Does someone know the plain formula which calculates the energy needed while running, based only on body weight and speed?
I googled around a lot but don't find this exact formula.
There is no “exact formula”, just estimations that generally seem to be more reliable than others. The following formula is one such example. While I don’t possess a peer reviewed study to pair with this formula, I personally trust the source. It comes from researcher Lyle McDonald’s “The Women’s Book Volume 1” (the formula is for males and females).
The formula is simply 2-3 kCal per pound of body mass for jogging and 3-4 kCal per pound of body mass for something like a six minute mile (both are on a per hour basis). These formulas are only considering the additional energy expended by the activity being performed, meaning that they aren’t including BMR or any other background metabolic process.