I need a gear calculator

[Modding_out]

Hey, I need some help finding a gear calculator. I have searched for a long time but I haven't been able to find what I'm looking for. Every single gear calculator I found is used for calculating Roll out and top speed. I need one that's going to tell me: ''gear A. + gear B. = gear ratio of X.'' I haven't been able to find it, and I was wonder if you guys could help me. thanks.