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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99.9% 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 99.9% SLA Mean?

A 99.9% SLA, also known as three nines of availability, guarantees that a service will be operational 99.9% of the agreed time. This translates to a maximum of 8.76 hours of downtime per year, 43.8 minutes per month, 10.1 minutes per week, or just 1.44 minutes per day. It is the most widely used production standard in the industry, offering a solid balance between cost and reliability for the majority of commercial applications.

Who Is It For?

The 99.9% SLA is ideal for production applications that require high availability without reaching critical levels. It is designed for SaaS platforms, e-commerce sites, public APIs, CRM/ERP systems, corporate websites, and mobile apps with steady traffic. It is also the recommended starting point for startups and SMBs launching their first production product, providing a solid foundation without the high infrastructure costs required by higher SLAs.

Infrastructure Requirements

Achieving a 99.9% SLA requires moderately redundant infrastructure. You need at least two servers or instances behind a load balancer, redundant power supplies (dual PSU), RAID 10 or equivalent storage, and internet connections from two different providers. Proactive monitoring with real-time alerts, a documented disaster recovery plan, and scheduled maintenance windows outside peak hours are also recommended.

Cost and Benefits

The 99.9% SLA represents the sweet spot between cost and availability. The required infrastructure costs approximately 2 to 3 times more than a basic single-server setup but remains affordable for most businesses. The main benefit is reducing downtime from days to hours per year, translating into higher revenue, better customer satisfaction, and compliance with client contracts that require minimum availability.

When to Upgrade to a Higher SLA?

You should consider upgrading to a higher SLA when your business depends critically on 24/7 availability, when losses per hour of downtime exceed the cost of redundant infrastructure, or when your clients demand stronger guarantees. If the allowed 43.8 minutes of monthly downtime is not enough, check our guide on 99.99% enterprise SLA for critical services, or if budget is tight, review the 99% SLA for development and testing as a more affordable alternative.

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