Most of them estimate body composition based on bioelectrical impedance - based on the fact that lean tissue conducts electricity differently than fat. Unfortunately, this method is very succeptible to error due to fluctuations in hydration, so workouts, caffeine, alcohol, etc. can alter the readings. In other words, they really can't be trusted.

JC

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Jonathan Cane
City Coach Multisport
http://www.citycoach.org

That fluctuation is inherent in it's design. It's measuring resistance to electrical flow in your lower body...as you stand up all day long, more fluid is collecting in your legs...therefore, your evening % will be lower. IN the AM, after lying down all night, that excess fluid will have returned to your vascular system...and perhaps dumped in your urine, and your AM % will be several points higher.

As far as day to day variations, it show me my hydration status...watch as your weight drops, it shows an increase in % body fat. That is really showing a decrease in hydration...drink more water!

This demonstrates how inaccurate "weight only" is for body compositon measurement.

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Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)

I have the older model and I understand the newer model is more accurate. That said I have had my % body fat checked several times and it ranges from 10.5-11.5%. The scale provides numbers from 5.5 to 8%. That said, it is useful in showing a trend and as Ktalon indicated your hydration status.

I own a Tanita and I like it a lot. Like everybody mentioned already, it's sensitive to more than just body fat, so if you're not careful, you'll get meaningless results.

I use it first thing in the morning: out of bed, pee, step on the thing. I get day to day values within 0.25%, with a body mass that's within 0.5 lbs. If I measure at night, it's about 2% off. Don't look for absolute values, but for trends: the unit is not accurate, but it's precise.

Dre'

I do the exact same thing as Dr. Dre'. Out of bed, pee, get on scale. I also don't expect a perfectly accurate measurement. I just use it as a tool to spot trends up or down. A day to day fluctuation of 1% doesn't concern me. I check to see a continual upward or downward trend and then adjust my diet and/or exercise if warranted.

I figure that just as resting HR can fluctuate by a small amount daily, so too can BF% depending on what I did or ate the previous day.

None of the methods for body fat measurement are 'accurate', but I am not sure that is what is important. As long as the conditions are consistant each time you measure, and the technology is consistant... you will get what you need... the ability to track relative shifts (just like a scale for weight). The actual number is only useful for bragging rights.

The explaination in the manual is surprisingly good.

Same as Dr. Dre also. I've using it for about 9 months now and I only get daily fluctations of about .5% and that I know is almost entirely hydration related. I know if I don't drink enough water the night before, or if my pee is dark that the reading will be on the high end. It is very consistent and repeatable as long as I am consistent and repeatable. It has been very easy to spot the trend.

A perfect tool for me needs, IMHO. I don't care about the absolute number. I just want to know if it is improving, week to week, and month to month.

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Justin in Austin, get it? :)

Cool races:
- Redman
- Desoto American Triple T