99.9% Uptime: How Much Downtime Does Three Nines Allow?
99.9% uptime — 'three nines' — allows 43 minutes and 50 seconds of downtime per 30-day month, 10 minutes 5 seconds per week, and 8 hours 46 minutes per year. It is the most common commercial SLA target because a well-run single-region deployment with health checks and fast rollbacks can hit it without multi-region infrastructure.
Per day
1.4 min
Per week
10.1 min
Per month (30d)
43.2 min
Per quarter
2.2 h
Per year
8.8 h
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Is 99.9% achievable for your team?
Usually yes, with three things in place: automated deployments that can roll back in under five minutes, health checks that remove a bad instance before customers notice, and an on-call rotation that actually responds. What breaks a 99.9% target is not exotic failure — it is a slow manual rollback, a database migration without a tested reverse, or an expired certificate nobody was alerted about. Each of those costs 30 minutes or more, which is most of the monthly budget in a single event.
Spending the 43-minute budget
Treat the allowance as an error budget. Every deployment, migration and experiment draws it down. While budget remains, ship; when it is gone, freeze feature work and spend the time on reliability. Set your internal SLO at 99.95% while committing to 99.9% externally, so you get a warning before you owe service credits.
Watch the dependency chain
Availability multiplies in series. If your service is 99.9% but depends on a payment provider at 99.9% and an identity provider at 99.9%, the customer-visible availability is 99.7% — 2 hours 11 minutes a month. Either make critical dependencies redundant, or degrade gracefully when they fail so their outage is not automatically yours.
99.9% uptime allowed downtime
| Period | Allowed downtime |
|---|---|
| Per day | 1m 26s |
| Per week | 10m 5s |
| Per 30-day month | 43m 50s |
| Per quarter | 2h 11m 30s |
| Per year | 8h 45m 57s |
How 99.9% compares
| Availability | Downtime per month | Downtime per year |
|---|---|---|
| 99% | 7h 12m | 3d 15h 36m |
| 99.9% | 43m 50s | 8h 45m 57s |
| 99.95% | 21m 55s | 4h 22m 58s |
| 99.99% | 4m 23s | 52m 36s |
| 99.999% | 26s | 5m 15s |