Use 2 ways: design HP or LP filter given f cutoff and db gain or solve fc and db
Use as a quick db calculator for 1st order active filters.
This app works in 2 modes. In result mode you enter all Rs and C and f. The app will return with the db gain at the frequency entered. In design mode you enter just desired cutoff fc, Ri, C and midband db gain. The app will return with the correct amp gain Rf and R for fc entered.