What is the formula for body fat? Methods and Calculations Explained

January 5, 2026 •

4 min read

Unlike BMI, which only considers height and weight, body fat percentage provides a more direct measure of a person's fat mass relative to total body mass. So, what is the formula for body fat and why isn't there just one? The answer depends entirely on the method used, as a single, universal equation does not exist.

Quick Summary

The formula for body fat varies significantly by the method used, with options ranging from simple equations based on BMI to more complex calculations requiring skinfold or circumference measurements.

Key Points

No Single Formula: There is no single universal formula for body fat; instead, different methods rely on different sets of calculations.
BMI-Based Estimates: The simplest formulas use your Body Mass Index (BMI), age, and gender, but they are often the least accurate, particularly for muscular individuals.
Skinfold Calipers: Caliper methods like Jackson-Pollock and Durnin-Womersley use a sum of subcutaneous fat measurements at specific sites, which is then used to calculate body density and fat percentage.
Circumference Method: Formulas like the U.S. Navy Method use tape measure readings of the waist, neck, and hips to estimate body fat, offering a simple at-home option.
Advanced Technology: Methods like Bioelectrical Impedance Analysis (BIA) and DEXA scans use sophisticated technology and proprietary formulas to provide more accurate assessments, though at a higher cost or requiring a clinic visit.
Consistency is Key: Regardless of the formula or method chosen, consistency is paramount. Always use the same technique and conditions to track your body fat changes over time.

The Basic Concept of Body Fat Calculation

At its core, body fat percentage is a ratio of your fat mass to your total body mass. The most fundamental formula is:

$Body Fat Percentage = (Fat Mass / Total Body Mass) x 100$

However, determining an accurate fat mass without specialized equipment is challenging. Consequently, various predictive formulas have been developed that use more accessible measurements like height, weight, age, gender, and specific body girths or skinfold thicknesses to estimate overall body density or fat percentage. Each method has a different formula and its own level of accuracy.

BMI-Based Body Fat Formulas

One of the simplest and most accessible methods for estimating body fat uses your Body Mass Index (BMI). While BMI is a flawed measure of body fat on its own, it can be combined with other factors like age and gender to produce a general estimate. These regression equations are not a replacement for more precise methods, but they are useful for a quick, at-home approximation.

For Adult Males: $Body Fat Percentage = (1.20 x BMI) + (0.23 x Age) - 16.2$

For Adult Females: $Body Fat Percentage = (1.20 x BMI) + (0.23 x Age) - 5.4$

These formulas are simple to use but can be inaccurate, particularly for individuals with high muscle mass. The BMI does not differentiate between weight from fat and weight from muscle, which can skew the results significantly.

Skinfold Caliper Methods

Skinfold calipers measure the thickness of subcutaneous fat (fat just under the skin) at several standardized points on the body. These measurements are then used in gender- and age-specific formulas to calculate body density, which is then converted into a body fat percentage using the Siri equation or a similar formula.

The Siri Equation (used to convert body density to body fat %): $Body Fat Percentage = (495 / Body Density) - 450$

The Jackson-Pollock Method

This is one of the most widely used skinfold methods, with different versions using 3, 4, or 7 measurement sites. The formulas calculate body density based on the sum of skinfold measurements and age.

Jackson-Pollock 7-Site Formula (for Men): $Body Density = 1.112 - 0.00043499(sum of 7 skinfolds) + 0.00000055(sum of 7 skinfolds)^2 - 0.00028826(Age)$

Jackson-Pollock 7-Site Formula (for Women): $Body Density = 1.097 - 0.00046971(sum of 7 skinfolds) + 0.00000056(sum of 7 skinfolds)^2 - 0.00012828(Age)$

Jackson-Pollock 3-Site Skinfold Sites:

Men: Abdominal, Chest, and Thigh
Women: Triceps, Suprailiac, and Thigh

The Durnin-Womersley Method

This method uses four skinfold sites and relies on logarithmic equations to calculate body density, which is then converted using the Siri formula.

