If you say you want to lose weight, you'll probably measure progress by stepping on a scale. But what you're trying to lose is fat, and the number on the scale may not reflect that. There are many ways to measure your body fat percentage, but they all come with different levels of (in)accuracy.
Knowing your body fat isn't just for tracking weight loss; it can provide a reality check against body mass index, which doctors and others use to tell whether you are overweight. BMI disagrees with body fat percentage in about 18% of people (although there's plenty of room for argument over those numbers). If you have a lot of muscle, you could show up as obese on the scale even if you don't carry much fat; and if you don't, you could have a high-ish body fat percentage while showing up as "normal" in the BMI charts.
This doesn't mean you need to track your body fat. For some of us, having an extra number to obsess over is a distraction from our real goals of getting healthier or stronger. If you're just curious where you fall, this photo guide can help. But if you really want the most accurate measure of your body fat that science can provide, read on.
Fancy Measurement Methods
In reality, there are only a handful of people who have experienced the most accurate method of body fat composition: cadaver dissection. Which means that these people are all dead. Scientists have used dissection to calibrate other methods like those described below.
Here's the problem: no method is guaranteed to be accurate, and even the methods considered very accurate can disagree with each other pretty substantially. Most of the research on accuracy was done on white male non-athletes, so that's an extra caveat if you don't fit the description.
Underwater Weighing is the classic method, and the basis for more complicated methods used in research. Your weight on land, your weight under water, and the amount of water you displaced, Archimedes-style, factor into an equation that gives your body fat percentage.
This method works because fat is lighter than muscle and bone. But those densities vary between people. In one study published in the Canadian Journal of Applied Sport Science, researchers weighed elite soccer players underwater, applied the standard formulas, and came up with negative body fat percentages for some of them, apparently because they had denser muscles and bones than the equations assumed. Age, sex and race also affect tissue density.
Dual X-Ray Absorptiometry (DXA) Scans are the newer gold standard. This is the medical scan more often used to measure bone density, but it's possible to use it for body fat measurement too. You lay on a table while the machine uses low-dose X-rays to construct an image of your body that can then be measured. This method is pretty good at estimating the density of bones and muscles, and figures out fat by subtraction. The cost starts at $US100, going up to the multiple hundreds (not covered by insurance) at some hospitals.
Here's the problem: The two gold standards — underwater weighing and DXA scans — disagree by an average of 7 per cent, according to a review in the European Journal of Clinical Nutrition. Other studies have found the discrepancies to be smaller. There's no way to say which is "correct", so you'll just have to live with the knowledge that accuracy is elusive.
The BodPod, which uses air plethysmography, is a density-based measurement that works on the same principle as underwater weighing: Knowing how much space you take up, and comparing that to your weight, we can calculate how much of your body is dense muscle and bone versus less-dense fat. The same caveats apply: you, personally, may have different body tissue densities than the equations assume.
Are any of these methods worth it? In my opinion, only if you're really curious about putting the most accurate possible number on your body fat percentage, and willing to live with the knowledge that you still won't know if it's right. I have a hard time imagining this person, but let's be honest: You're out there somewhere.
More Accessible Methods
Rather than chasing accuracy that doesn't exist, the best way to use body fat measurements is probably the same way you use the scale: to track changes over time. If your weight on the scale stays the same but your body fat decreases, it doesn't matter exactly what number your body fat is — the important thing is that you've managed to make that number go down.
The most practical measurements will be the ones that will reflect changes in your body composition, while still being cheap and easy enough that you can do them often. Here are your best options.
Skinfold Callipers, wielded by someone with experience, are probably your best bet. The consistency of measurements you get depends a lot on the person doing the measuring: are they picking the exact same area of skin to pinch each time? Are they using the same amount of pressure? It takes 50 to 100 practice measurements, depending on who you ask, for somebody to become really good at it. If your gym offers this service, and you can get the same person each time, definitely take advantage.
Measuring yourself isn't recommended, because it's hard to see the numbers on the callipers when you're pinching your own body. There is a brand of cheap callipers meant for self-measurement, though they're probably not as reliable as going to a pro.
You'll also need to decide which body parts to measure, and what to do with the calliper numbers once you get them. You can take your pick of equations to calculate body fat. Some ask for three measurements (chest, abdomen, and thigh is one popular combo) and some ask for seven or more. Especially if you are young and athletic, you'll want to make sure you're including thigh and calf measurements, since your upper and lower body may have drastically different amounts of fat.
Rather than using a calculator to estimate body fat based on the measurements, some people just add the calliper measurements together, and track the total. If it goes up or down, you know your amount of fat is changing — and you don't have to worry about whether it's reflecting your "real" body fat percentage or now.
Bioelectrical Impedance Devices, including those scales with the silver foot pads, have a lot of drawbacks. They send a gentle electric current through your body, with the idea that fat has a different electrical resistance than other tissues. It's not a bad theory, but in practice there is a lot of variation, even day-to-day.
Your hydration status can change your bioelectrical reading. That means that your body fat reading may change from the beginning to the end of the workout; for women, it may change depending on when in your monthly cycle you measure. If you want to use this method, you have to be extremely consistent: measure each time at the same time of day, under the same conditions (say, after breakfast but before your workout) and same time of the month.
The type of device you use also matters: a scale with foot pads is only putting current through your legs; a handheld device is only using your upper body. And even when you're using a device consistently, the results can still vary. So if you seem to be losing fat but the device isn't showing a big change in numbers, don't be discouraged.
Tape-measure methods use the size of different body parts to calculate your body fat. The standard caveats apply: the equation may be based on people who are different than you, and variation in body shape can result in a large difference in the results. The tape measure also can't tell the difference between muscle and fat. If you gain inches in your thigh, for example, is that a good thing (more muscle) or a bad thing (more fat)? Callipers don't have this problem because pinching the skin separates fat from muscle.
The Bottom Line
Expensive, get-it-done-in-a-lab methods are the most accurate, but none will give you a guaranteed correct number for your body fat percentage. If you want to track your body fat on the cheap, callipers probably give the best tradeoff between convenience and accuracy. Don't forget that you can also gauge your progress by that old standby: noticing whether your clothes are getting looser or tighter.