Uptime & SLA Calculator

Frequently Asked Questions

How much downtime does each "nine" allow?

Per year: 99% (two nines) ≈ 3.65 days, 99.9% ≈ 8.77 hours, 99.99% ≈ 52.6 minutes, 99.999% (five nines) ≈ 5.26 minutes. Each extra nine cuts allowed downtime by about 10×.

How is allowed downtime calculated?

Allowed downtime = period length × (1 − SLA fraction). For 99.9% over a 30-day month: 30 × 24 × 60 × 0.001 ≈ 43.2 minutes.

How does monthly downtime compare to yearly?

At 99.95%, a 30-day month allows 30 × 24 × 60 × 0.0005 ≈ 21.6 minutes, while a full year allows about 4.38 hours. Many SLAs measure per calendar month for credit purposes.

What happens to a composite SLA across dependencies?

For services in series, multiply their availabilities: two 99.9% components yield 0.999 × 0.999 ≈ 99.8%. Redundancy in parallel improves it: two 99% paths give 1 − (0.01 × 0.01) = 99.99%.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Service-level agreements are usually quoted as a percentage of uptime - the famous "nines." 99.9% is "three nines," 99.99% is "four nines," and so on. The percentage alone is abstract, so engineers translate it into an error budget: the maximum time the service may be unavailable over a given window. This calculator does that translation both ways. In SLA mode you enter a target percentage and it returns the allowed downtime per day, week, month, quarter, and year - the budget you have to spend on deploys, incidents, and maintenance. In reverse mode you enter how many minutes you were actually down over a period and it computes the SLA percentage you achieved. Site-reliability teams use these numbers to set realistic targets, decide whether to ship risky changes (do we have budget left?), and write contracts whose penalties they can actually meet.

Formula / Method

The allowed downtime fraction is (100 − SLA) ÷ 100. Multiply that fraction by the number of minutes in the window to get the downtime budget: a day is 1,440 minutes, a week 10,080, a 30.42-day average month 43,800, a quarter 131,400, and a 365-day year 525,600 minutes. So allowed downtime per year = 525,600 × (100 − SLA) ÷ 100, and similarly for shorter windows. The reverse calculation inverts this: achieved SLA = (1 − downtimeMinutes ÷ periodMinutes) × 100. We use an average-length month (525,600 ÷ 12 = 43,800 min) so monthly and annual budgets are internally consistent; a specific calendar month of 28–31 days will differ slightly.

Worked Example

Target three nines, 99.9%. The allowed downtime fraction is (100 − 99.9) ÷ 100 = 0.001. Per year that is 525,600 × 0.001 = 525.6 minutes ≈ 8.77 hours. Per month it is 43,800 × 0.001 = 43.8 minutes, per week 10.08 minutes, and per day just 1.44 minutes. Step up to four nines (99.99%) and the annual budget shrinks tenfold to about 52.6 minutes - barely time for a single rough deploy. Reverse it: if you were down 60 minutes in a month (43,800 minutes), your achieved SLA was (1 − 60 ÷ 43,800) × 100 ≈ 99.863%, which falls short of three nines for that month.

Common Mistakes

Conflating monthly and annual SLAs - an SLA met over a year can still be badly missed in a single bad month.

Forgetting planned maintenance counts - unless the contract excludes it, scheduled downtime spends the same budget as an outage.

Adding nines without counting the cost - each extra nine roughly tenfolds the engineering effort and redundancy required.

Measuring availability at the wrong layer - a healthy server with a broken dependency is still down from the user's view.

Using a 30-day month everywhere - mixing 30-day months with a 365-day year makes the budgets inconsistent.

Tips & FAQ

How much is each nine? 99% allows about 3.65 days/year, 99.9% about 8.77 hours, 99.99% about 52.6 minutes, and 99.999% ("five nines") about 5.26 minutes - each nine cuts the budget by 10×. What is an error budget? The flip side of an SLO - the downtime you are allowed to spend; while budget remains you can ship faster, and when it runs out you freeze risky changes. SLA vs SLO vs SLI? The SLI is the measured indicator, the SLO is your internal target, and the SLA is the externally promised number with penalties, usually set looser than the SLO. Should I aim for five nines? Rarely - it is extremely expensive and most products are fine at three or four; match the target to user impact and cost. Does latency count as downtime? Often yes - many SLOs treat "too slow" responses as unavailable. Which window matters? Shorter windows are stricter because a fixed outage is a larger fraction of a smaller budget. Use these figures to set targets you can actually keep and to write SLAs with honest penalties.

Frequently Asked Questions

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