What amount of blood do you lose in a below knee amputation?

Depends: Blood loss in any surgical procedure depends on the circumstances of the surgery and the skill of the surgeon. So, it is possible to have a below the knee amputation and lose very little blood. It is also possible, especially in an emergency situation where there is already trauma, to lose a large amount of blood. In a non-emergency situation bleeding should be well controlled.

Kept at a minimum: The doctor will make an effort to keep the bleeding to a minimum. Hopefully there will only be a limited amount of blood loss, otherwise one may need to possibly get a transfusion.