How to Convert BMI to Body Fat Percentage: Formula Comparison
Quick Take
The Deurenberg formula converts BMI to body fat percentage by accounting for age and sex. It is simple to use but less accurate than circumference-based methods like the US Navy formula. Choose Deurenberg when you only have height and weight; use the Navy method when you can measure a few body circumferences.
BMI is a useful screening tool, but it has a well-known limitation: it does not tell you how much of your weight is fat versus muscle. This is why many people want to convert their BMI number into a body fat percentage. The conversion is not a simple lookup table — several validated formulas exist, each with different assumptions and accuracy tradeoffs.
This article walks through the three most common approaches: the Deurenberg formula (pure BMI-to-body-fat conversion), the US Navy method (circumference-based), and a few alternative formulas. We will look at how each works, what input data it needs, and how accurate you can expect the result to be.
For hands-on calculations, the Body Fat Calculator on VivMetric implements both the Deurenberg formula and the US Navy method side by side, so you can compare results instantly with your own measurements.
Why Convert BMI to Body Fat in the First Place?
BMI was never designed to estimate body fat. It is a simple ratio of weight to height squared that correlates loosely with adiposity at the population level. For individuals, BMI can be misleading — a muscular football player might have a BMI of 28 with only 15 percent body fat, while a sedentary office worker with the same BMI could carry 35 percent body fat.
Converting BMI to an estimated body fat percentage gives you a more interpretable number for fitness and health planning. Body fat percentage maps more directly to health risk categories than raw BMI, especially when evaluating individuals outside the "average" body composition range.
The Deurenberg Formula: The Classic BMI-to-Body-Fat Conversion
The Deurenberg formula, published in 1991, remains the most widely used method for converting BMI to body fat because it requires only four inputs: BMI, age, sex, and (optionally) whether the person is athletic. The formula was developed using data from over 800 adult subjects with direct body fat measurements via densitometry.
The Formula
Body Fat % = 1.2 × BMI + 0.23 × Age - 10.8 × Sex - 5.4
Where "Sex" equals 1 for men and 0 for women. The calculation works with BMI as a unitless number (already calculated from height and weight in either imperial or metric units). The output is estimated body fat percentage.
Worked Example
Let us take a 35-year-old man with a BMI of 26. Using the Deurenberg formula: 1.2 × 26 + 0.23 × 35 - 10.8 × 1 - 5.4 = 31.2 + 8.05 - 10.8 - 5.4 = 23.05 percent body fat.
For a 35-year-old woman with the same BMI of 26: 1.2 × 26 + 0.23 × 35 - 10.8 × 0 - 5.4 = 31.2 + 8.05 - 0 - 5.4 = 33.85 percent body fat. Note how the sex term shifts the estimate up by about 10.8 percentage points for women — reflecting the biological difference in body fat distribution between sexes.
Strengths and Weaknesses
The Deurenberg formula works well for the general adult population. It is fast, requires no tape measure, and has a reported accuracy within about 4 percent body fat of DEXA measurements for most users. However, it struggles with two groups:
Athletes and very muscular individuals. The formula tends to overestimate body fat for people with significant muscle mass because their higher BMI is driven by lean mass, not fat. The optional "athlete" adjustment (subtracting 1 to 2 percentage points) can help, but it is a rough correction.
Elderly adults. The formula may underestimate body fat in older individuals because it does not account for age-related loss of lean mass, which effectively increases body fat percentage even when total weight stays the same.
Alternative Formulas: Comparing the Options
Several alternative formulas have been developed for specific populations or use additional inputs. Here are the most relevant ones for practical use.
US Navy Method (Hodgdon & Beckett)
Developed by the US Navy in 1984 and updated in 2008, this method uses height plus two or three circumference measurements (waist, neck, and for women, hip) to estimate body fat. It is significantly more accurate than the pure Deurenberg formula because it accounts for body shape — a proxy for fat distribution.
For men: Body Fat % = 86.010 × log10(waist - neck) - 70.041 × log10(height) + 36.76
For women: Body Fat % = 163.205 × log10(waist + hip - neck) - 97.684 × log10(height) - 78.387
The US Navy method has a typical accuracy of about 2.5 to 3.5 percent body fat compared with DEXA — notably better than Deurenberg for most body types. The tradeoff is that you need a tape measure and the ability to measure waist, neck, and hip circumferences consistently.
Jackson-Pollock 3-Site Formula
The Jackson-Pollock formula uses three skinfold measurements (chest, abdomen, thigh for men; triceps, suprailiac, thigh for women) plus age. It is more accurate than the Deurenberg formula when skinfolds are measured correctly, but it requires calipers and training to measure consistently. This is more of a clinical or personal-training tool than a home-use method.
BMI-Based Simplified Estimate
For a very rough approximation, some sources use a simplified version: body fat percentage roughly equals BMI plus 5 to 10 for men and BMI plus 10 to 15 for women, depending on age. This is not a validated formula but can give a quick mental estimate when you have nothing else. We do not recommend relying on this for anything beyond casual curiosity.
Accuracy Comparison: What the Research Shows
Multiple studies have compared body fat estimation formulas against DEXA, the clinical reference standard. Here is a summary of typical accuracy ranges from peer-reviewed research:
| Formula | Inputs Required | Typical Error vs DEXA | Best For |
|---|---|---|---|
| Deurenberg | BMI, age, sex | ± 3.5 – 5.0% | General population, quick estimates |
| US Navy | Height, waist, neck, hip | ± 2.5 – 3.5% | Active adults, reasonable accuracy |
| Jackson-Pollock | 3 skinfolds, age, sex | ± 2.0 – 3.0% | Trained personnel, athletes |
| BIA scales | Weight, height, age, sex | ± 3.0 – 5.0% | Daily tracking at home |
When to Use Which Method
There is no single "best" formula — the right choice depends on what you have to work with and how precise you need the result to be.
Use Deurenberg when: You only have BMI (height and weight) and want a quick, reasonable estimate. This is ideal for casual self-assessment or when comparing against population norms where perfect accuracy is not critical.
Use US Navy when: You can take waist and neck (plus hip for women) measurements and want a more accurate result. This is the method we recommend for most home users. The Body Fat Calculator on VivMetric supports both approaches and lets you compare them side by side.
Use clinical methods when: You need precise body fat data for medical diagnosis, competitive bodybuilding, or research. DEXA, CT, and MRI remain the gold standards and cannot be replaced by any formula-based estimate.
Important Limitations to Remember
No formula-based conversion accounts for body fat distribution — where your fat is stored matters for health risk, and formulas do not capture this. Someone with 25 percent body fat concentrated around the abdomen has a very different metabolic risk than someone with 25 percent body fat distributed in the hips and thighs.
This is why we always recommend combining body fat estimation with waist circumference or waist-to-height ratio measurements. The Waist-to-Height Ratio Calculator provides central adiposity context that body fat percentage alone cannot.