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Body composition is the split between fat mass and everything else that makes up body weight, including muscle, organs, bone, and body water. That split often matters more than scale weight alone, because two people can weigh the same yet face very different training, nutrition, and health questions.
This calculator turns a scale reading, height, age, and tape measurements into a structured estimate of body fat. It compares the U.S. Navy circumference method, Relative Fat Mass (RFM), and a BMI plus age formula, then reports a headline Body-Fat % together with BMI, Lean Mass, Fat Mass, and a Waist-to-height risk band.
That makes it useful for ordinary check-ins when you want more context than body weight can provide on its own. A coach can compare methods before planning a cut, a lifter can see whether a stalled scale still hides a shrinking waist, and someone tracking long-term habits can use the same landmarks every few weeks to watch trend direction rather than chase day-to-day noise.
The package also shows how much the built-in models disagree. A narrow Method spread suggests the three estimates are telling a similar story. A wide spread is a sign to slow down, because the issue may be measurement technique, body shape, or the fact that circumference and BMI-based formulas are only approximations of body fatness.
That boundary matters. A lean-looking result does not diagnose health, a high result does not measure visceral fat directly, and no anthropometric equation can replace a clinical assessment or a direct method such as DXA. Pregnancy, edema, very unusual muscularity, and inconsistent tape placement can all make the estimate less trustworthy, so this tool is best treated as an informational aid rather than a diagnosis.
A good first pass is the default median focus with careful measurements taken under repeatable conditions. Enter height and weight, then measure neck and waist with the tape held level and relaxed. Female entries add hip circumference for the Navy formula. That gives you a headline estimate that is less dependent on any one model while still keeping each individual method visible in Coaching Metrics and Method Concordance.
This tool is strongest when you want repeatable comparison, not a one-time verdict. Use it for weekly or monthly check-ins, for reviewing a waist change that happened faster than scale weight changed, or for planning a target body-fat range before a diet phase. It is a poor fit when you need a medical diagnosis, a precise sports-lab number, or a result from rushed measurements taken at different landmarks each time.
The sensible next step is usually another measurement, not another interpretation. Recheck the tape landmarks, compare the three model outputs, and then decide whether the trend in Body-Fat % and Waist-to-height ratio matches what you expected from recent eating, training, or recovery changes.
The package calculates three independent body-fat estimates from the same input set. The Navy route uses circumferences and height, RFM uses only height and waist, and the BMI plus age route applies the Deurenberg body-fat equation to the unadjusted BMI base. The selected Estimate focus can be Navy, RFM, or a median across the available methods.
Once a headline body-fat value exists, the rest of the core outputs are derived deterministically. Fat Mass is body weight multiplied by the chosen body-fat fraction, Lean Mass is the remainder, Waist-to-height ratio is waist divided by height, and the target planner backs into a projected body weight from the current or adjusted lean mass and the chosen Target body-fat %.
The advanced controls change different parts of the pipeline, which is easy to miss if you only look at the summary badge. Body-fat scale and Body-fat add modify the selected headline body-fat estimate after the package chooses the focus model. BMI add shifts the displayed BMI, but the BMI + age estimate still comes from the original BMI calculation. Hip circumference only matters for female Navy entries, and age only matters for the BMI plus age route.
The package converts length inputs to centimeters and inches as needed, and it clamps extreme outputs so impossible values do not spill through the charts. These are the main equations behind the visible numbers.
| Symbol | Meaning in this package | Field or unit |
|---|---|---|
| h | Height used by the selected model | inches for Navy, centimeters for RFM |
| w | Waist circumference | inches for Navy, centimeters for RFM and WHtR |
| n | Neck circumference | inches |
| hp | Hip circumference for female Navy entries | inches |
| s | RFM sex offset | 0 for male, 12 for female |
| a | Age in years | Age |
| mul, add | Post-selection adjustment controls | Body-fat scale, Body-fat add |
Using the default male sample from the package makes the flow concrete. At 170 cm, 70 kg, waist 80 cm, neck 38 cm, and age 35, the three built-in methods return about 13.7 percent, 21.5 percent, and 20.9 percent. The median focus lands at 20.9 percent, which then yields about 14.6 kg of Fat Mass, 55.4 kg of Lean Mass, and a Waist-to-height ratio of 0.47.
