Relative Fat Mass Calculator
Use this Relative Fat Mass calculator to estimate body-fat percentage from your height and waist. The 2018 RFM equation does not require bodyweight.
Measure at the top of the right hip bone after a normal breath out, keeping the tape level. Use the same landmark and technique whenever you compare results.
See your estimated body-fat percentage and how a small waist-reading difference changes it. Add bodyweight for estimated fat mass and fat-free mass, meaning the rest of your bodyweight after estimated fat mass, then save a same-method baseline.
What Your Relative Fat Mass Estimate Means
Your result is a Relative Fat Mass estimate from the Woolcott and Bergman 2018 equation. The method is named beside the percentage so you can repeat the same measurement protocol later. It uses height, waist and the selected male or female formula; it is not a direct body scan measurement.
If you entered bodyweight, the calculator also estimates fat mass and fat-free mass from the same percentage. Fat-free mass is the rest of your bodyweight after estimated fat mass; both values come from the same waist-based estimate.
Calculate above to see your result and the measurements behind it.
How to Measure Your Waist for RFM
Stand upright and place a flexible tape around the top edge of the right hip bone. Keep it horizontal, snug without compressing the skin, and record the measurement after a normal breath out.
Use the same unit for height and waist. Bodyweight is optional and affects only the mass breakdown; the RFM percentage itself uses height, waist and the selected male or female equation.
- Height and waist unit
- Choose one unit and use it for both your height and waist measurements.
- Height
- Enter your standing height without shoes.
- Waist measurement
- Measure around your waist at the top of your hip bones, keeping the tape horizontal and snug without pressing into your skin.
- Sex used by the body-fat equation
- Choose the male or female equation required by the published body-fat method.
- Waist comparison range
- Choose a small measurement difference to see how it changes the estimate. This is a comparison, not a confidence interval.
- Bodyweight
- Enter your current bodyweight.
- Weight unit
- Choose the unit shown on your scale.
How Waist Measurement Changes the Estimate
The chart keeps height and the selected RFM formula fixed while changing only the waist measurement. It shows how much a small tape difference would move the estimate.
The center point is your entered waist. Nearby points are measurement scenarios, not a confidence interval, health range or prediction of future body-fat change.
Vertical axis: Estimated body fat (%)
On a small screen, swipe the graph sideways or open its data table.
Horizontal axis: Assumed waist (cm)
- Waist-measurement sensitivity
View all chart values and details
| Series | Assumed waist (cm) | Estimated body fat (%) | Interpretation |
|---|---|---|---|
| Waist-measurement sensitivity | 89 | 23.55 | Scenario |
| Waist-measurement sensitivity | 89.5 | 23.78 | Scenario |
| Waist-measurement sensitivity | 90 | 24 | Scenario |
| Waist-measurement sensitivity | 90.5 | 24.22 | Scenario |
| Waist-measurement sensitivity | 91 | 24.44 | Scenario |
How the RFM Body-Fat Formula Works
RFM uses the ratio of height to waist. The male equation is 64 − 20 × (height ÷ waist). The female equation adds 12 to that result. Height and waist must use the same unit.
If bodyweight is entered, estimated fat mass equals bodyweight × (RFM percentage ÷ 100), and estimated fat-free mass is the remainder. Both values are derived from the same RFM estimate.
See the equations and calculation details
| Result | How it is calculated |
|---|---|
| Male RFM estimate | 64 − 20 × (height ÷ waist) |
| Female RFM estimate | 64 − 20 × (height ÷ waist) + 12 |
| Estimated fat mass | Bodyweight × estimated body-fat percentage ÷ 100 |
| Estimated fat-free mass | Bodyweight − estimated fat mass |
A Complete RFM Body-Fat Example
For a 180 cm height and 90 cm waist using the male equation, height ÷ waist equals 2. The calculation is 64 − 20 × 2, which gives an estimated body-fat percentage of 24%.
With the same height and equation, an 89 cm waist gives about 23.55% and a 91 cm waist gives about 24.44%. That comparison shows why repeating the same tape placement matters.
| Input or result | Value |
|---|---|
| RFM formula | Male |
| Height | 180 cm |
| Waist | 90 cm |
| Estimated body fat | 24% |
What This Body-Fat Estimate Can and Cannot Tell You
RFM was developed and evaluated with US adult NHANES data, with the main validation focused on adults aged 20–69. Accuracy for a specific athlete or body type may differ.
A small change in the calculated percentage does not prove that body composition changed. Tape placement, breathing, posture and normal measurement variation can move the result.