Uptime Percentage Calculator

Convert an uptime percentage into the downtime it permits across each period.

About Uptime Percentage Calculator

Availability is easier to reason about as an allowance of minutes. Three nines sounds close to perfect but permits nearly nine hours of outage a year, which is why SLAs are negotiated in nines rather than percentages. An SLA number only becomes actionable once converted into the outage budget it grants, and into the mean time between failures your website must actually achieve.

Formula

Uptime Percentage

Downtime = period × (1 − availability ÷ 100)

availability: Uptime percentage promised or measured
period: Window the figure is measured over

Worked example

With the default values below, Uptime Percentage Calculator returns Allowed Downtime of 43.20. Change any input above and every figure here is recalculated in your browser.

Inputs used

Target or measured availabilityaccepts 0–100 %: 99.9 %
Measurement periodaccepts 1–366 days: 30 days
Number of outages in the periodminimum 0: 2

Results

Allowed Downtime: 43.20 minutes
Allowed Downtime: 0.720 hours
Equivalent Annual Downtime: 8.76 hours
Availability Class: Three nines — 8.8 hours a year
Mean Time Between Failures: 360.0 hours

Step by step

1Values used
2Uptime Percentage
3Allowed Downtime= 43.20 minutes
4Allowed Downtime= 0.720 hours
5Equivalent Annual Downtime= 8.76 hours
6Availability Class= Three nines — 8.8 hours a year
7Mean Time Between Failures= 360.0 hours

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Uptime Percentage calculated?

Downtime = period × (1 − availability ÷ 100) Availability is easier to reason about as an allowance of minutes. Three nines sounds close to perfect but permits nearly nine hours of outage a year, which is why SLAs are negotiated in nines rather than percentages.

Why does Uptime Percentage matter?

An SLA number only becomes actionable once converted into the outage budget it grants, and into the mean time between failures your website must actually achieve.

What values do I need to enter?

This calculator takes 3 inputs: Target or measured availability, Measurement period, Number of outages in the period. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.