The velocity ratio of a pulley is the ratio of the distance moved by the rope to the distance moved by the effort.

For a single fixed pulley, the velocity ratio is 1:1, since the rope moves the same distance as the effort.

For a single movable pulley, the velocity ratio is 2:1, since the rope moves twice the distance as the effort.

For a block and tackle with n movable pulleys, the velocity ratio is 2^n:1.

For example, a block and tackle with 4 movable pulleys has a velocity ratio of 2^4:1, or 16:1.

This means that for every 1 unit of effort, the load will move 16 units.