Hi Hayley,

I am confused regarding body fat percentage and BMI.   How much attention should I pay to the numbers and how accurate are they?

I had my reading done at the gym on their new machine (the one you stand on, not the calipers), so I assumed the number would be pretty accurate. Then I found out the percentage!   33%!   My home scale also measures body fat and it read 28.5 the same morning!  And my BMI is 24.7.  What is the difference between BMI and body fat?
Now, my second question is; how much does my body fat fluctuate and how much should I worry about this number?  My understanding is that it reads differently if you are dehydrated, for instance. In other words, I'm hoping it wasn't accurate. But the brand spanking new machine at the gym should be right, no?

I know I'm not in Olympic athlete shape, but I do have a fair bit of muscle. I run, I do weight training and I spin, etc.  I'm 49, 5’7" and weigh 150 lbs - 50 lbs of fat!?!  Aghhhh. I work out 5 days a week, and consider myself in pretty decent shape.  My trainer insisted this number was correct and that ideally I should be between 21-24.  33% ?! Really?  


Apparently-I'm-Fat Liz


Your body fat percentage should not fluctuate day to day, like your weight will on a scale, as it is a measurement of how much of your total body mass is composed of fat and how much of it is made of lean body mass (muscle, bones, organs, cells, etc).  Scales, on the other hand, will include how much water your body is storing, among other things, which will not only vary day to day but also hour to hour.

You are correct in assessing that body fat measuring scales will provide you with a number that will fluctuate based on many conditions, including how hydrated you are. These scales are good to get a basic understanding of your body composition but they are far from accurate and your trainer needs to realize that as well.

Our bodies need fat to function, and women require more fat than men. Men require 3-5% of their bodies to be comprised of fat, where as women need a minimum of 8-12%. What this means is if your body does not have the essential fat that it needs you cannot survive. According to the American Council on Exercise an average woman will measure in the range between 25 and 31% and we are considered fit if we measure between 21-24%. Athletes will be in a lower range of around 14-20%.

Your BMI is a simple equation that is measurement of your total body weight and your height. The appropriate range for a woman is between 18 and 25. The problem with this number is that is does not take into consideration what is lean body mass and what is fat body mass so it is quite possible to measure high when you are healthy and low when you are not.

If you would like to start making adjustments, do not try and do it all at once, as that can be quite overwhelming, but as I stress time and time again start making small changes each week. Stay away from the body fat scale for at least 12 weeks and focus on other means to track your progress such as your measurements, your improvement in strength and cardiovascular fitness, how your clothes are fitting and how you are feeling each day. 

In 12 weeks you should have made enough of an improvement to see a change in what the scale is reading, regardless of how accurate it is. You also must understand that as you become more fit, you may not see a huge change in your current weight but what will change is what that weight consists of.  Right now you are measuring at roughly 100 pounds of lean body mass and 50 pounds of fat mass. In 12 weeks you could be reading at 110 pounds of lean body mass and 35 pounds of fat mass.  Even though the normal weight scale will only be reading a 5 pound reduction in weight loss your body fat will be significantly less, reading 24%, well in the healthy range and a very important thing to keep in mind.

Attached – Hilary Duff leaves the gym