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Height + weight Waist + hip Neck + age Metric profile
This selects published equation coefficients; it is not a statement about identity.
Enter 20–100 completed years.
years
Use one repeatable method for trends; consensus is the median of the three published estimates.
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Enter 1–70%; use the same source and protocol across trend checks.
%
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Choose one or two decimal places without changing any screening result.
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Read the measures together

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Estimate agreement

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Use with context

These are adult population screening equations, not a diagnosis or direct measurement. Measurement landmarks, muscle mass, pregnancy, edema, ethnicity-specific guidance, medicines, disability, illness, and clinical history can change interpretation.

Chart renderer unavailable. The metric ledger remains available.
Body Composition Calculator Estimate body-fat percentage from tape measurements and compare Navy, RFM, and BMI-age methods with lean-mass and uncertainty context.
Body Mass Index (BMI) Calculator Calculate adult BMI from height and weight, compare three reference systems, and add waist-to-height and healthy-band weight context.
BMI-for-Age Percentile Calculator Calculate a child’s BMI-for-age percentile from measurements or known BMI and interpret it with WHO or CDC curves, z-scores and P95 checks.
Fat-Free Mass Index (FFMI) Calculator Calculate raw or height-normalized FFMI from body-fat or fat-free-mass readings, with sensitivity ranges and goal projections.

Scale weight cannot distinguish fat, muscle, bone, fluid, or where body fat is carried. Height, waist, hip, and neck measurements add different clues, but each answers a narrower question. Reading several measures together can reveal why one number changed without pretending that any equation directly measures health.

QuestionUseful measureImportant limit
How does weight compare with height?Body mass index (BMI)Does not separate fat from lean tissue.
How large is the waist relative to the body?Waist-to-height and waist-to-hip ratiosDepends on consistent tape placement and tension.
What share of weight might be fat?Body-fat equationsPopulation equations can disagree for one person.
How are estimated fat and lean mass distributed for height?Fat mass index (FMI) and lean-mass index (FFMI)Inherits error from the selected body-fat estimate.

Trends are most useful when the measurement method stays fixed. Moving a tape a few centimetres, changing posture, measuring after a meal, or switching from one body-fat method to another can create an apparent change that is mostly procedural. Record the same landmarks, conditions, units, and method each time.

Body-fat estimates are not interchangeable measurements. Circumference equations respond strongly to tape values, Relative Fat Mass uses height and waist, and the BMI-plus-age equation uses weight, height, age, and a published coefficient path. Their spread is useful evidence of model uncertainty rather than a puzzle that must be averaged away.

These metrics are screening and tracking aids for adults, not diagnoses. Pregnancy, growth, edema, major muscle development, illness, disability, and population differences can reduce the usefulness of general reference bands. Clinical decisions should use direct assessment and professional context.

How to Use This Tool:

Use one measurement session and one repeatable tape protocol so the estimates remain comparable.

  1. Select the published Formula path and enter age from 20 through 100 completed years. This choice selects equation coefficients; it is not a statement about identity.
  2. Measure height and weight, then measure waist, hip, and neck with a level tape at the landmarks required by the source protocol. Choose centimetres or inches for each length and kilograms or pounds for weight.
  3. Choose the Body-fat headline. The consensus option is the median of the Navy circumference, Relative Fat Mass, and BMI-plus-age estimates.
  4. If selecting Known body-fat percent, enter a value from a repeatable external method. Keep the same method and protocol for later comparisons.
  5. Review BMI and waist signals first, then compare the body-fat estimates and their spread. Recheck tape measurements when one estimate is unexpectedly far from the others.

Interpreting Results:

BMI, waist-to-height ratio, waist-to-hip ratio, and waist circumference describe related but different patterns. Agreement strengthens a screening signal; disagreement is a reason to inspect measurement technique and individual context, not to choose whichever label seems preferable.

The body-fat headline controls fat mass, lean mass, FMI, and FFMI. The method-spread value is the highest minus the lowest of the three equation estimates in percentage points. A large spread warns that derived mass values are method-sensitive.

Display precision changes only the number of decimal places. It does not change thresholds or make an estimate more accurate. Use the unrounded trend and a consistent protocol rather than reacting to a small displayed difference.

Technical Details:

All measurements are converted to centimetres and kilograms before calculation. The model then produces size ratios, reference bands, three body-fat estimates, derived mass indices, Mosteller body surface area, and the weight interval corresponding to BMI 18.5–24.9.

Formula Core

BMI and the two waist ratios are direct divisions. H is height in metres for BMI and centimetres for waist-to-height ratio; W is weight in kilograms.

BMI=WH2WHtR=waistheightWHR=waisthip

The Navy circumference estimate uses inches and base-10 logarithms. For the male coefficient path, A = waist − neck. For the female coefficient path, A = waist + hip − neck.

BFmale=86.01×log10(A)−70.041×log10(height)+36.76 BFfemale=163.205×log10(A)−97.684×log10(height)−78.387

Relative Fat Mass (RFM) uses centimetres. The female coefficient path adds 12 percentage points.

RFM=64−20×heightwaist+s

The BMI-plus-age estimate uses s = 10.8 for the male coefficient path and 0 for the female path.

BF=1.2×BMI+0.23×age−s−5.4

Consensus is the median of those three estimates. Fat mass equals weight × headline body-fat percent ÷ 100; lean mass is the remaining weight. FMI divides fat mass by height in metres squared, and FFMI applies the same operation to lean mass.

BSA=Hcm×Wkg3600

Rule Core

MeasureRangeLabel
BMI< 18.5Underweight
BMI18.5 to < 25Healthy weight
BMI25 to < 30Overweight
BMI30 to < 35Obesity class 1
BMI35 to < 40Obesity class 2
BMI≥ 40Obesity class 3
Waist ÷ height< 0.4Below reference corridor
Waist ÷ height0.4 to < 0.5Healthy central adiposity
Waist ÷ height0.5 to < 0.6Increased central adiposity
Waist ÷ height≥ 0.6High central adiposity

Waist-to-hip ratio is at or above the screening threshold at ≥0.85 for the female path or ≥0.90 for the male path. Waist circumference reaches the increased action point at ≥80 cm for the female path or ≥94 cm for the male path, and the high action point at ≥88 cm or ≥102 cm respectively.

The adult model accepts height 120–230 cm, weight 35–250 kg, waist 45–180 cm, hip 60–200 cm, neck 20–60 cm, BMI 12–65, and body-fat inputs or estimates from 1% through 70%. Results outside those domains are withheld rather than clamped.

Limitations and Accuracy Notes:

  • General BMI and waist bands are screening references, not individual diagnoses or treatment targets.
  • Circumference equations are sensitive to landmark choice, tape tension, posture, and breathing.
  • The three body-fat equations were developed from different populations and can be biased for people unlike those samples.
  • Known body-fat percent is accepted as entered; its quality depends on the external method and protocol.
  • Fat mass, lean mass, FFMI, and FMI inherit any error in the selected headline body-fat estimate.

References: