Uptime SLA Calculator
Each nine divides the allowance by ten.
Work out Uptime SLA. Each nine divides the allowance by ten. Names the misconception directly.
Permitted downtime a year
8.77 hours
99.9% · Three nines
Each additional nine divides permitted downtime by ten, which is why the step from 99.9% to 99.99% is a different engineering problem rather than a slightly harder version of the same one. At three nines a single 40-minute incident consumes most of a month's budget. At four, a single incident of any length that requires a human to wake up and log in has already blown it — which is why four nines and above demand automated failover rather than a faster on-call rota. Note also that a monthly SLA and an annual one are different promises: monthly resets forgive a bad month in a way an annual budget does not.
How the Uptime SLA Calculator works
Permitted downtime per day, week, month, quarter and year for any availability target. Each additional nine divides the allowance by ten, which is why the step from three nines to four is a different engineering problem rather than a harder version of the same one.
Also known as: uptime · sla · availability · downtime allowance · nines · three nines · four nines · five nines · 99.9 uptime · how many minutes is 99.9 · downtime per month · sla credits
What a nine actually buys
99% permits 3.65 days of downtime a year. 99.9% permits 8 hours 46 minutes. 99.99% permits 52 minutes, and 99.999% permits 5 minutes 15 seconds.
Each nine divides the allowance by ten, which is what makes the steps discontinuous rather than incremental. At three nines a single 40-minute incident consumes most of a month's budget. At four, any incident requiring a human to wake up, log in and diagnose has already exceeded the whole year.
So four nines and above is not a harder version of three — it is a different architecture. It demands automated failover and automated recovery, because the human response loop is longer than the entire allowance.
Monthly and annual are different promises
A monthly SLA resets. A bad month is forgiven at the end of it, and the following month starts with a full allowance. An annual budget carries the damage forward for the rest of the year.
Providers usually offer monthly and customers usually assume annual, which is a gap worth noticing before signing. Twelve months at exactly 99.9% monthly is a very different experience from 99.9% measured annually, because the monthly version permits twelve separate bad days.
Allowances are also cumulative within the period rather than requiring consecutive downtime, which many buyers assume wrongly. Frequent short outages breach an SLA exactly as effectively as one long one.
The exclusions are where the availability goes
Planned maintenance is excluded from almost every agreement, and for some services that is a substantial number of hours. Dependency failures, force majeure and anything attributed to the customer's own configuration are commonly excluded too.
Measurement is negotiable in the same way. Who checks, from where, how often, and what counts as down — an outage shorter than the polling interval may never be recorded at all.
Which is why comparing two providers on the headline percentage alone is close to meaningless. And what an SLA guarantees, in practice, is a service credit: typically a small percentage of the monthly fee, applied on request, capped far below what the outage cost you. Availability is an engineering problem rather than a contractual one.
Where to go next
The Uptime SLA question rarely arrives on its own. These are the ones that usually come with it:
- Error Budget Calculator — An unspent budget means you are shipping too cautiously.
- Downtime Cost Calculator — Lost revenue is rarely the largest line.
- MTTR and Availability Calculator — Detection is usually the biggest share and the cheapest to fix.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How much downtime does 99.9% allow?
About 43 minutes in a 30-day month and roughly 8 hours 46 minutes a year. A single incident that takes 40 minutes to resolve consumes most of a month's allowance.
How much downtime does 99.99% allow?
About 4 minutes 19 seconds a month and 52 minutes a year. Any incident requiring a human to wake up, log in and diagnose has already exceeded it, which is why four nines demands automated failover.
What is five nines?
99.999%, which permits about 5 minutes 15 seconds a year. It is achievable only with full redundancy and automated recovery, and it is quoted far more often than it is delivered.
Is a monthly or annual SLA better?
They are different promises. A monthly SLA resets, so a bad month is forgiven; an annual budget carries the damage forward. Providers usually offer monthly and customers usually assume annual.
Does planned maintenance count against the SLA?
Usually not, and that exclusion is where a lot of the real availability goes. Read what the agreement excludes before comparing two providers' numbers — the exclusions differ more than the percentages.
What does an SLA actually guarantee?
A service credit, in almost every case. The credit is typically a small percentage of the monthly fee and rarely approaches the cost of the outage to you, which is why availability is an engineering question rather than a contractual one.
How is uptime measured?
Usually by external synthetic checks at a fixed interval, which means an outage shorter than the interval can go unrecorded. Who measures, from where and how often are all negotiable and all affect the number.
What is the difference between an SLA, an SLO and an SLI?
The indicator is what you measure, the objective is the internal target, and the agreement is the contractual promise with penalties. The objective should be stricter than the agreement so problems surface before credits are owed.
Do outages have to be consecutive to breach an SLA?
No — allowances are cumulative over the period, which is what makes frequent short outages as damaging as one long one. Many buyers assume the opposite.
What availability do the big cloud providers offer?
Commonly 99.9% to 99.99% for individual services, with higher figures for multi-zone deployments. Read the exclusions, since planned maintenance and dependency failures are frequently carved out.
Does higher availability always cost more?
Steeply and non-linearly. Each nine roughly multiplies the infrastructure and operational effort, which is why the right question is what availability the business actually needs rather than how high it can go.
Is 100% uptime possible?
Not as a promise. Anyone offering it is either excluding enough in the definitions to make it meaningless or has not yet had the outage. Every serious provider quotes a figure below it.
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