Durnin-Womersley 4-Site Skinfold Sites:

Biceps
Triceps
Subscapular
Suprailiac

Circumference Measurement Methods

The U.S. Navy Method is a popular example of a circumference-based estimation. It uses a tape measure to take several body measurements, which are then plugged into a gender-specific formula. While not as precise as calipers, it's easier for self-measurement.

U.S. Navy Formula (for Men): $Body Fat % = 86.010 x log{10}(abdomen - neck) - 70.041 x log{10}(height) + 36.76$

U.S. Navy Formula (for Women): $Body Fat % = 163.205 x log{10}(waist + hip - neck) - 97.684 x log{10}(height) - 78.387$

Bioelectrical Impedance Analysis (BIA)

BIA devices, such as smart scales and handheld monitors, pass a weak electrical current through the body. The formula used for BIA is proprietary to the manufacturer but is based on the principle that electrical currents move faster through fat-free mass (which contains more water) than through fat mass. These devices use complex algorithms involving resistance, reactance, height, and weight to predict body composition. Accuracy can be affected by hydration levels, time of day, and food intake.

Comparing Body Fat Measurement Methods

Here is a comparison of different body fat assessment methods to help you choose the right one for your needs.


Method	Accuracy	Cost	Convenience	Considerations
BMI-based Formula	Low	Free	Very High (requires only BMI)	Does not distinguish between muscle and fat. Highly inaccurate for athletes.
Skinfold Calipers	Moderate to High	Low	Moderate (requires practice)	Technician skill is critical for accuracy. Requires privacy and access to calipers.
Circumference Measurements	Moderate	Very Low	High (requires only tape measure)	Easier for self-assessment but less precise than calipers. Affected by measurement consistency.
BIA (Smart Scales)	Variable	Moderate	High (at-home convenience)	Accuracy heavily depends on hydration levels. Can be inconsistent. Proprietary formulas.
DEXA Scan	Very High	High	Low (clinic visit required)	Gold standard for body composition. Expensive and less accessible.

Conclusion

In summary, there is no single universal formula for body fat; instead, there are multiple methods, each with its own underlying formula and level of accuracy. The best approach depends on your priorities regarding accuracy, cost, and convenience. While BMI-based equations and circumference methods offer a convenient estimate, skinfold calipers provide a more hands-on and relatively accurate measurement for those willing to practice. For the most precise results, advanced clinical methods like a DEXA scan are the gold standard. When assessing your body composition, consistency is key, regardless of the formula you choose. Always use the same method and conditions over time to track changes most effectively.

For more information on the most accurate methods available, you can consult authoritative resources like the National Institutes of Health.

Frequently Asked Questions

No, there is no single standard formula. The calculation used for body fat percentage varies depending on the measurement method, with different equations for BMI-based estimates, skinfold calipers, and circumference measurements.

BMI-based formulas offer a quick estimate but are not highly accurate, especially for individuals with different body compositions. Because BMI doesn't distinguish between muscle and fat, these formulas can significantly overestimate fat in muscular people.

The Siri equation is a formula used to convert a person's body density into an estimated body fat percentage. It is typically used as the final step after calculating body density from skinfold measurements or other methods.

Skinfold calipers measure the thickness of subcutaneous fat at several designated sites on the body. These measurements are then summed and entered into a specific formula, like the Jackson-Pollock or Durnin-Womersley equation, to estimate body density and then body fat.

The U.S. Navy body fat formula is a simple and accessible method that uses a tape measure. While it is more accurate than a basic BMI calculation, its reliability can be affected by human error in taking measurements.

Yes, hydration levels significantly affect the accuracy of some methods, particularly Bioelectrical Impedance Analysis (BIA), which is found in many smart scales. Since the devices use the body's conductivity to estimate body composition, dehydration can lead to inaccurate readings.

Advanced clinical methods, such as Dual-Energy X-ray Absorptiometry (DEXA) scans, are considered the most accurate way to measure body fat. While less accessible and more expensive, they provide a very precise and detailed body composition analysis.