Uptime / SLA Calculator
An availability target like "99.9%" is easy to say and hard to picture. Enter one below to see exactly how much downtime it allows over a day, a week, a month and a year.
How each extra "nine" shrinks the allowance
Allowed downtime per year at common targets, drawn on a logarithmic scale — each step down is about ten times less downtime. Your target is highlighted.
Work it backwards
Measured some downtime and want the uptime percentage it produced?
The "nines" at a glance
| Uptime | Name | Downtime / month | Downtime / year |
|---|---|---|---|
| 99% | two nines | 7h 12m | 3d 15h 36m |
| 99.5% | — | 3h 36m | 1d 19h 48m |
| 99.9% | three nines | 43m 12s | 8h 45m 36s |
| 99.95% | — | 21m 36s | 4h 22m 48s |
| 99.99% | four nines | 4m 19s | 52m 34s |
| 99.999% | five nines | 26s | 5m 15s |
Why each extra nine is so expensive
Every additional nine cuts your downtime budget by about 90%. Going from 99.9% to 99.99% means the same yearly outage allowance drops from nearly nine hours to under an hour. In practice that requires redundancy across failure domains, automated failover, tighter deploy processes, on-call coverage and constant testing — each of which adds cost and complexity. Most products do not need more than three nines; picking a target higher than your users require mostly buys you stress.
SLA, SLO and actual uptime
- SLA — the promise in the contract, often with refunds or credits if missed.
- SLO — the internal goal the team works to, usually set stricter than the SLA to keep a buffer.
- Actual uptime — what you measured. The gap between this and your SLO is your early-warning signal.
Frequently asked questions
How much downtime does 99.9% uptime allow?
About 8 hours 45 minutes per year, or roughly 43 minutes 12 seconds per 30-day month. "Three nines" sounds strict but still leaves close to a full working day of outage over a year.
What is the difference between 99.9%, 99.99% and 99.999%?
Each extra nine cuts allowed downtime by about 10×. 99.9% allows ~8.8 hours a year, 99.99% ("four nines") about 52 minutes, and 99.999% ("five nines") about 5 minutes 15 seconds. The cost of infrastructure and process to reach each nine also rises sharply.
Does scheduled maintenance count against an SLA?
It depends on the contract. Many SLAs exclude announced maintenance windows from the downtime calculation, so read the definition of "downtime" and "excluded events" carefully. This calculator shows raw allowed downtime; adjust for exclusions your agreement allows.
What month length does this calculator use?
It uses fixed periods: 30 days for a month, 90 days for a quarter and 365 days for a year, which is the common convention in SLA documents. A calendar month with 31 days would allow slightly more downtime.
What is the difference between an SLA, an SLO and actual uptime?
The SLA is the contractual promise (often with penalties). The SLO is the internal target a team aims for, usually stricter than the SLA. Actual uptime is what you measured. Teams typically set the SLO tighter than the SLA so they have a safety margin.