| Output | Boundary logic | Displayed result |
|---|---|---|
| Body-Fat Category for male entries | <6 Essential, 6 to <14 Athlete, 14 to <18 Fitness, 18 to <25 Average, >=25 Obese | Badge in the summary and Coaching Metrics |
| Body-Fat Category for female entries | <14 Essential, 14 to <21 Athlete, 21 to <25 Fitness, 25 to <32 Average, >=32 Obese | Badge in the summary and Coaching Metrics |
| BMI | <18.5 Underweight, 18.5 to <25 Normal, 25 to <30 Overweight, >=30 Obese | BMI and the Adiposity Risk Map |
| Waist-to-height risk band | <0.50 Lower central risk, 0.50 to <0.60 Increased central risk, >=0.60 High central risk | Waist-to-height risk band |
Validation is deliberately visible. If the geometry is impossible, such as a male waist that is not larger than neck, the warning box is supposed to stop you before you trust the charts. The package also falls back when a selected method cannot be computed, which is helpful for continuity but another reason to read warnings before reading the headline number.
Use the page as a short measurement workflow, not as a one-click verdict.
The first question is whether the headline estimate and the supporting context agree. A Body-Fat % in a broad maintenance band is easier to trust when Waist-to-height ratio stays below 0.50 and Method spread is small. When the models disagree sharply or the waist-height band looks worse than the body-fat badge suggests, central adiposity is probably the more urgent clue.
The best verification step is a repeat measurement under the same conditions. Use the same tape landmarks, similar hydration and timing, and the same Estimate focus if you want to compare one session against the next.
A male entry with Age 35, Height 170 cm, Weight 70 kg, Neck 38 cm, and Waist 80 cm produces a headline Body-Fat % of about 20.9. The same run shows BMI 24.2, Waist-to-height ratio 0.47, Body-Fat Category Average, Lean Mass 55.4 kg, and Fat Mass 14.6 kg. With Target body-fat % left at 18.0, the planner projects a Projected target weight near 67.5 kg and a Fat-mass change needed of about -2.5 kg.
A female entry with Age 42, Height 165 cm, Weight 68 kg, Neck 34 cm, Waist 82.5 cm, and Hip 98 cm yields a Waist-to-height ratio of exactly 0.50. That is enough for the package to show Waist-to-height risk band as Increased central risk because the lower band is strictly below 0.50. The same inputs produce Body-Fat % about 34.2, BMI about 25.0, and Body-Fat Category Obese, which is a good reminder that a threshold edge still counts as the higher band.
Suppose a male entry is typed as Neck 40 cm and Waist 39 cm while Estimate focus is set to Navy. The warning box reports that the waist must be larger than the neck, and the package falls back because the selected focus is unavailable. That fallback can leave Body-Fat % at an unrealistic 0.0 while still filling other fields, so the correct action is not to interpret the chart. It is to remeasure the waist, clear the warning, and then recalculate.
The median focus is the safest default because it softens the effect of one model running unusually high or low. Switch to Navy or RFM when you specifically want that method to drive the headline Body-Fat %.
Because the three built-in models respond to different signals. Navy cares about tape geometry, RFM depends on height and waist, and the BMI plus age route depends on weight, height, age, and sex. A wide Method spread means those assumptions are not converging tightly for your measurements.
No. BMI add changes the displayed BMI, but the package computes the BMI + age estimate from the original BMI base before that additive offset is applied.
The Navy circumference formula needs a positive log term. For male entries that means waist must be larger than neck. For female entries, waist plus hip must be larger than neck. If that relationship fails, the warning is telling you the tape inputs need to be checked before the Navy estimate can be trusted.
Yes. The visible calculation path runs in the browser, and this package has no upload step or server-side body-composition helper for the entered measurements.