Which is the Most Accurate Body Fat Calculator / Measurement Method?
September 08 2009
I get often asked about the most accurate body measurement method/calculator. Here we will look at the common body fat percent measurement methods to find the most accurate one
Body Mass Index (BMI)
Body Mass Index (BMI) is just simply weight (kg) divided by height (msquare) and is often used to classify people into categories based on disease risk. Click here to find your BMI
Problem 1. BMI just assumes your body be a tub of fat. So if you are an athlete or someone who lift weights, it just ignores all that hard-earned muscle and might classify as “overweight or obese”. How unfair is that?
Just to be sure, BMI do not estimate bodyfat. Equations are being developed to estimate bodyfat from BMI but the current equations have large errors (>5%) in estimating bodyfat.
Underwater (Hydrostatic) Weighing
Underwater weighing is considered as the Gold Standard among body fat measurement methods/calculators. It is based on the fact that density of muscle is higher than fat and hence muscle sinks and fat floats.
Problem 1. It is based on a two-component model and hence divides your body into two compartments: Fat Free Mass & Fat Mass. Fat free mainly mass includes bone, muscle, water (also glycogen, soft tissues) apart from body fat as shown below.
* Figure shows a two-compartment model and how it divides total body mass.
It assumes fat free mass and fat mass to have a constant density. But it is pretty clear that the density of your muscles and bones do change depending on age, illness, activity, and ethnicity.
If you are 18 and lift weights, your bone and muscle density will be a lot different from a 45 year old sedentary individual.
Problem 2. Fat-free mass is made up of proteins, water, & bone minerals. So if you drink a lot of water before measuring, it can’t help but think it is all muscle (lucky you).
Hydrostatic weighing is used as a standard to even validate other bodyfat measurement techniques. Now tell me how screwed up is that?
Skin Calipers
Skin calipers measures the fat under the skin (subcutaneous) in certain areas to estimate bodyfat. It assumes that subcutaneous fat is proportional the fat inside your body.
Problem 1.Body fat distribution & how well it correlates with the fat inside your body varies based on age, fatness levels , physical activity, and genetics. Some might have more on the thighs whereas others on their abs.
Problem 2. It is based on a two compartment model and hence also has all the problems mentioned above.
Problem 3. There are equations which measures skin thickness from 3, 7, & 9 sites. But, generally, as the measurements increases so does the errors.
Bioelectrical Impedance Analysis (BIA)
BIA is based on the fact that muscle is 72% water and hence is less resistant to the flow of current than fat. Usually BIA is measured by attaching electrode to your wrist and feet and passing a current through.
* Picture shows BIA with electrodes connected and the commonly used hand held & scale BIA.
Problem 1. As you might have guessed everything from drinking a lot of water, emptying your bowels, having more salt in your diet, having a high- carb diet can change the water levels, and along with it your bodyfat.
Problem 2. The commonly seen hand held or scales are called segmental body fat analysers -measures one half of the body -and the equations used in these are yet to be validated.
Problem 3. Since they are based on a two-compartment model they suffer from all the problems associated with a two compartment model.
DEXA
DEXA is slowly replacing to be the gold standard for body composition methods. DEXA is based on a three-compartment model than a 2-compartment model. It can measure the density of fat, bone minerals, and water & protein combined.
Problem 1. This is lot more accurate than the two compartment models but still assumes the density of water & protein. It also has a lot more assumptions which are too technical to make any sense.
Conclusions
- As you can see there are lots of errors & assumptions in the body fat methods/calculators we commonly use. So what really matters more than accuracy is reliability. Can the body fat method you use consistently keep track of your body fat changes “accurately”?
- Find a method that is cost effective, reliable, and easily accessible.
- Skin calipers (measured by the same technician) and BIA (measured under similar conditions) are cost effective, consistent, and easily accessible.
