The Corpus blog
Body composition
Body composition is the split between fat mass and fat-free mass, muscle, bone, organs, water, that make up your total weight. Two people can weigh the same and have completely different compositions, which is exactly why weight alone is such an incomplete signal for physique progress.
The two numbers worth tracking are body fat percentage (how much of your weight is fat) and FFMI (how much fat-free mass you carry relative to your height). Neither one is precise outside a lab, DEXA scans, calipers, bioelectrical scales, and photo-based estimates all carry real error, but a consistent method, read as a trend rather than a single snapshot, is genuinely useful.
The guides below cover what these numbers mean, how to calculate them, and how Corpus estimates both from a single guided photo check-in.
The Navy Body Fat Formula: How It Works and How Accurate It Is
The Navy formula estimates body fat from height, neck, waist and hips. Studies against DEXA found it reads about 4 to 6 points low and can miss weeks of fat loss.
DEXA Scan Alternatives You Can Use at Home
The DEXA scan alternatives worth using are the ones you can repeat: smart scales, calipers, tape and photo apps. What studies show each gets right and wrong.
Are Smart Scales Accurate for Body Fat?
Smart scales weigh you accurately but estimate body fat poorly. What studies comparing them with DEXA found, why hydration skews the number, and how to use one for a real trend.
Body Fat Percentage Chart for Men and Women (by Age)
Healthy body fat percentage ranges for men and women by age, from the study most charts are built on — plus what each range tends to look like, when abs show, and why charts disagree.
Cutting vs. Bulking vs. Recomp: How to Choose
Should you cut, bulk, or recomp? The evidence-based rates for each phase, a simple way to choose based on body fat and training experience, and how to track whether it's working.
How Much Muscle Can You Gain Naturally?
An honest answer to 'how much muscle can I gain in a month?' — what the research can and can't say, the FFMI data behind 'natural limits,' realistic rates of weight gain, and how to see slow progress.
Skinny Fat: What It Means and How to Fix It
Skinny fat means a normal weight with relatively high body fat and little muscle. What the research calls it, why BMI misses it, how to measure it, and what usually works.
DEXA vs. Calipers vs. BIA vs. Photo: How Accurate Is Each Body Fat Method?
A straight comparison of the four common ways to estimate body fat percentage, DEXA, skinfold calipers, bioelectrical impedance, and photo-based estimates, and what each one actually gets wrong.
What Body Fat Percentage Actually Looks Like
Body fat percentage numbers mean little without a visual reference. Here's what the common ranges actually tend to look like, and why the same number can look different on different people.
FFMI, Explained Without the Bro Science
Understand Fat-Free Mass Index, how to calculate it, why body-fat estimates affect it, and how to use an FFMI trend with appropriate caution.
Common questions on this topic
What is the Navy formula for body fat percentage?
It's a regression equation developed at the Naval Health Research Center in San Diego that estimates body fat from height and neck and waist circumferences, plus hip circumference for women. A calculator does the logarithm math for you.
Full guide: The Navy Body Fat Formula: How It Works and How Accurate It Is →Is the Navy body fat formula different for women?
Yes. Women's version adds hip circumference and measures the waist at its narrowest point. In 62 women aged 40–60, the Navy equation averaged 30.3% body fat against 29.5% by underwater weighing, but individual results didn't agree well.
Full guide: The Navy Body Fat Formula: How It Works and How Accurate It Is →What is the best alternative to a DEXA scan?
For a single accurate reading, nothing at home replaces DEXA. For tracking change, the best alternative is a method you can repeat under identical conditions every week, such as a smart scale used at the same time each morning, the same person taking skinfolds, a tape measure, or a photo-based app.
Full guide: DEXA Scan Alternatives You Can Use at Home →Is a smart scale as good as a DEXA scan?
For weight, close: three smart scales were within a median of 0 to 0.3 kg of DEXA. For body fat, no: the same scales underestimated fat mass by a median of 2.2 to 4.4 kg, and the authors concluded they should not replace DEXA in patient care.
Full guide: DEXA Scan Alternatives You Can Use at Home →Are smart scales accurate for body fat percentage?
Not very. In a 2021 study, three consumer smart scales underestimated fat mass compared with DEXA by a median of 2.2 to 4.4 kg, even though they measured body weight within a median of 0 to 0.3 kg.
Full guide: Are Smart Scales Accurate for Body Fat? →Which type of smart scale is most accurate?
In a 2023 study of fifteen impedance devices, octapolar models — the ones with hand grips as well as foot pads — and some foot-to-foot scales performed best, particularly for tracking change over time. Performance varied widely between models.
Full guide: Are Smart Scales Accurate for Body Fat? →What is a healthy body fat percentage for a woman?
For white and African American women aged 20–39, a large study linking body fat to healthy BMI limits found a range of roughly 21–33%. It rose to about 23–34% for ages 40–59 and 24–36% for ages 60–79.
Full guide: Body Fat Percentage Chart for Men and Women (by Age) →What is a healthy body fat percentage for a man?
For white and African American men aged 20–39, the same study found roughly 8–20%, rising to about 11–22% for ages 40–59 and 13–25% for ages 60–79.
Full guide: Body Fat Percentage Chart for Men and Women (by Age) →