What does 99.9% SLA mean in practice?

99.9% SLA (Three Nines) means 0.1% of the time can be downtime. That works out to 43.8 minutes per month or 8.76 hours per year. Most production APIs and SaaS tools operate at this level.

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

99.99% (Four Nines) allows only 4.38 minutes of downtime per month versus 43.8 minutes for 99.9% (Three Nines). Achieving Four Nines typically requires redundancy, automated failover, and zero-downtime deployments.

What is an error budget?

An error budget is the amount of downtime or errors a service is allowed before breaching its SLA. If your SLA is 99.9%, your monthly error budget is 43.8 minutes. SRE teams track error budget consumption to balance reliability with feature velocity.

How do I calculate SLA for services in series?

For services that must all be available (in series), multiply their availability percentages. Two 99.9% services in series = 99.9% × 99.9% = 99.8% composite availability. This is why microservice architectures carefully manage dependency chains.

What SLA should I target for my service?

99.9% is standard for most internal tools and B2B SaaS. 99.95% for production APIs with paying customers. 99.99% for mission-critical financial or healthcare systems. Five Nines (99.999%) is rare and expensive — typically only for telecom infrastructure.

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SLA Uptime Calculator

Calculate allowed downtime for any SLA percentage. Daily, weekly, monthly, yearly allowances for 99.9%, 99.99%, and 99.999% SLAs. Free SRE uptime calculator.

SLA Percentage

1.4m

Daily downtime

10.1m

Weekly downtime

43.2m

Monthly downtime

8.76h

Yearly downtime

💡 At 99.9% SLA, you can afford 43.2m of downtime per month. Error budget per quarter: 2.16h

About This Tool

Calculate the maximum allowed downtime for any SLA (Service Level Agreement) percentage. Enter your SLA target — 99.9%, 99.95%, 99.99%, or any custom value — and instantly see how much downtime you can afford per day, week, month, and year. Essential for SRE teams, DevOps engineers, and anyone managing service reliability. Use presets for the most common SLA tiers or enter a custom percentage.

How It Works

Select a preset SLA tier — 99.9% (Three Nines), 99.99% (Four Nines), 99.999% (Five Nines) — or enter a custom percentage

Instantly see the allowed downtime converted to seconds, minutes, hours, or days for each time window

Use the monthly budget figure to track your error budget consumption

Calculate remaining error budget for the quarter by multiplying monthly by 3

Frequently Asked Questions

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