What the fall factor means
fall factor = fall height / length of rope that absorbs the fall. The fall height is twice the distance above the last protection plus any slack. A long fall on a lot of rope can be softer than a short fall right above the belay: 4 m fall on 20 m of rope is factor 0.2, a 4 m fall on 2 m of rope is factor 2 – the maximum possible in normal climbing, when the leader falls past the belay without any protection.
The force depends on the fall factor, not on the length of the fall, because a longer rope stretches more. The estimate uses the simple spring model of the rope that is behind the UIAA test: the rope's rated impact force is measured with an 80 kg mass falling 4.6 m on 2.6 m of rope – a fall factor of about 1.77.
Real forces are lower than this estimate: the belayer moves, the rope slips a little through the belay device, knots tighten, the body and harness absorb energy. But friction in quickdraws and over edges makes it worse, because less rope can stretch. The top piece of protection holds both rope strands: the climber's side and the belayer's side. Without friction that is almost twice the force on the climber; because the rope rubs over the carabiner, the belayer's side is lower and the total is typically about 1.6 times.
For understanding only. Learn belaying and leading from a qualified instructor.
Mountains are not a calculator. Weather, conditions, fitness and experience change everything. Use these numbers for planning, and turn back in time.