You have a data center with 99.9% availability requirements. How many hours of downtime (unplanned) is allowed to meet this availability requirement?

Question

Options

• A6.8 hours
• B3.4 hours
• C8.6 hours
• D8.8 hours

• A
  3% (1)
• B
  8% (3)
• C
  3% (1)
• D
  86% (32)

Explanation

Option D (8.8 hours) is correct because 99.9% availability means 0.1% downtime is permitted. Multiplying 8,760 hours (hours in a year) by 0.001 gives 8.76 hours, which rounds to 8.8 hours.

Why the distractors are wrong:

• A (6.8 hours) corresponds to no standard availability tier - it's simply not the result of any common availability percentage calculation.
• B (3.4 hours) is approximately half the correct value, which would reflect ~99.96% availability, not 99.9%.
• C (8.6 hours) is close but slightly low - likely a rounding trap designed to catch off-by-one errors in the calculation.

Memory tip: Anchor on the formula - (1 − availability%) × 8,760. For each "nine" added (99% → 99.9% → 99.99%), divide the downtime by 10. Since 99% = 87.6 hours, one more nine gives 8.76 ≈ 8.8 hours.

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