Relative Fat Mass (RFM) Calculator

Estimate adult body fat percentage from height and waist circumference using the Woolcott–Bergman equation, with measurement guidance and live steps.

Calculation inputs

Make the waist measurement reproducible

1
Use the NHANES waist site
Identify the uppermost lateral border of the right iliac crest. The illustration is not a landmark guide.
2
Keep the tape level and snug
Use a non-stretch tape without compressing the skin; read after normal exhalation.
3
Match the length units
Height and waist must both use cm or both use inches. Age only checks the adult scope.

Measurement and method sources are linked below. Illustrations explain the workflow; they do not supply measurement data.

Explore the calculation model

Height-to-waist calculation

Calculation results

Estimated body fat · RFM25.1%Woolcott & Bergman (2018) · adult estimate

Height / waist1.944

Waist / height0.514

Equation constant64

Step-by-step solution

Choose the formula
$RFM (%) = 64 - 20 \times \frac{height}{waist}$
Substitute your measurements
$64 − 20 × 175 ÷ 90$
Calculate in order
$Height ÷ waist = 1.94444444; multiply by 20 = 38.88888889$
Write the result
$25.1%$

Representative adult mannequin, stadiometer and separate measuring tape; written protocol defines the waist site — Representative illustration · not a scan, patient measurement or image to scale.

1
Put measurements in the same unit
175 cm ÷ 90 cm = 1.944444
2
Apply the equation coefficient
20 × 1.944444 = 38.888889
3
Use the selected equation constant
64 − 38.888889 = 25.1%

The equation estimates total body-fat percentage. It does not show where fat is stored or measure visceral fat.

For adults aged 20+; not validated here for pregnancy, children, edema or conditions that substantially change body shape. A clinician must interpret the estimate.

Validated originally using US adults aged 20+; accuracy varies between individuals and populations. The figure is not a direct DXA measurement or a visceral-fat measurement.

Measurements & calculation details

25.1%

Quantity	Value
Equation	64 − 20 × height / waist
Height	175 cm
Waist circumference	90 cm
Age (scope check only)	30 years
Estimated body fat	25.111111%

Try a worked example

Choose an example to load its measurements into the calculator. Each result uses the same calculation engine as your own inputs. Back to your calculation

Understand the calculation

Concept & meaning

RFM estimates whole-body fat percentage using a height-to-waist ratio and the sex variable from the original model. It does not require body weight and cannot locate fat around internal organs. A waist measurement is not a direct body-fat measurement.

Formula & special cases

With matching units, male RFM = 64 − 20 × height/waist and female RFM = 76 − 20 × height/waist. Converting both height and waist from centimetres to inches leaves their ratio unchanged. Age checks the adult scope and does not alter the equation.

Scope & limitations

The 2018 model was developed and validated against DXA using US NHANES adults aged 20+. Individual error remains and performance can differ by population and body shape. Outputs outside 0–100% are rejected with an explanation; apparently plausible percentages still do not guarantee accuracy.

RFM (%) = 64 - 20 \times \frac{height}{waist}

Result from your current inputs

Estimated body fat · RFM: 25.1%

Frequently asked questions

Where should I measure my waist?

For consistency with NHANES, use the iliac-crest protocol described above, keep the tape horizontal and read after a normal exhalation. Different waist sites can change the estimate.

Does the estimate equal a DXA result?

No. DXA was the reference method in validation; that does not make the equation a DXA measurement for an individual.

Why does age not change the calculated percentage?

Age is not a term in the final 2018 equation. The age field limits this interface to the adult scope used here.

Can this measure visceral fat or diagnose obesity?

No. It estimates total body fat percentage from an anthropometric model. It does not measure a fat depot or establish a diagnosis.

Show all questions (5 more)Show fewer questions

What does the Relative Fat Mass (RFM) calculate?

Estimate adult body fat percentage from height and waist circumference using the Woolcott–Bergman equation, with measurement guidance and live steps.

Which values should I enter?

Use the labeled fields above. The original model uses two sex-specific equations; it is not individualized for hormone therapy. Values outside the supported range clear the result and show an explanation.

What formula does this calculator use?

RFM (%) = 64 − 20 × height/waist + 12 × sex; male = 0, female = 1

Why might my result differ from another source?

Results can differ because measurement methods, rounding and assumptions vary. This calculator is an estimate and is not a medical diagnosis.

Does the result update automatically?

Yes. Inputs and presets update the result, teaching diagram and expandable steps together. Invalid input clears the previous result.

Original equation, explicit units

Calculation	Formula	Interpretation
Male coefficients	64 − 20 × height/waist	Height and waist must have the same length unit.
Female coefficients	76 − 20 × height/waist	Same ratio; a different model constant.
Age	Adult scope check only	Not an equation coefficient.

Sources & References

Woolcott & Bergman · 2018Original RFM derivation and validationRead the reference

CDC / NCHS · NHANESWaist measurement protocol at the iliac crestRead the reference

References document the methods and their limitations; links and logos do not imply institutional endorsement or expert review.