Enter the total uptime (hours) and total downtime (hours) into the calculator to determine the % availability of a system.

% Availability Formula

The following formula is used to calculate the percentage availability of a system. 

A = UT / ( UT + DT ) *100
  • Where A is the percentage availability (%)
  • UT is the total uptime (hours)
  • DT is the total downtime (hours)

To calculate an availability percentage, divide the total uptime by the sum of the uptime and downtime, then multiply by 100.

Availability Definition

What is a percentage availability? A % availability is a ratio of the total time a system is running, known as uptime, and the total possible time a system could be running. The total time the system could potentially run is known as the sum of the uptime and downtime.

Example Problem

How to calculate % availability?

  1. First, determine the total uptime.

    For this problem, we are looking at the total uptime and downtime of a web server over a month period. The total uptime is found to be 690 hours over the month.

  2. Next, determine the total downtime.

    The total downtime of the server was measured to be 30 hours last month.

  3. Finally, calculate the percent availability.

    Using the formula, the percent availability is found to be 690 / (690+30) *100 = 95.83%.

About Availability

Can availability be negative? An availability can never be negative. This would require either the total uptime or downtime to be negative, which is not possible.

Does availability include planned downtime? Availability should include both planned and unplanned downtime. While a planned downtime can be expected and understood by customers, it is still a portion of time in which the customer does not have access to a system. The system is still unavailable.

Where is availability used? Availability is a term that can be used in many fields, but for the purpose of this page, the term is used to describe the total percentage of possible time that a service is available to a customer or user.