SLA Uptime Calculator

Calculate downtime percentage and uptime for any SLA

Use this free uptime calculator and downtime calculator to convert any SLA percentage (99.5%, 99.9%, 99.99%) to allowed downtime per day, week, month, and year. Learn how to calculate downtime percentage and understand what "three nines" or "five nines" really means for your service. Essential for SREs and DevOps engineers. 100% client-side. Your data never leaves your browser.

%
Quick presets:

Allowed Downtime

Per Year
—
Per Quarter
—
Per Month
—
Per Week
—
Per Day
—

SLA "Nines" Quick Reference

Uptime % Name Downtime/Day Downtime/Month Downtime/Year
99% Two nines 14m 24s 7h 18m 3d 15h 36m
99.5% Two and a half nines 7m 12s 3h 39m 1d 19h 48m
99.9% Three nines 1m 26s 43m 50s 8h 45m 36s
99.95% Three and a half nines 43s 21m 55s 4h 22m 48s
99.99% Four nines 8.6s 4m 23s 52m 34s
99.999% Five nines 0.86s 26s 5m 15s

Click any row to calculate its downtime

Understanding SLA Uptime

What is SLA Uptime?

SLA (Service Level Agreement) uptime is a contractual promise about how available a service will be. It's expressed as a percentage of total time. For example, 99.9% uptime means the service is expected to be available 99.9% of the time, allowing for 0.1% downtime.

The remaining percentage (100% - uptime%) is your error budget. The maximum amount of downtime you can have before breaching your SLA.

How to Calculate Downtime Percentage

Calculating downtime percentage is straightforward once you understand the relationship between uptime and downtime:

  1. Start with your uptime percentage: For example, 99.5% uptime
  2. Calculate downtime percentage: Downtime % = 100% - Uptime % = 100% - 99.5% = 0.5%
  3. Convert to actual time: Multiply the downtime percentage by the total time period
  4. For a year (365 days): 0.5% × 365 days × 24 hours = 43.8 hours of downtime allowed per year
  5. For a month (30 days): 0.5% × 30 days × 24 hours = 3.6 hours of downtime allowed per month

Formula: Allowed Downtime = (100% - Uptime%) × Total Time Period

For example, with 99.5% uptime over a year: (100% - 99.5%) × 8,760 hours = 0.5% × 8,760 = 43.8 hours per year.

Use this calculator above to instantly convert any uptime percentage to downtime for different time periods (day, week, month, year).

The "Nines" Explained

In the industry, we refer to uptime levels by counting the number of 9s:

  • Two nines (99%): ~3.65 days downtime per year. Basic availability, acceptable for internal tools.
  • Three nines (99.9%): ~8.76 hours downtime per year. Standard for most SaaS products and APIs.
  • Four nines (99.99%): ~52 minutes downtime per year. Enterprise-grade, requires significant investment.
  • Five nines (99.999%): ~5 minutes downtime per year. Mission-critical systems, extremely expensive to achieve.

Each additional nine is exponentially harder to achieve and typically costs 10x more in infrastructure and engineering effort.

Error Budgets in Practice

Error budgets help teams balance reliability with feature velocity:

  • Budget remaining: Deploy new features, run experiments, take risks.
  • Budget exhausted: Focus on reliability, fix issues, reduce change velocity.
  • Monthly reset: Most teams track error budgets on a rolling 30-day window.

This creates a data-driven way to decide when to prioritize stability over new features.

SLA Calculator FAQ

What does 99.9% uptime mean?

99.9% uptime (three nines) allows approximately 8 hours and 45 minutes of downtime per year, or about 43 minutes per month. This is a common SLA target for web services, APIs, and SaaS products.

How do I calculate downtime percentage?

To calculate downtime percentage, subtract your uptime percentage from 100%. For example, if you have 99.5% uptime, your downtime percentage is 100% - 99.5% = 0.5%. Then multiply this percentage by the total time period to get actual downtime. For a year: 0.5% × 8,760 hours = 43.8 hours of allowed downtime. Use this calculator above to instantly convert any uptime percentage to downtime.

What is 99.5% uptime in hours?

99.5% uptime allows approximately 43.8 hours of downtime per year, which equals about 3.6 hours per month, 1.2 hours per week, or 10.3 minutes per day. This is a common SLA target for many services, offering a balance between reliability and cost. Use the calculator above and select 99.5% to see the exact breakdown.

How much downtime is allowed for 99.5% SLA?

With a 99.5% SLA, you're allowed 0.5% downtime, which translates to approximately 43.8 hours per year, 3.6 hours per month, 1.2 hours per week, or 10.3 minutes per day. This is often called "two and a half nines" and is a popular choice for services that need good availability without the extreme cost of four or five nines.

What is the difference between 99.9% and 99.99%?

That extra nine makes a huge difference. 99.9% allows 8.76 hours of downtime per year, while 99.99% allows only 52 minutes. Achieving four nines typically requires redundant infrastructure, automated failover, and 24/7 on-call support.

What is an error budget?

An error budget is the maximum allowed downtime for a given period based on your SLO. With a 99.9% monthly SLO, your error budget is 43 minutes. If you use it all on incidents, you should pause risky deployments until the budget resets.

Can any service achieve 100% uptime?

No. 100% uptime is practically impossible. Even the most reliable services (AWS, Google Cloud) experience some downtime. This is why SLAs always specify a percentage below 100% and include compensation terms for breaches.

How do I choose the right SLA target?

Start with your current performance, consider your dependencies (you can't exceed their SLAs), and factor in cost. Going from three nines to four nines typically requires 10x the infrastructure investment. Match your SLA to your customers' actual needs.

What's the difference between SLI, SLO, and SLA?

SLI is what you measure (e.g., uptime percentage). SLO is your internal target (e.g., aim for 99.9%). SLA is the contractual promise to customers with penalties (e.g., refund if below 99.5%). Learn more →