What is the Drag Equation for Particles

For particles, I apply drag, I see it’s effect, but I need to know what is the equation being used to create the effect? Is there some default air density or drag force? Or is it just reducing the speed by a set amount every frame? That’s what I’m assuming, but I can’t work out what that equation would look like because I’m not smart I guess. Any assistance would be very much appreciated.