In physical assessments, when the body fat percentage was not entered in the assessment, the one shown next to the BMI is an estimate made by a formula from the assessment's measurements, not a measurement. MyFitnessClient calculates it every time the assessment opens and does not save it in the Fat field. This page covers which data go into the calculation, which method is used, the formulas, the sources and the categories.
Where the estimate appears
- On the web, in the view of an assessment, the BMI card shows the BMI and its category. When there is an estimate, the Body composition row shows the category by body fat percentage and, next to it, the percentage and the method's name, for example Jackson-Pollock 7 skinfolds. With the percentage entered in the assessment, it appears as “Fat”, without the method's name. The card only appears when the assessment has weight and height.
- A client with access to assessments sees the same card in their own assessments, on the web.
- In the app, the assessment shows the percentage among the highlights: Est. body fat when it is an estimate, or Fat when it was entered in the assessment. A note underneath gives the estimate's method and says that the values are indicative.
- The assessment PDF shows the percentage from the method saved in the assessment or, with no saved method, the one from Auto.
What the estimate needs
- Weight and height in the assessment. Without height there is no BMI, no estimate and no category, even with the percentage entered.
- Gender Male or Female in the client's profile. With Other, or with no gender, there is no estimate and no category by body fat percentage.
- Date of birth in the client's profile, for the skinfold methods. The age used is the age on the day of the assessment. The entered value and US Navy don't need it.
On the web, in the assessment opened by the professional, Gender, Age and Birth date appear under the client's name. Anything missing shows as Not set, with a warning that says what stops being calculated and the Fill in option, to enter it in the client's profile. The client can also fill in their gender and date of birth in their personal details: see Update your phone, tax ID and personal details.
Because the calculation runs every time the assessment opens, correcting the gender or the date of birth changes the estimate and the category of all of that client's assessments, including old ones.
Which method is used
With Auto, the default choice, MyFitnessClient uses the first method in this table that has all the data. Except for the entered value, a result only counts if it is greater than 0 and less than 70%; if not, it moves on to the next method.
| Order | Method | What it needs |
|---|---|---|
| 1 | Body fat % entered | The value entered in the assessment's Fat field, greater than 0, for example one from a bioimpedance scale. |
| 2 | Jackson-Pollock 7 skinfolds | All 7 skinfolds, each greater than 0: Chest, Midaxillary, Triceps, Subscapular, Suprailiac, Abdominal and Thigh. Uses age. |
| 3 | Jackson-Pollock 3 skinfolds | Men: Chest, Abdominal and Thigh. Women: Triceps, Suprailiac and Thigh. Uses age. |
| 4 | US Navy | Height, Neck and Waist, with the waist greater than the neck; for women, also Hips. Does not use age. |
The formulas
In the skinfold methods and in US Navy, the formula gives the body density D, and the Siri equation turns the density into a percentage: body fat % = (4.95 ÷ D − 4.5) × 100. For skinfolds, S is the sum of the skinfolds in mm and age counts in years; in US Navy, the measurements are in cm. The result is rounded to 1 decimal place.
| Method | Men | Women |
|---|---|---|
| Jackson-Pollock 7 skinfolds | D = 1.112 − 0.00043499 × S + 0.00000055 × S² − 0.00028826 × age | D = 1.097 − 0.00046971 × S + 0.00000056 × S² − 0.00012828 × age |
| Jackson-Pollock 3 skinfolds | S = Chest + Abdominal + Thigh. D = 1.10938 − 0.0008267 × S + 0.0000016 × S² − 0.0002574 × age | S = Triceps + Suprailiac + Thigh. D = 1.0994921 − 0.0009929 × S + 0.0000023 × S² − 0.0001392 × age |
| US Navy | D = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height) | D = 1.29579 − 0.35004 × log10(waist + hips − neck) + 0.22100 × log10(height) |
Categories by body fat percentage
The Body composition row classifies the percentage by sex and, from Normal up, also by age. At the lean end, the limits are the same at any age:
| Category | Men | Women |
|---|---|---|
| Underweight | Less than 6% | Less than 14% |
| Athlete | From 6 to less than 14% | From 14 to less than 21% |
| Fit | From 14 to less than 18% | From 21 to less than 25% |
From there up, Normal runs to where Overweight starts, and the Overweight and Obese limits rise with age:
| Sex and age | Normal | Overweight | Obese |
|---|---|---|---|
| Men, under 40 | From 18 to less than 20% | From 20 to less than 25% | 25% or more |
| Men, 40 to 59 | From 18 to less than 22% | From 22 to less than 28% | 28% or more |
| Men, 60 or older | From 18 to less than 25% | From 25 to less than 30% | 30% or more |
| Women, under 40 | From 25 to less than 33% | From 33 to less than 39% | 39% or more |
| Women, 40 to 59 | From 25 to less than 34% | From 34 to less than 40% | 40% or more |
| Women, 60 or older | From 25 to less than 36% | From 36 to less than 42% | 42% or more |
Without a date of birth, the category uses the under-40 row. Age counts because the same percentage changes category with age: a man with 26% is classed as Obese at 30 and as Overweight at 55.
