How do body fat analysis scales calculate body fat % for women?

I've just bought some body fat analyser scales (Salter ones). When I weigh me and my boyfriend his body fat is 15% and mine is 26%. If I set myself as male it drops to 16%. if I set my boyfriend's setting to female, his rises to 25%. If I get the measurement done at the gym with calipers, he is 15%, i am 16%. Why the disparity between male/female settings on the scales? What is right?

Public Comments

1. i
2. sgr
3. Men and women store fat differently. (the differences are due to different activity habits, NOT genetic tendencies, so it isn't odd that your scales measure you more accurately 'as a man' if you are just as active as 'the average man') Some of these scales use an electrical signal that passes from the front to the back of your foot (basically, measuring the fat/wet characteristics of your foot). The fat pad in the bottom of your foot does vary, but mainly relates to your bodyweight (it's an impact cushion). Even adjusting for average size differences, women usually have smaller feet per bodyweight than men, so their fat pad will often be thicker to compensate for more concentrated pressure on the ground. this difference needs to be incorporated in the calculation. Some scales pass a signal from one foot to the other. This signal goes up your leg and down the other, so it should be able to measure a response in your whole lower body. Because women generally store more fat in their lower body (around the hips) than men (who often store more around the waist, because they use their waist less when walking), the scales will be programmed to calculate differently from the measured response . Most of these electronic machines can be pretty inaccurate, because lots of lifestyle factors (tiredness, coffee, alcohol, basically anything that can dehydrate you) will influence the reading massively. Some of the more 'professional' machines can produce more reliable results, but the most accurate method is manual 'skinfold' measurement, where the actual skinfat thickness in several places can be matched against charts (or a computerised version of them) by a trained operator.
4. I'm not sure, I work at Boots and I know at most Boots stores they have a machine which does your whole BMI and gives you information on your actual weight bmi! :) It's all confusing stuff isn't it.
5. To calculate your body fat percentage, write down how much you weigh but you have to be honest. Remember, no one will see this but you. Multiply your weight by 703. Next, write down your height, in inches. Multiply by that same number. Then you will divide your weight number by your height number. That is your BMI. For example, if your weight were 150 pounds x 703, your weight answer would be 105,450. If your height is 5’4”, that would be 64 inches x 64 for a total of 4,096. Taking the 105,450 divided by 4,096, you come out with a BMI of 25.7.