Last week, I discussed the reasons why I prefer to use body fat percentage instead of body mass index (BMI) as a measure of health. The body mass index calculates the ratio between your weight and your height to place you in a certain weight status category. If your ratio is 18.5-24.9, you are categorized as “normal weight.” If you are 25.0 or above, you are considered “overweight” or “obese.”
The formula cannot determine how much of your weight is lean mass (muscle, water, bone, blood, etc.) and how much is fat mass. So, use caution if using BMI to determine if you are the correct weight or how much weight to lose. Also, if you have a large amount of muscle mass, you may be considered overweight, according to your BMI. This is why measuring your body fat percentage is a preferred method.
Using Body Fat Percentage
Your body fat percentage measures the percentage of your body that’s fat mass and the percentage that’s lean mass. Our bodies need essential fat for insulation, energy production, nutrient absorption, and to make hormones. But too much body fat leads to increased risk for diabetes, heart diseases, high blood pressure, kidney disease, cancers, infertility, lowered immune function, and the list goes on and on.
There are various ways to measure your body fat, including bioelectrical impedance (handheld devices and scales), calipers, underwater weighing, and DEXA scans, to name a few. Here’s a great article describing different measurement methods and their pros and cons.
Once you calculate your body fat percentage, you will know how many pounds of fat you are carrying and whether or not you are carrying the recommended amount.
Women naturally carry more body fat than men, just in case we conceive, so our bodies can be ready for a healthy pregnancy.
The general recommendation is for women to have less than 25% body fat and for men to have less than 18%. Above these levels puts you at increased risk for developing chronic diseases. Also note, as you age, you can carry a higher percentage of body fat and still be considered healthy.
So how do you use these numbers to calculate how much weight to lose? Let’s do the math:
Take a 200lb woman, named Melissa, whose body fat percentage is 35%.
To find the pounds of fat, multiply 200 by 35%. 200 pounds X .35= 70 pounds of fat
To find the pounds of lean mass, subtract the pounds of fat from her total weight. 200 total pounds – 70 fat pounds= 130 lean pounds.
Melissa has 130 pounds of lean mass and 70 pounds of fat.
If her goal is to reach 25% body fat, we can guesstimate how much fat she needs to lose.
Stay with me, we are about to throw in a little algebra!
If Melissa maintained 130lbs of lean mass while losing fat, her lean mass would be 75% of her total weight (25% is fat mass, so 100% minus 25% equals 75% lean mass).
We now divide 130lbs by 75% to calculate what her total weight would be at 25% body fat. 130 pounds lean mass ÷ .75= 173 pounds total weight.
FINAL STEP! We subtract Melissa’s starting weight of 200 pounds minus 173 pounds and get 27 pounds.
So, Melissa would need to lose about 27 pounds of fat to get to 25%. That’s assuming she doesn’t gain any lean mass. Because her lean mass may increase slightly if she’s exercising, she may be able to lose less than 27lbs to get to 25% body fat.
…Are you still here lol?
I know… that was a lot of math, but it’s not nearly as complicated as it seems.
The great thing about this formula is that it does not depend on your height. It just says hey, this is how much fat you have and here’s how much fat you need to lose. That’s it.
Now, let’s compare this to the body mass index.
Let’s say Melissa is 5’7. Her BMI is 31.3, which is considered overweight.
To be classified as “normal weight,” she would need to weigh 159lbs, which would make her BMI 24.9. Melissa needs to lose 41lbs according to the BMI chart.
But wait, according to her body fat percentage, she doesn’t need to lose 41lbs! She only needs to lose 27lbs to have a healthy body fat percentage.
If she lost 41lbs, and all of those pounds came from fat, she’d actually be at 18% body fat (She started with 70 pounds of fat, minus 41 pounds of fat lost= 29lbs of fat. Now, divide 29 pounds of fat by 159 total weight to get the body fat percentage).
Depending on the look Melissa wants to achieve, she may be leaner than she desires, but she may think she needs to be that size to be considered healthy. Wrong. Because of the amount of lean mass she started out with, she may not need to lose 41lbs. She can just lose 27 pounds to be in a healthy range. Make sense?
What happens when a person aims to lose weight and does not measure body fat percentage along the way? That person may also be losing MUSCLE while losing fat, but the person wouldn’t know.
When you lose muscle, you decrease your metabolic rate, making your body burn LESS calories throughout the day and making it harder to lose weight. Not so hot! Plus, muscle mass affects how your body looks and helps to create shape. And muscle is needed for strength, bone density, mobility, improved posture, and stability.
I recommend getting your body fat tested before setting weight loss goals. This will help you determine a realistic goal and plan accordingly to prioritize keeping your lean mass as you shred the fat. And you can make adjustments along the way.
It will also help you set healthy weight GAIN goals when aiming to increase lean mass. Many bodybuilders and gym enthusiasts go through “bulking” phases, a period when they increase their caloric intake to build more muscle. But many of them also unintentionally add on a large amount of fat in the process. Measuring your body fat percentage during that phase will help you determine what you are really gaining and minimize fat gained along the way.
I offer body fat testing through my personal training company, which is a complimentary service for clients and a nominal fee for non-clients.
If you would like a more accurate assessment, CompositionID Houston offers a DEXA scan, which also measures your bone mass and bone density, and it will also display how symmetrical (or non-symmetrical) you are. Most importantly, a DEXA scan can measure the amount of visceral fat you have internally, which is the fat covering your organs. Visceral fat is the fat most associated with chronic diseases.
I highly recommend getting a DEXA scan to assess your overall body composition. Use code “diggdeep25” to receive $25 off this service from CompositionID Houston.
Calculate Your Body Mass Index. National Heart, Lung, and Blood Institute website. https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm
Dietary Fats. American Heart Association website. March 23, 2014. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/dietary-fats
Health Risks of Being Overweight. National Institute of Diabetes and Digestive and Kidney Diseases website. https://www.niddk.nih.gov/health-information/weight-management/health-risks-overweight
The 10 Best Ways to Measure Your Body Fat Percentage. Tinsley, Grant. Healthline.com website. April 29, 2018. https://www.healthline.com/nutrition/ways-to-measure-body-fat
Body Fat Calculator. Active.com website. https://www.active.com/fitness/calculators/bodyfat