Not to be confused with BMI
The category next to the BMI number is always the classic BMI category, which uses only weight and height: BMI = weight (kg) ÷ height (m)², with 1 decimal place.
| BMI category | BMI |
|---|---|
| Underweight | Less than 18.5 |
| Normal | From 18.5 to less than 25 |
| Overweight | From 25 to less than 30 |
| Obese | 30 or more |
The category in the Body composition row comes from the body fat percentage and can be different. On the web, the Waist/Hip row of the same card, when the assessment has both circumferences, shows the waist divided by the hips, with 2 decimal places and no category.
Choosing the method in an assessment
On the web, when more than one method gives a result, the options Auto, Manual, JP7, JP3 and US Navy appear under the Body composition row, only the ones that can be calculated. Manual is the value entered in the Fat field.
- The professional chooses an option and it is saved in that assessment, with no save button. A team member needs permission to edit assessments. The PDF then uses that method.
- If the saved method no longer has data, Auto applies again.
- The client, on the web, can switch options to see another method, but their choice is not saved.
- In the app there are no options: when the percentage was entered in the assessment, the app shows that value; otherwise, it shows the estimate from the saved method or, without one, from Auto.
Fat mass, lean mass and height in meters
In the assessment forms, on the website and in the app, when the weight and the body fat percentage are entered, with body fat from 0 to 100, MyFitnessClient fills in on its own:
- Fat mass = weight × body fat % ÷ 100, with 1 decimal place.
- Lean mass = weight − fat mass, with 1 decimal place.
The two fields are only filled in while they are empty or were filled in by the app. A value typed by hand, for example one from a bioimpedance scale, is never replaced. These calculations only use the value entered in Fat, never the skinfold or US Navy estimate. A height entered in meters, up to 3, becomes centimeters on save: 1.75 becomes 175.
Sources
- Jackson-Pollock, men: Jackson AS, Pollock ML. Br J Nutr. 1978;40:497-504. Men aged 18 to 61. Standard error of about 0.0073 g/ml in body density. PubMed 718832
- Jackson-Pollock, women: Jackson AS, Pollock ML, Ward A. Med Sci Sports Exerc. 1980;12:175-81. Women aged 18 to 55. Standard error of 3.6 to 3.8 percentage points of body fat. PubMed 7402053
- Siri equation: Siri WE, 1961. Converts body density into body fat percentage.
- US Navy: Hodgdon JA, Beckett MB. Reports 84-11 (men) and 84-29 (women), Naval Health Research Center, San Diego, 1984.
- Normal, Overweight and Obese: Gallagher D et al. Am J Clin Nutr. 2000;72:694-701, table 4, the body fat percentages that correspond to a BMI of 25 and of 30 in adults aged 20 to 79. PubMed 10966886
- The three leanest categories: the Athlete and Fit limits, from the American Council on Exercise (ACE).
Questions
Why did the percentage of an old assessment change?
The estimate is calculated every time the assessment opens. If the client's gender or date of birth changed in the profile, or if the professional chose another method, the number and the category change too.
Why doesn't the body fat percentage appear?
Something is missing: weight and height in the assessment, gender Male or Female in the profile, or the data for at least one method (the value entered in Fat, the skinfolds with the date of birth, or the neck and the waist, with the hips for women). It also doesn't appear if the waist is not greater than the neck or if the result does not fall between 0 and 70%.
Which method is the most accurate?
None is exact. To compare results over time, compare assessments made with the same method, by the same person and under the same conditions.
Why is the category different from the BMI one?
BMI uses only weight and height. The body composition category uses the body fat percentage, sex and age, so the two may not match.
Does the website's body fat calculator give the same number?
With the same measurements, the same sex and the same age, yes: the Body fat calculator uses the same formulas and the same categories. There the method is chosen by hand, and the calculator has its own limits and warnings that Assessments don't have.