5 min read · Published August 24, 2026 · Vast Networks
The conversion table worth memorising
Per year: 99% allows about 3 days 15 hours 36 minutes. 99.5% allows about 1 day 19 hours 48 minutes. 99.9% allows about 8 hours 45 minutes 57 seconds. 99.95% allows about 4 hours 22 minutes 58 seconds. 99.99% allows about 52 minutes 34 seconds. 99.999% allows about 5 minutes 15 seconds.
Per month the same figures become roughly 7 hours 18 minutes, 3 hours 39 minutes, 43 minutes 50 seconds, 21 minutes 55 seconds, 4 minutes 23 seconds, and 26 seconds respectively.
Why the measurement window changes everything
An annual window means a single nine-hour outage exhausts a 99.9% commitment for the whole year and the provider owes nothing further no matter what happens in the remaining months. A monthly window resets the allowance twelve times, so that same outage breaches one month and the rest of the year is measured fresh.
Monthly measurement is generally better for the customer and is worth asking about specifically, because the percentage alone tells you nothing about which applies.
What "available" is measuring
Almost always the circuit, from the provider handoff outward — not your firewall, your switch, your power, or your application. A circuit that is up while your router is down counts as available, and that is the correct definition, but it means circuit availability is not service availability.
End-to-end availability is the product of every component in the path. Two components each at 99.9% in series give roughly 99.8% combined. This is why organizations that genuinely need high availability build redundancy rather than buying a larger number on one circuit.
What a single fiber cut does to the arithmetic
Repairs to a cut fiber take hours: locating the fault, getting a crew and equipment there, exposing the cable, splicing, and testing. A single cut therefore comfortably exceeds a 99.99% annual allowance on its own.
That is not an argument against the commitment; it is an argument for understanding what the commitment protects. A four-nines figure on a single path describes an intention and a credit schedule. Four nines in practice, over years, generally requires two physically diverse paths.