How to use the sla uptime calculator
The formula
Allowed downtime = (100 − uptime %) × the period. So 99.9% uptime allows 0.1% downtime: 0.001 × 2,592,000 seconds (30 days) = 2,592 seconds = 43.2 minutes a month. Each additional nine divides the downtime by ten.
Reading an SLA like a pro
Ask three questions: is the percentage measured monthly or yearly, what counts as 'down' (full outage vs degraded), and what service credits apply when it is missed? The percentage alone is only half the contract.
Example
99.9% uptime = at most 43.2 minutes down per month (and about 8 hours 46 minutes per year). 99.99% tightens that to just 4.3 minutes a month — which is why 'four nines' business services cost what they do.
Frequently asked questions
How many 'nines' do I actually need?
For a home connection, 99.9% is typical. Businesses that lose money when offline usually contract 99.95% or 99.99% — each extra nine cuts allowed downtime roughly tenfold, and costs rise just as fast.
Does planned maintenance count against the SLA?
Usually not — most contracts exclude agreed maintenance windows and count only unplanned outages, often with a minimum incident length. Always read your contract's definition; this calculator shows the raw time budget.
Why does the monthly figure use 30 days?
A standard convention for comparison. A 31-day month allows about 3% more downtime than the figure shown; February allows less.
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