Short version - don't buy one.
Here's the longer version.
You've all seen them, and I'm sure many of you have them - the scales where you take your shoes off and magically it tells you your body fat percentage.
The scales work via bioimpedance analysis which involves a small electrical current being passed through your body from foot to foot. The signal travels through the various components of our bodies (fat, muscle, bone) and the time it takes is measured by the scale. The more water there is in the tissue, the faster the transmission and because fat has less water content than muscle, the more fat you have, the slower the electrical current travels and the more fat is registered by the scale.
A good body fat percentage scale will have an error range of at least 4% and frankly you can affect more error by varying your hydration level.
So why don't I like them? Wouldn't it be a good thing to know your body fat percentage? Umm, why would that be a good thing? Would it change anything? Would you live any differently or choose any differently if you didn't see those numbers going down?
The answer to the above questions should be "no", because regardless of your body fat percentage what I'd be recommending you be doing is living the best lifestyle that you can enjoy.
What many folks don't know buying a body fat percentage scale is that body fat percentage changes slowly - very slowly and frankly aside from exercising and losing weight, there's really nothing for you to do to bring that number down. Add in the wide range of error that these scales tend to have and now you've got a tool in your bathroom with the potential of providing you with daily discouragement utilizing a number that you can't do anything specific about to change. Not so smart.
A good practice for a physician is not to order any test that you're not prepared to act on or where the results will not add to the person's health and well being. In my mind body fat percentage is just such a test.
I'll reiterate the goal that I tell everyone in my program is the only goal worth having,
"Live the best life you can enjoy, not the best life you can tolerate."Sure there will be days with more calories and days with less exercise, but that's real life and in real life, if you're living a life you don't enjoy (a diet for example), eventually you'll decide to stop living that way. Knowing your body fat percentage will only confuse that goal.