SLA & Uptime Calculator
What an SLA allows in downtime
Pick a target or type one. A month here is 30.44 days, one twelfth of a year.
Composite SLA of a dependency chain
If your service is up only when every dependency is up, availabilities multiply.
Error budget
How much downtime your target allows, and how much of it you have already spent.
Turn nines into minutes, stack dependency SLAs, and watch your error budget.
The arithmetic behind the nines
Three rules cover most availability conversations.
Each nine costs ten times more
Going from 99.9% to 99.99% cuts allowed downtime from about 8.8 hours a year to 53 minutes. The engineering effort tends to rise as fast as the downtime falls.
Chains multiply, and only downward
A product that needs five dependencies, each at 99.9%, ships an effective 99.5%. Your availability is bounded by the product of everything you rely on.
Redundancy multiplies the other way
Two independent 99% instances give 99.99% together, because both must fail at once. The catch is independence: shared regions and pipelines fail together.
SLA calculator FAQs
What does 99.9% uptime actually mean?
It means the service may be down for at most 0.1% of the time: about 43.8 minutes in a month or 8 hours 46 minutes in a year. The calculator shows the allowed downtime for any target across a day, week, month, quarter and year.
How do I work out the SLA of several services combined?
When your product needs every dependency to be up at once, multiply their availabilities. Two services at 99.9% give 99.9% x 99.9% = 99.8%, which is nearly 9 hours more downtime per year than either service alone. Chains get worse fast, and the calculator does the multiplication for you.
Why is my real availability worse than my cloud provider's SLA?
Because your users experience the product, not a single component. Compute, database, DNS, CDN and third-party APIs each carry their own availability, and the serial combination is lower than any one of them. This is the main reason architects add redundancy for the weakest links.
What is an error budget?
The downtime your target still permits in the current period. A 99.9% monthly target gives a budget of roughly 44 minutes; if incidents have consumed 20, you have about 24 left. Teams following site reliability engineering practice use the remaining budget to decide when to ship risky changes and when to freeze.
Does redundancy improve availability?
Substantially, when failures are independent. Two independent instances at 99% availability fail together only 0.01% of the time, which is 99.99% availability. The catch is the word independent: shared dependencies, shared regions and shared deploy pipelines fail together.
Why do month figures differ slightly between calculators?
Some tools define a month as exactly 30 days, others as the calendar average. This calculator uses one twelfth of a 365.25 day year, which is 30.44 days. The difference is about one percent.