Uptime & SLA Calculator
Enter your SLA uptime percentage and this calculator instantly shows the allowed downtime across five reporting periods, from daily through yearly. Switch to business-hours mode to calculate only against your operational window, or use composite mode to see the effective SLA when two services depend on each other. The reverse mode works backwards: enter a downtime budget in minutes and get the matching uptime percentage.
What is uptime and why does it matter?
Uptime is the percentage of time a service is available and functioning correctly. It is the core metric in a Service Level Agreement (SLA), the contract between a provider and a customer that defines the minimum acceptable reliability. When a service falls below its committed uptime, penalties such as service credits or refunds typically apply. For a business that depends on a third-party API, cloud database, or CDN, understanding the uptime percentage and its corresponding downtime allowance is essential for risk planning and architecture decisions. A difference of just one decimal place - say, 99.9% versus 99.99% - cuts the allowed monthly downtime from about 44 minutes to under 5 minutes.
How uptime percentage is converted to downtime
The formula is straightforward: allowed downtime = period length x (1 - uptime / 100). For a 30-day month the period length is 2,592,000 seconds, so at 99.9% uptime the allowed downtime is 2,592,000 x 0.001 = 2,592 seconds, or about 43 minutes 12 seconds. The same ratio applied to a 365-day year gives 31,536 seconds, or roughly 8 hours 45 minutes. "N nines" is a shorthand: three nines means 99.9%, four nines means 99.99%, five nines means 99.999%, and so on. Each additional nine reduces the allowed downtime by roughly a factor of ten.
Composite SLA: what happens when services depend on each other?
Modern applications rarely depend on a single service. If your app needs both a database (SLA: 99.9%) and a CDN (SLA: 99.95%) to function, the effective uptime is the product of the two probabilities: 0.999 x 0.9995 = 0.9985, which is 99.85%. This is always lower than either individual SLA. The more dependencies your system has, the more aggressively the composite SLA degrades. For three services each at 99.9%, the combined SLA is 0.999^3 = 99.7%. This is why high-availability architectures add redundancy at each layer rather than relying on a chain of individually reliable services.
Business-hours SLA vs. 24/7 SLA
Some SLAs apply only during business hours, for example 8 hours a day across 5 working days. In that case the measurement window is narrower, so the absolute downtime allowance is smaller even for the same percentage. An 8x5 business-hours window is 2,080 hours per year versus 8,760 hours for continuous operation. A 99.9% uptime SLA over an 8x5 window allows about 2.08 hours of downtime per year, compared to 8.75 hours under a 24/7 SLA. When reviewing a vendor SLA, always check whether the percentage applies to calendar time or to a defined operational window, since the difference is substantial.
Common SLA tiers and their allowed downtime
| Nines | Uptime % | Downtime / month | Downtime / year | Typical use case |
|---|---|---|---|---|
| Two nines | 99% | 7h 18m | 3d 15h 39m | Internal tooling, dev/test environments |
| Two and a half nines | 99.5% | 3h 39m | 1d 19h 49m | Non-critical business apps |
| Three nines | 99.9% | 43m 12s | 8h 45m 36s | Standard SaaS, cloud VMs |
| Three and a half nines | 99.95% | 21m 36s | 4h 22m 48s | Premium SaaS tiers |
| Four nines | 99.99% | 4m 19s | 52m 33s | Enterprise cloud, payment gateways |
| Four and a half nines | 99.995% | 2m 9s | 26m 17s | High-availability financial systems |
| Five nines | 99.999% | 25.9s | 5m 15s | Carrier-grade, life-critical systems |
Based on a 30-day month (2,592,000 seconds) and a 365-day year (31,536,000 seconds).
Frequently asked questions
What does "three nines" of uptime mean?
99.9% uptime, also called "three nines" because three 9s appear after the decimal point if you write it as 99.9%. At this level, a 30-day month allows roughly 43 minutes and 12 seconds of downtime, and a full year allows about 8 hours and 45 minutes. Three nines is the most common SLA tier for standard SaaS products and cloud virtual machines.
How do I calculate allowed downtime from an uptime percentage?
Multiply the period length in seconds by (1 minus the uptime fraction). For a 30-day month: 2,592,000 x (1 - uptime/100). For 99.9% that is 2,592,000 x 0.001 = 2,592 seconds, or about 43 minutes. For a 365-day year use 31,536,000 seconds instead. This calculator does all of this instantly for any percentage you enter.
What is a composite SLA?
When your service depends on two or more external services, the effective uptime is the product of their individual uptime fractions. If service A has 99.9% uptime and service B has 99.95% uptime, the combined SLA is 0.999 x 0.9995 x 100 = 99.85%. Each additional dependency reduces the composite SLA, which is why architects design for redundancy at each layer rather than assuming individual service SLAs add up.
What is the difference between uptime, availability, and SLA?
Uptime is the raw percentage of time a service is operational. Availability is a broader concept that includes uptime but can also factor in performance degradation and partial outages. An SLA is the formal contract that commits a vendor to a minimum uptime or availability level and specifies remedies such as service credits if the commitment is not met. In practice the three terms are often used interchangeably when discussing reliability targets.
How many hours of downtime does 99.99% uptime allow per year?
99.99% uptime allows 31,536,000 x 0.0001 = 3,153.6 seconds per year, which is approximately 52 minutes and 33 seconds. By contrast, 99.9% allows about 8 hours 45 minutes and 99.999% allows only about 5 minutes 15 seconds per year. Each additional nine reduces annual allowed downtime by roughly a factor of ten.
How does a business-hours SLA differ from a 24/7 SLA?
A business-hours SLA counts downtime only during a defined operational window, for example 9am-5pm Monday through Friday. Because the measurement window is shorter, the absolute downtime allowance is smaller. A 99.9% business-hours SLA over a standard 8x5 week (about 2,080 hours per year) allows around 2.08 hours of downtime per year, rather than the 8.75 hours allowed under a 24/7 99.9% SLA. Always check whether a vendor SLA is 24/7 or limited to business hours.