Three nines (99.9%)

Reliability targets are commonly described by their number of nines. Three nines is 99.9%, four nines is 99.99%, five nines is 99.999%. The shorthand is convenient because each additional nine cuts the allowed downtime roughly tenfold, so the gap between adjacent targets is much larger than it looks.

Why it matters: 'three nines' is the most common baseline target for production web services because it is meaningfully reliable yet achievable for a competent team without exotic architecture. It translates to a concrete budget — about 8 hours 45 minutes of downtime per year, or roughly 43 minutes per 30-day month — which makes it easy to reason about and to verify.

A common misconception is that the jump from 99.9% to 99.99% is a small, incremental improvement. It is not: four nines allows under an hour of downtime per year versus nearly nine hours for three nines — about ten times stricter. Each extra nine typically demands disproportionately more engineering, redundancy, and cost, which is why teams choose the lowest number of nines that genuinely meets user needs.

Hitting and proving a three-nines target also depends on measurement resolution. With only a coarse monitor you cannot credibly claim 99.9%, because short outages slip between checks; frequent checks are what let you both achieve and defend the number.