A Quick Guide To Estimating Body-fat Percentage

Finding your body-fat percentage is an imprecise endeavor.

The methods that we generally have available to measure it range from being ‘acceptably accurate’ to ‘very poor,’ and they are nearly all useless for tracking changes over time. – They’re all marketed as being really accurate of course, but that’s just because people are after your dollars.

The lack of a guide on this site to help you find this has been something that has bugged me for some time, however it seems like I’ve found a fix for that – a quick calculation to help you to get a good estimation of your body-fat percentage that can be used to help you set up your diet more precisely.

Estimating Body-fat Percentage

It doesn’t really matter what your body-fat percentage is, you just need to get an estimation of it for your calorie intake calculations and setting your fat and protein intake targets. We don’t need to be exact, we just need to not be too far off.

Here’s a rundown of the methods we typically have available to us for measuring body fat, from most to least accurate. I’m basing this on their standard error of estimate (SEE) – which basically means how accurate they are for most people:

1. Die in a non-too-messy way that requires an autopsy.
2. DEXA scan, ~1-2% (Expensive, inconvenient.)
3. The US Navy Equation based on body measurements below. ~3% (Convenient, free.)
4. BodPod / Underwater weighing, ~3% (Expensive, inconvenient.)
5. Body-fat caliper measurements. Skilled practitioner, ~3%. Non-skilled practitioner ~5%.
6. BIA machines (Omron, Tanita), usually found in commercial gyms, 5-8% (Best avoided.)

So, if you use a BIA machine and get a reading of  20%, your actual body fat could be anywhere from 12-28%. If you use the US Navy equation and get a reading of 20%, your actual body fat could be anywhere from 17-23%. This is the difference between being useless and helpful.

Here’s that US navy formula. Take a look and then we’ll get into some practical recommendations:

Measurement Guidelines

• Height – Get someone else to do it for you if possible.
• Stomach – Measure at the navel. Have a relaxed stomach, exhaled but, don’t forcefully push it out!
• Neck – Keep your head straight, look forward. Keep your shoulders down and relaxed.
• Measure three times for each and average the three.

Simple as that.

Recommendations

The US Navy equation wins out on convenience and cost, this is what I’d recommend for most people. Notice that the reading is heavily determined by the measurement at the navel. Two important things to point out in regards to this:

1. Fat comes off of the stomach/torso generally from the top down. Past the point of 10% body fat, the mid-stomach measurement (that at the navel, which the equation uses) will change very little as the person gets leaner, because most of the fat is coming off from the lower abs and back at this point. Therefore, if you’re already very lean, this won’t work well for you. It will be best to estimate from pictures, or use calipers.
2. It will give those with particularly thick (well developed) abs, or a bloated abdomen higher readings than reality.

Now, let’s put vanity concerns aside for a moment and focus on practical concerns of getting body-fat percentage incorrect. What does that mean for your calculations?

Well, let’s say that you are a 80 kg (176 lb) male, at 15% body fat, but you calculated it to be 20%. You’ll underestimate your calorie needs by ~115 kcal. You’ll eat ~10g less protein than may be ideal, and your fat intake may be 5-8g lower than it should.

Is this a big deal? No. So I really wouldn’t worry about it too much.

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Anticipated Questions

Do you have the calculation for women?

Sure thing. The reason it wasn’t included is because I know that the majority of the site’s readers are men. Here are the full, imperial calculations for both men and women:

• Men: Body-fat % = 86.010 x log10(abdomen – neck) – 70.041 x log10(height) + 36.76
• Women: Body-fat % = 163.205 x log10(waist + hip – neck) – 97.684 x log10(height) – 78.387

“This looks inaccurate for me…”

Yes, you may be right. It’ll work well for the majority of people, but you could be the exception to the rule. If this can be used to get close to what is correct, then that will do, because it’s as good as or better than whatever else most people have available.

If you believe that you have a better measurement from elsewhere then by all means, please use that. Remember, this is not being suggested a means of tracking your progress but just a start point for which to base calculations, which will need to be adjusted based on real world progress anyway.

Now, the comment section is going to develop a natural selection bias towards people complaining about this calculation being wrong because people rarely write to say that something worked out right, and no-one ever complains if they are told they are leaner than they thought. So, before writing to tell me that, here are some reasons for inaccuracies that may be worth considering:

1. Measurement error,
2. Comparison with some other measurement device that was also inaccurate.
3. You are the relatively rare exception to the people that it will work well for.

If you’d like to read the full US MoD paper on the way the formulas were developed so you can make your own decision, you can get that here.

How do you gauge body-fat percentage when setting things up for clients?

I just do it by eye, looking at photos, but I’m an experienced coach so I know what to look for and ask for when I get people to take those photos. Obviously most people reading this don’t have that available as an option, hence this guide.

If I post a photo link in the comments can you tell me what you think my body-fat percentage is?

Sorry, but no. I’ll just be flooded with requests if I do and that’s not how I want to spend my time. Besides, I think talking about specific numbers puts the focus on the wrong thing – what matters is how the body measurements combined with scale weight, strength and appearance are changing over time. Make sure you don’t screw up your chances of being successful, check out my detailed progress tracking guide here.

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Hope this was helpful. Questions welcomed in the comments. – Andy