Determine the single numerical value for each of your interval ratios by dividing the first number in the ratio by the second number in the ratio. For example, for interval ratios 3:2, 4:3 and 9:8, the single numerical values would be 1.5, 1.3 and 1.125, respectively.
Add together each of the numerical values you calculated. In the example, the sum of our numerical values would be 3.925.
Divide your result by the number of numerical values you began with. In the example, we would divide 3.925 by 3, and our result would be 1.3083. This value is your mean interval ratio.