The general rule of thumb for anchor rope length is that the length of the rope should be seven times or ten times longer than the depth of the water. For instance, if you are looking for an anchor rope that you’ll be using in 20 foot water, multiple 7 by 20, for a 140 foot rope. If you’d prefer to err on the side of caution, you can multiply 10 by 20, for a 200 foot anchor rope.