Uptime & SLA Calculator

Turn an availability target like 99.9% into the downtime it allows per day, week, month, quarter and year. See exactly what each “nine” costs. Calculates in your browser.

Allowed downtime per year

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Per day-
Per week-
Per month-
Per quarter-

Show the math

Allowed downtime is the share of each period the service may be unavailable:

downtime = period × (1 − availability ÷ 100)

Periods use a 365-day year, so a month is one twelfth of a year and a quarter is one quarter of it. “Three nines” (99.9%) allows about 8h 45m of downtime a year; each extra nine cuts that roughly tenfold. Everything is calculated in your browser.

What this does

An uptime / SLA calculator turns an availability target like 99.9% into the downtime it allows per day, week, month, quarter and year.

How to use it

  1. Enter your availability target.
  2. Read the allowed downtime per period.
  3. Add a nine to compare targets.
  4. Plan your maintenance budget.

How it works

Each extra nine cuts the allowed downtime roughly tenfold. Periods use a 365-day year, so a month is a twelfth and a quarter is a quarter of it.

downtime = period × (1 - availability)

Understanding your result

The headline is how little downtime each nine actually buys: 99.9 percent still allows about 43 minutes a month, while 99.99 cuts that to roughly four. Read the monthly figure, not the yearly one, when judging a real incident, and remember a published SLA is a contractual floor with credits, not a promise, so design for the downtime it permits rather than assuming none.

Example

99.9% allows about 8 h 45 m of downtime per year, or roughly 43 minutes a month.

Sources & methodology

Last updated .

Frequently asked questions

How much downtime does 99.9% allow?

About 8 hours 45 minutes per year, or roughly 43 minutes a month. Each additional nine, 99.99%, 99.999%, reduces the allowed downtime by about a factor of ten.

How are the periods worked out?

Downtime equals the period length times one minus the availability fraction. Periods use a 365-day year, so a month is a twelfth of the year and a quarter is a quarter of it.

Is anything sent to a server?

No. The figures are calculated entirely in your browser.