how to calculate terminal velocity?

We have the balance of forces on a free falling body as f = MA = W - D; where M is the mass, A is its acceleration, W = Mg is the weight, and D = kV^2 is the drag force. The body reaches terminal speed when f = MA = W - D = 0, which is to say there is no more acceleration and the body is falling at a constant speed called the terminal velocity.

Note the drag D increases as the speed of the body falls faster. At some point the speed is such that the drag is equal but opposite to the weight. And then we have W = mg = kV^2 = D; so that Vterm = sqrt(mg/k) where k = 1/2 rho Cd A and rho is air density, Cd is drag coefficient, and A is X-sectional area.

So you need to know the factors that make up k. And you need to know the body's weight W = mg.

But this is tricky in that air density rho is a variable. It's not fixed; it gets less dense the higher the body is found in elevation. Of course in the extreme, rho = 0 in space. So in space there is no drag and thus no terminal speed.