Weight to height ratio only tells the weight to height ratio.
It can not be 100% fat due to the fact that our bodies contain water, muscle, bone, various organs, etc, etc, etc, etc...
Weight doesn't tell you how much is x or y or z, just the total weight.
If you wanna know your fat %, then there's ways to measure that.. most health clubs and gyms have tools that allow you to do that.
Mind you, even if your % is high, that doesn't automatically mean you're "fat" or that you're unhealthy.
Many skinny models have a staggeringly high fat% in proportion to their bodies.