How is uptime percentage calculated?

Uptime percentage is calculated using the formula: Availability (%) = (Total Time - Downtime) / Total Time × 100. For example, if a service ran for 720 hours in a month with 43 minutes of downtime, the availability would be 99.9%.

How much downtime does a 99.9% SLA allow?

A 99.9% SLA (three nines) allows approximately: 43.8 minutes per month, 10.1 minutes per week, 1.44 minutes per day, or 8.76 hours per year. This is the standard for most production services.

What is the difference between 99.9% and 99.99% availability?

99.9% (three nines) allows ~43.8 minutes of downtime per month, suitable for standard production applications. 99.99% (four nines) allows only ~4.38 minutes per month, requiring redundant infrastructure and high availability. The difference can cost significantly more in infrastructure.

What is an SLA and how does it relate to downtime?

An SLA (Service Level Agreement) is a contract that defines the expected service level, including the guaranteed uptime percentage. For example, a 99.9% SLA means the provider guarantees the service will be available 99.9% of the time, and any downtime below that threshold may result in credits or penalties.

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99.9999% SLA - Downtime Calculator

Select an SLA level or enter downtime minutes to see the allowed downtime per period.

SLA 99% SLA 99.9% SLA 99.95% SLA 99.99% SLA 99.999% SLA 99.9999%

Availability & SLA Calculator

Calculate availability from downtime or downtime from SLA requirement

Downtime minutes per month

Enter the downtime minutes for each month to get a more accurate annual average.

Select SLA level

Maximum allowed downtime

SLA Level	Day	Week	Month	Quarter	Year	Typical use
99%	14 min 24 s	1 h 41 min	7 h 12 min	21 h 36 min	3 d 16 h	Dev / testing
99.9%	1 min 26 s	10 min 5 s	43 min 12 s	2 h 10 min	8 h 46 min	Standard production
99.95%	43 s	5 min 2 s	21 min 36 s	1 h 5 min	4 h 23 min	High availability
99.99%	9 s	1 min	4 min 19 s	12 min 58 s	52 min 34 s	Enterprise
99.999%	< 1 s	6 s	26 s	1 min 18 s	5 min 15 s	Mission critical
99.9999%	< 1 s	< 1 s	3 s	8 s	32 s	Fault tolerant

Downtime cost calculator

Hourly revenue ($)

What is availability?

Availability measures the percentage of time a service is operational. It is calculated by subtracting downtime from total time divided by total time. It is the most common metric used in SLA contracts to guarantee service quality.

Availability (%) = (Total Time - Downtime) / Total Time × 100

Example: 30 days = 720h. If the service was down 7.2h: (720 - 7.2) / 720 × 100 = 99% availability

What is an SLA?

An SLA (Service Level Agreement) is a contract between provider and customer that defines the minimum acceptable service level. It typically includes guaranteed uptime percentage, maximum incident response time (MTTR), and penalties for non-compliance.

Why percentiles matter

The gap between 99.9% and 99.99% is massive: the former allows 43 minutes of downtime per month, the latter only 4.3 minutes. For an e-commerce site earning $100k/day, 1 hour of downtime costs ~$4,166. Choosing the right SLA is a business decision, not a technical one.

Mean time metrics

MTBF (Mean Time Between Failures) = Total Operational Time / Number of Failures
MTTR (Mean Time To Repair) = Total Repair Time / Number of Repairs
MTTA (Mean Time To Acknowledge) = Total Time to Acknowledge / Number of Incidents

SLA Service Credit

Typical penalty when SLA is not met:

• Below 99.9% but ≥ 99.0% → 10% credit
• Below 99.0% → 25% credit

What Does a 99.9999% SLA Mean?

A 99.9999% SLA —also called six nines— allows only 31.5 seconds of downtime per year. That is roughly 10x more stringent than a 99.999% (five nines) SLA. At this level, the system must be fully fault-tolerant: no single hardware failure, software crash, or network partition can cause a service interruption. There is no scheduled maintenance; every upgrade or patch must happen live with zero observable impact. This is the practical ceiling of availability engineering.

Who Is It For?

A six nines availability target is designed exclusively for critical national infrastructure and life-safety systems: air traffic control (systems like FAA's ERAM cannot tolerate more than seconds of downtime), power grid management (SCADA systems to prevent blackouts), defense and military command-and-control networks, and emergency services like Next Generation 911 and public alerting systems.

Infrastructure Requirements

Delivering a fault tolerant SLA demands infrastructure that is orders of magnitude beyond standard high-availability setups: N+3 redundancy or higher where every component has at least 3 active backups, duplicate data centers hundreds of miles apart in active-active mode with synchronous replication, sub-second automatic failover with convergence under 50 ms, and weekly failover testing with controlled failure injection.

Cost

A 99.9999% SLA costs 5-10x more than five nines, which itself costs 20-50x more than 99.9%. This level is beyond any standard commercial enterprise — only governments and critical infrastructure organizations can justify it. A duplicated air traffic control center 300 miles away costs hundreds of millions, but a 5-minute outage could cost human lives and billions in legal liabilities.

When to Upgrade?

Moving from five nines to six nines is an existential decision, not an incremental improvement. You should only consider this if your service ranks in the top 0.0001% of criticality — where an outage means loss of life, irreversible environmental harm, or a national security breach. Infrastructure cost jumps 5-10x compared to 99.999%. Before taking this step, verify you have consistently met a 99.999% mission critical SLA for at least 12 consecutive months.

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