This app calculates the fuel required for an inflight diversion, taking into account distance, true airspeed, wind data and track. You can adjust the fuel flow multiplier based on the number of operating engines — for example, use 1 for single-engine operation or 2 for both engines. If a reserve fuel value is entered, it will automatically be added to the diversion fuel total.

Functional Demo: https://www.theairlinepilots.com/apps/diversion-fuel-planning.php