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The actual relationship between % util and FICO points deducted is a trade secret, but it must be mathematical. Thus, one must assume what mathematical relationship best matches experience by consumers to attempt to duplicate, and thus project, the relationship. The upper and lower point limits for revolving %util must first be assumed, and then the mathematical relationship between the two must be assumed.
Here are my model assumptions with the related points, for your consideration and comments.
I first assumed that of the 30% total assigned by FICO to the total %util category, 25% is revolving %util, and 5% is installment %util, the two together making for the total of 30% for this category. This is only my assumption, but it seems to work. However, the math can be done for any other assumption. From this, the match flows. 25% of 850 total FICO equals a total of 212.5 points for this factor.
Next, the formula between min. util of 0% and max util of 100%. I have plotted two equations between these limits. One is linear (straight line) assumption, and the other is exponential (parabolic) assumption, with points varying as a function of the square of %util at each level.. Exponential seems to better project experience. Here are the mathematical values for each:
%Util Exponential Linear
10% -2.1 -21.3
20% -8.5 -42.6
40% -34.0 -85.2
60% -76.5 -127
80% -136.0 -170
100% -212.5 -212.5