99.5% uptime SLA
99.5% uptime allows 3 hours 39 minutes 8 seconds of downtime a month, 7 minutes 12 seconds a day and 1 day 19 hours 49 minutes 44 seconds a year, using a 365.2425-day year. Detecting an outage with 1-minute checks and one confirming retry takes up to 2 minutes, 0.9% of that monthly budget, before anyone starts fixing it.
How much downtime 99.5% allows, and what it takes to actually meet it.
Allowed downtime
| Period | Allowed downtime | Required uptime |
|---|---|---|
| Daily | 7m 12s | 23h 52m 48s |
| Weekly | 50m 24s | 6d 23h 9m 36s |
| Monthly | 3h 39m 8s | 30d 6h 49m 57s |
| Quarterly | 10h 57m 26s | 90d 20h 29m 51s |
| Yearly | 1d 19h 49m 44s | 363d 9h 59m 27s |
Year = 365.2425 days; month = year ÷ 12 (30.44 days); quarter = year ÷ 4. Rounded down to the second.
What 99.5% takes in practice
Detection eats into the budget
Monitoring can't alert before it has seen the failure. Worst-case detection is roughly the check interval times the number of failed checks it takes to confirm (confirmation avoids paging on a single network blip).
| Check interval | Failed checks to alert | Worst-case detection | Share of the monthly budget |
|---|---|---|---|
| 5 minutes | 1 | 5m | 2.3% |
| 5 minutes | 2 | 10m | 4.6% |
| 1 minute | 1 | 1m | 0.5% |
| 1 minute | 2 | 2m | 0.9% |
| 30 seconds | 1 | 30s | 0.2% |
| 30 seconds | 2 | 1m | 0.5% |
Dependencies multiply
If your service needs two components that are each 99.5% available (say, the app and its database), the best you can expect is 99.5% × 99.5% ≈ 99.0025%, which is 7h 17m 11s of downtime a month. Every hard dependency on the request path lowers the ceiling.
Other SLA levels
| SLA | Per month |
|---|---|
| 99% | 7h 18m 17s |
| 99.5% | 3h 39m 8s |
| 99.9% | 43m 49s |
| 99.95% | 21m 54s |
| 99.99% | 4m 22s |
| 99.999% | 26s |
Know your real uptime, not just the promised one
Vigil checks every minute from several locations and confirms a failure before alerting, so detection stays a small slice of your budget. Monthly SLA reports and a public status page come included.