Calculate allowed downtime for any SLA uptime percentage
Use this free SLA calculator to convert uptime percentages to allowed downtime per day, week, month, and year. Understand what "three nines" or "five nines" really means for your service. Essential for SREs and DevOps engineers. 100% client-side — your data never leaves your browser.
| Uptime % | Name | Downtime/Day | Downtime/Month | Downtime/Year |
|---|---|---|---|---|
99% |
Two nines | 14m 24s | 7h 18m | 3d 15h 36m |
99.5% |
Two and a half nines | 7m 12s | 3h 39m | 1d 19h 48m |
99.9% |
Three nines | 1m 26s | 43m 50s | 8h 45m 36s |
99.95% |
Three and a half nines | 43s | 21m 55s | 4h 22m 48s |
99.99% |
Four nines | 8.6s | 4m 23s | 52m 34s |
99.999% |
Five nines | 0.86s | 26s | 5m 15s |
Click any row to calculate its downtime
SLA (Service Level Agreement) uptime is a contractual promise about how available a service will be. It's expressed as a percentage of total time. For example, 99.9% uptime means the service is expected to be available 99.9% of the time, allowing for 0.1% downtime.
The remaining percentage (100% - uptime%) is your error budget — the maximum amount of downtime you can have before breaching your SLA.
In the industry, we refer to uptime levels by counting the number of 9s:
Each additional nine is exponentially harder to achieve and typically costs 10x more in infrastructure and engineering effort.
Error budgets help teams balance reliability with feature velocity:
This creates a data-driven way to decide when to prioritize stability over new features.
99.9% uptime (three nines) allows approximately 8 hours and 45 minutes of downtime per year, or about 43 minutes per month. This is a common SLA target for web services, APIs, and SaaS products.
That extra nine makes a huge difference. 99.9% allows 8.76 hours of downtime per year, while 99.99% allows only 52 minutes. Achieving four nines typically requires redundant infrastructure, automated failover, and 24/7 on-call support.
An error budget is the maximum allowed downtime for a given period based on your SLO. With a 99.9% monthly SLO, your error budget is 43 minutes. If you use it all on incidents, you should pause risky deployments until the budget resets.
No. 100% uptime is practically impossible. Even the most reliable services (AWS, Google Cloud) experience some downtime. This is why SLAs always specify a percentage below 100% and include compensation terms for breaches.
Start with your current performance, consider your dependencies (you can't exceed their SLAs), and factor in cost. Going from three nines to four nines typically requires 10x the infrastructure investment. Match your SLA to your customers' actual needs.
SLI is what you measure (e.g., uptime percentage). SLO is your internal target (e.g., aim for 99.9%). SLA is the contractual promise to customers with penalties (e.g., refund if below 99.5%). Learn more →