Uptime / SLA Calculator
Convert an SLA uptime percentage like 99.9% into allowed downtime per day, month or year free in your browser with this uptime calculator.
Enter Values
How to use this calculator
- Enter the uptime percentage (e.g. 99.9).
- Press Calculate for the allowed downtime.
How it works
Downtime is the time the service is allowed to be unavailable: (1 − uptime%) of the period. Each extra '9' cuts the downtime roughly tenfold.
Worked example
99.9% uptime. About 526 minutes (8.8 hours) of downtime a year.
Common mistakes
- Reading 99.9% as nearly perfect — it still allows about 8.8 hours of downtime a year.
- Comparing SLAs without the measurement window; monthly and yearly percentages differ.
Frequently asked questions
How much downtime does each SLA allow?
99% allows about 3.65 days a year, 99.9% about 8.8 hours, 99.99% about 52 minutes, and 99.999% (‘five nines’) about 5 minutes.
Is a higher SLA always worth it?
Each extra nine costs disproportionately more in redundancy and engineering, so match the target to what downtime actually costs your service.
How much downtime does each '9' allow?
Each extra nine cuts allowed downtime ~10×: 99% ≈ 3.65 days/yr, 99.9% ≈ 8.8 hours, 99.99% ≈ 53 minutes, 99.999% ≈ 5 minutes.
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Cite this tool
Zerdly. (2026). Uptime / SLA Calculator. Retrieved from https://www.zerdly.com/tools/uptime-sla-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.