Uptime / Downtime Calculator
Convert any uptime % (like 99.9%) into allowed downtime per day, week, month or year — with a full nines table.
Formula last reviewed 4 August 2026 · How we verify our calculators
Allowed downtime / month
43m
- In minutes
- 43
- In hours
- 1
- Downtime per year
- 8h 46m
Updates live as you type
Frequently asked questions
Downtime = period × (100 − uptime%). Over a 30-day month, 99.9% allows about 43.2 minutes; over a year, about 8.76 hours.
Multiply the total time in the period by the failure fraction (100 − uptime%) ÷ 100. The tool does this for any uptime value, including ones not on the standard nines table like 99.95% or 99.97%.
Each nine is an availability tier: 99% (two nines), 99.9% (three), 99.99% (four), 99.999% (five). Adding a nine reduces allowed downtime roughly tenfold.
An SLA often states an uptime percentage, so this tells you the downtime that percentage permits. But the SLA is the contract; your internal SLO is usually set tighter so you breach it first as a warning.
It depends on the agreement. Many SLAs exclude announced maintenance windows from downtime, so read the specific terms before assuming this figure is the whole story.
What downtime does your uptime target allow?
“99.9% uptime” sounds precise until you convert it to real outage time. This calculator does exactly that: pick any target uptime — the slider isn’t limited to the standard nines, so 99.95% or 99.97% work fine — and a period (day, week, month or year), and it returns the allowed downtime in a friendly format plus minutes, hours, and the per-year figure.
The maths is simple: downtime = total time × (100 − uptime%). At the defaults — 99.9% target uptime over a month — the calculator takes the 30-day period (43,200 minutes) and multiplies by (100 − 99.9) ÷ 100 = 0.001, giving 43.2 minutes, displayed simply as "43m." The same 99.9% target over a full year instead permits 8.76 hours. Tightening to 99.99% drops the monthly allowance to about 4.32 minutes — a tenth of the three-nines figure — while loosening to 99% over a 28-day month allows roughly 6h 43m, matching the commonly cited "~7 hours a month at two nines."
Use it to sanity-check an SLA you're being offered, set a realistic internal SLO, or size the redundancy and on-call coverage a target implies. Remember that many SLAs exclude scheduled maintenance, and that an internal SLO is usually set tighter than the contractual SLA so it trips first as an early warning — by the time the contractual SLA is actually at risk, the internal team should already know and be responding.
The period selector matters as much as the percentage itself: the same 99.9% target permits a very different absolute number of minutes depending on whether you are looking at a day, a week, a month or a full year, so always check which period an SLA or internal target is quoted against before comparing two numbers that look similar on paper.