I'm sorry to tell you that the best way to measure body fat is expensive and very inconvenient. It's also at odds with your training goals, whatever they might be, because to undergo it you have to be dead.

When scientists want to be really precise, they dissect a cadaver, putting the fatty tissue in a bucket and setting the non-fatty tissue aside. They weigh, they measure, they calculate, and finally they can give a pretty darn good estimate of how much of the cadaver's body was made of fat. Obviously, this is no use to the cadaver. But it's invaluable to the scientists, because they can compare the results to pre-dissection measurements of the bodies, and figure out how such-and-such caliper reading relates to the actual amount of body fat.

Bottom line, don't ask what's the most accurate way to measure body fat. Now you know, and you don't want it.

There are other ways to measure your body fat percentage, but here's the rub: each of them is a calculated estimate, and it's pretty much impossible to know how much fat is on your actual living body. I'll give you some information about how a few different methods compare to each other, and you can decide which you'd like to use—if any.

The 'gold standards'

Once upon a time, the gold standard of body composition analysis was underwater weighing. Some people say it still is, although most research has moved on to using DXA scans. And then the Bod Pod people say theirs is the gold standard. There's no clear answer, especially when, say, underwater weighing gives answers that are 7 percent different from DXA. (That study claimed that DXA was wrong by 7 percent, but you could flip it around and say that underwater weighing is wrong, or that they both are.)

Underwater weighing involves, literally, weighing a person while they are underwater and the water is pushing up on their body making it lighter. You plug this weight into an equation along with the person's weight on land, the volume of water they displaced, and a handful of correction factors (even if they breathed out all the air in their lungs, as directed, how much gas are we guessing was in their intestines?) and out pops a body fat percentage. This depends on density, of course, and the notion that fat is lighter than muscle and bone.

The hiccup here is that the exact density of fat-free body mass is not the same for everybody; one study calculated body fat percentages for football players that were in the negative numbers. The researchers pulled out their calipers and verified that the players did have fat deposits on their body, so their muscle and bone must have been significantly denser than the equations assumed. Since you can measure body density in water, it was only a matter of time before somebody came up with a way of doing it in air. That's the method that you'll call "air plethysmography" if you're trying to impress your friends, or "the Bod Pod" if you're trying to spell it correctly. A review of accuracy studies found that Bod Pod, DXA, and underwater weighing agreed to within an average of 1 percent for the adults they tested. That sounds great, but some studies had larger discrepancies, up to 4 percent. They suggest that some of the techs didn't know how to work their machines properly, and that some people's bodies are harder to measure. So that's a great result on average, but you as an individual might end up with one of the less-accurate readings. The subjects in the accuracy tests were described as "healthy men and women," so they weren't necessarily athletes. The same density issues that the football players ran into are very likely to be a factor for the Bod Pod, too.

So what is DXA anyway? Sometimes spelled DEXA, it's a body scan based on X-rays (but with less radiation exposure than a conventional X-ray). It literally gives you a picture of your bone, muscle, and fat tissue, and if your right arm is bigger than your left (true for most righties, just because we use it more) you'll see that reflected in the numbers. You don't have to get into a swimsuit, so it's quicker and easier than underwater weighing or air plethysmography, but it's also the most expensive. (Prices vary, but you're looking at $100 or more for DXA, vs. $50ish for a visit to the Bod Pod.)

Like the other "gold standard" methods, DXA gives great results on average, but the numbers for any one individual (for example, you) could still be off by a lot – up to 4 percent in one study of bodybuilders.

Bioelectric sensors

So maybe you don't want to spend big bucks on a test that has questionable accuracy, even if it is really awesome (seriously, I've never been in a Bod Pod, but I bet you feel like a naked astronaut). For the lazy and/or budget-conscious, there are some lower tech options.

First, I've got to mention those scales with the metal footpads. There are also handheld devices and, for the serious researcher, machines with electrodes. They work by bioelectrical impedance analysis – basically, the idea that electricity flows more freely through fat-free tissues than through fat.

These devices have a reputation for not being very accurate, although the science behind them isn't so bad. The problem is that your hydration status can throw off the readings—if you are dehydrated, you'll show up as having more body fat, and if you're well hydrated, the number will be low.

Your workout itself can also affect your results, so if the scale lives at the gym, step on it before your workout rather than after. It can also read differently after you've had a meal.
Since you're not shelling out big bucks every time you step on the scale, you can use it a little differently than you might use one of the gold-standard methods above. Get into a routine so that you use it consistently (always before a meal, before a workout, when you're well-hydrated, for example). Then, rather than trying to guess what your body fat percentage is in absolute terms, you can use the scale's reading to monitor progress: is it going up or down? By a lot or a little?

Calipers

Finally, here's the dirt on calipers. They're widely hailed as cheap, simple, and pretty darn accurate. Are they your best bet?

You won't be surprised to hear that their accuracy can vary a lot, and in this case much of the variation comes from the person wielding the calipers. Are they picking the same spot for a skin fold every time? Are they using just the right amount of pressure?

It takes 50 to 100 practice measurements (depending on who you ask) before someone is really good at doing caliper measurements. It's possible to buy calipers to use on yourself or a friend, but unless you're committed to practicing, you're better off getting measured by somebody who really knows their skinfolds. If it were me? I'd try to get the same person every time, since everybody will be slightly different in their technique.

Which brings us to another source of error: which body parts do they measure? One classic set of measurements is the bicep, tricep, subscapular (shoulder blade), and iliac crest (the ridge on the edge of your pelvis). Since this only includes your upper body, it's turned out to be very inaccurate for athletes whose legs are well developed, and even generally for healthy young people. Adding a thigh and a calf fold improves accuracy by a lot.

But there's still the question of turning skinfold measurements into a percentage of body fat, which is trickier than it sounds. There are a variety of equations available; take your pick. A study of elite soccer players found that all of the caliper equations (except for the seven-site one) resulted in "unacceptable high overestimation" of body fat. Once again, we're in the realm of tests designed for non-athletes, so take the measurements with a grain of salt.

That study found another interesting source of variation: not only does the accuracy of caliper calculations vary from body part to body part, but it also varies based on where the player is in their training cycle (pre-season, competitive season, etc.). This could be a factor for other athletes, too.

But one really useful thing came out of this study: although it won't give you a number for body fat percent, you can add up your skinfold caliper measurements and use that as your number to monitor during training. The researchers verified that if you gain or lose body fat, the combined measurement will reflect that.

Whatever method you choose, you have to accept that you'll never know your exact percentage for sure. But that's okay. Pick a method that works for you, and focus on consistency. If you choose a bioimpedance method, follow the same routine every time with hydration, meals, and workout timing. If you choose calipers, get an experienced person to measure you, and make sure you jot down the actual numbers rather than taking their word for a percentage. And if you want to splurge on a Bod Pod or DXA scan, go for it, but don't mistake the results for absolute truth. You can only get that when you're dead.