Are the two comparable at all?
If a person's body fat percentage is calculated with the hand held device (I believe it is called bio-impedance) and then their BMI is calculated using the formula, would you expect the two numbers to agree? If so, what are the similarities? And if not, what is the difference?
Your bodyfat percentage and Body Mass Index are actually two very different numbers that can have very little to do with each other depending on several factors.
Your body fat percentage (regardless of the source you get it from, be it the handheld bio-impedance electrical machine or through skinfold calipers) is a measure of how much fat you have on your body compared to lean mass in form of muscle, bone, organs, etc.
For example, if you weigh 200 pounds at 10% bodyfat, you are carrying 20 pounds of fat on you.
Your Body Mass Index is a measure of weight compared to height. It actually has very little to do with fat or muscle.
Using our same example, a person who weighed 200 pounds at 10% bodyfat could be categorized as obese unless they were at least 6 foot 4.
Body Mass Index is a simple, quick gauge of your body based on the combined average data of the measurements of millions of people. It does not necessarily apply to an individual. Your bodyfat percentage is a far more useful number.
I would expect these two numbers to agree only if the person being tested were completely average, which is hard to expect in the real world.
As an aside, the bio-impedance method of testing bodyfat may be convenient but it is not all that accurate. The readings can vary greatly during the course of a day and can be affected by how much water you have in you as well as what you've eaten during the day.
For best results, use it at the same time every day (preferably first thing in the morning before doing anything else). This will give you a standardized way to compare your results.
Personally, those standing bodyfat scales are pretty much a waste of money. I actually have one (Tanita Ironman scale that's supposedly tuned for athletes) and it pegs me at anywhere between 20% to 36% bodyfat even though I'm about 8 to 10% or so.
What IS interesting to think of is that the scales you stand on are really only measure bodyfat in your LOWER body. The current goes from one foot to the other the quickest route possible. They may have formulas to estimate upper body fat and get a total but this fact makes it very inaccurate for total body testing.
For best results, you need to go from right or left hand to opposing foot to get a cross-section of the whole body.
As for hydration, the better hydrated you are, the lower your bodyfat levels will record. Water is a good conductor and more of it means the current goes through easier (and muscle has more water than fat). Also, if you're hydrated, you'll weigh more than if you are dehydrated, which will result in lower readings.
Same goes for eating salami (or any sodium-rich processed foods). The sodium conducts the current well and gives lower readings.
These bodyfat scales are good as a curiosity but I wouldn't put any stock in them in terms of giving legitimate numbers, to be honest.
The one I have cost me $120 and is a supposedly good quality BF scale and it still doesn't work well for me. If you get a cheap one, you'll really be guessing. You're better off with a regular scale and tape measure to throw around your waist in terms of feedback on fat loss.
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