How to Calculate Body Fat Percentage

When I meet a new personal training client, part of the initial consultation is measuring their body fat percentage. Every personal trainer in London does this (or should!) with each new client, as it is a significant statistic of health & fitness, and enables you to monitor progress.

An optimum range for men is 12 – 18%, and for women it’s 22 – 28% as women require more body fat than men for a range of biological reasons. One of my male personal training clients in east London was 42% body fat when I started with him, and I got him down to 28%, below the danger zone. Males with body fat of 30% and over are technically obese, and for women it’s 40% and over.

To determine body fat percentage I use an Omron body composition monitor, but you can also use calipers and take readings from six points on the body. However, the caliper test requires skill on the part of the personal trainer, and two different people measuring the same client could come up with two different results.

Calculating muscle gain/loss

Once you know someone’s weight and body fat percentage, you can work out their lean mass (body weight minus fat, ie muscle, bones, water, organs).

Example 1: Male weighs 210 lbs and his body fat percentage is 30%.

His fat mass is 210 x 30% = 63 lbs.
His lean mass is 210 – 63   = 147 lbs.

His personal trainer puts him through a three month regime of exercise and good nutrition, and he is tested again.

His new weight is 195 lbs, and his body fat percentage is 25%.

How much fat has he gained or lost, and how much muscle has he gained or lost?
If the personal trainer did a good job, he should have lost significant body fat, and gained some muscle.

His new fat mass is 195 x 25% = 49lbs.
He has lost 14 lbs after 3 months training, which is 1 stone, not a bad result.

His new lean body mass is 195 – 49 = 146lbs.

This is 1 lb less than before he trained, not ideal because as every personal trainer knows, part of any improvement in physical health and fitness is an increase in muscle mass. He has probably been on an exercise regime with too much cardio and light resistance training, rather than muscle-building exercise. However, to lose 14 lbs in fat is a good result.

Example 2: Female weighs 150 lbs and has 30% body fat. 

Her initial body fat mass is 150 x 30% = 45 lbs.
Her lean body mass is 150 – 45 = 105 lbs.

Her personal trainer puts her on a low calorie diet, and a regime of just cardio with no muscle-building exercise at all, and after a month she is measured again.

Her new weight is 130 lbs, and new body fat percentage is 27%. So she’s lost 20 lbs, and her body fat percentage is down 3 percentage points.

Her new fat mass is 130 x 27% = 35 lbs. So she’s lost 10 lbs of fat.

Her new lean mass is 130 – 35 = 95 lbs.

So she’s lost 10 lbs of lean mass, which is not good, as this is mainly muscle loss (and likely loss of glycogen stores and water). This is typical of low calorie starvation diets, and fasting regimes. The problem is that you do lose weight, but half this weight-loss in this case is muscle loss. The less muscle mass you have, the lower your resting metabolic rate, which means you’re less able to burn fat. This is why people who come off starvation diets put on all the fat they had lost and more on top, and worse still they are weaker than before they started the starvation diet.

To calculate the ideal weight for a particular body fat percentage

Example: a female weighs 145 lbs and her body fat percentage is 32%. This is just outside the healthy range, but certainly not obese. She wants to know what her ideal weight should be if her body fat were 21%.

Her current lean mass percentage is 68%
Her ideal lean mass percentage is 79%

Therefore her ideal weight is 68/79 x 145 = 0.86 x 145 = 125 lbs.

Dominic Londesborough is a personal trainer in London and author of the blog.