How much downtime does 99.9% allow per month?

For a 30-day month, 99.9% uptime allows about 43.2 minutes of downtime — a helpful reference point often called the 'three nines' availability tier.

Why does 99.99% allow so much less downtime than 99.9%?

Each additional 'nine' cuts allowed downtime by roughly 10×, so 99.99% ('four nines') allows only about 4.3 minutes per month versus 43.2 minutes for 99.9%.

Allowed Downtime Calculator

Calculate exactly how much downtime is allowed over a given period while still meeting a target uptime percentage (like 99.9% or 99.99%) — useful for understanding SLA commitments in concrete minutes rather than abstract percentages.

Inputs

Results

Allowed Downtime: 43.2 minutes

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Formula

Allowed Downtime = Total Period × (1 − Target Uptime % / 100)

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