SLA and uptime calculator

What you can do with SLA & Uptime Calculator

• Availability to downtime per day, week, month, quarter and year.
• Reference table for 99%, 99.5%, 99.9%, 99.95%, 99.99% and 99.999%.
• Downtime to availability, with the equivalent number of nines.
• Combined availability for components in series (all required) or in parallel (redundant).
• Instant results as you type, in your own language.

How to use it

1. Enter the availability in your SLA, for example 99.9.
2. Read the allowed downtime for the period your contract measures.
3. Use Downtime to availability to check how much a past outage consumed.
4. Add your service's components under Combined availability.
5. Choose serial or parallel to see whether the design can meet the SLA.

Serial systems lower availability

A web server at 99.95% that depends on a database at 99.9% gives the service about 99.85%, lower than either part. Every component that must be up multiplies in. This is why an SLA should be checked against the whole dependency chain, not a single server.

Redundancy raises it, if failures are independent

Two 99.9% firewalls in parallel give about 99.9999% in theory. That only holds if they do not share a power feed, a switch or a configuration error, so treat parallel results as an upper bound.

Limitations

Months are averaged at 30.44 days. Contracts may measure differently and exclude planned maintenance, so check the definitions in your agreement.

Frequently asked questions

How much downtime is 99.9% uptime?

About 43 minutes 50 seconds per month, or 8 hours 46 minutes per year.

How much downtime is 99.99%?

About 4 minutes 23 seconds per month, or 52 minutes 36 seconds per year.

How do I calculate availability for dependent systems?

Multiply the availabilities of components that are all required. For redundant components, subtract the product of their unavailabilities from one.

Does planned maintenance count?

It depends on your contract. Many SLAs exclude announced maintenance windows.