 1 − 30 ÷ 43,200 ≈ 99.93%. The other way round: allowed downtime = monitored time × (1 − uptime).
With and without planned maintenance
Many agreements do not count planned maintenance as downtime. Then the time in the maintenance window is deducted from both the downtime and the monitored time. That gives two figures: uptime in total and uptime without planned maintenance. Show both, so nobody can say something is hidden. Tegra Uptime calculates both (service windows are part of Pro).
How high should uptime be?
- 99% is too little for most business sites (over 7 hours of downtime a month).
- 99.9% is a common target for a website or web shop.
- 99.99% and above needs redundancy, several servers and often several data centres, and costs accordingly.
Remember that the number also depends on how you measure: how often, from how many places, and when a failure counts as downtime. See how to choose an uptime monitor and what uptime monitoring is.