An interval scale is a scale (of measurement) created by units of equal size.

When dealing with an interval scale, the difference between any two values can be calculated by using subtraction.

Ratios of values have no meaning, because the value of zero is arbitrary.

Some examples of variables that use interval scales would be time, temperature (Celsius), temperature (Fahrenheit), etc.

When using a twelve hour clock, we can compare the time of 4:00 in the afternoon to 8:00 in the evening. It is possible to say that the difference in time is four hours (8:00 − 4:00). Please see the illustration below.

On a comparable twenty-four hour clock, we can compare the time of 16:00 in the afternoon to 20:00 in the evening. It is possible to say that the difference in time is four hours (20:00 − 16:00). Please see the illustration above.

If we look at the ratios of these numbers [4/2 = 2, and 18/16 = 1.125], the ratios are different, indicating that these ratios have no meaning.