Body Fat (BIA) Calculator
The formula
What the body fat (bia) tells you
Tape-meat body composition for body fat (bia): the US Navy circumference method — the field standard gyms and physicals actually use — with fat and lean mass in kilograms.
Calculation: US Navy: BF% = 86.010·log₁₀(waist−neck) − 70.041·log₁₀(height) + 36.76 (men); +1.08·…−84.52 women. Enter your inputs and every substitution is shown below the result.
Reading the result
Track the trend monthly under the same conditions; ±2 points of tape error is normal.
Limitations and best practice
Hydration and recent meals shift centimetres — measure fasted, morning, same spot.
This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Frequently asked questions
How accurate is tape-meat body fat?
Which formula is best for me?
Source & methodology
US Navy Body Composition Program method.
How Body Fat (BIA) works — and what your result really means
You want to know where you stand, and a computed figure beats a remembered one every time. Body Fat (BIA) is the kind of number people argue about from memory. This page ends that argument with arithmetic you can watch happen: enter your figures, get Body fat in real time, and see every substitution printed below.
Health figures in Health & Medical describe populations, not individuals — Body Fat (BIA) places you on a distribution built from bodies like yours, and the 25.5% answer is a starting point for a conversation, not a diagnosis. That framing is the difference between using a health metric and being misled by one.
There is a habit that separates people who get useful answers from Body Measurements numbers: they decide what question the figure answers before they compute it. Body Fat (BIA) answers “Body fat?” — and nothing else, which is precisely why the result is trustworthy.
What makes a calculation page trustworthy is that its copy is derived from its own arithmetic: here body fat = 25.5%, recomputed every time a field changes, and the sensitivity section ranks the inputs that move the result — Waist / hip circumference first, at ≈28.4 % of the headline for a ten-percent error either way in that input — which is the thread the rest of this guide pulls on.
How Body Fat (BIA) is computed, line by line
Here is the complete model this tool runs — US Navy: BF% = 86.010·log₁₀(waist−neck) − 70.041·log₁₀(height) + 36.76 (men); +1.08·…−84.52 women — with each input explained below. If a competitor’s page gives you a different answer from the same inputs, this is the text that settles which of you is right.
Weight (kg) (example value 125) is a quantity rather than a category — precision here is cheap and worth having. Stay in one unit system top to bottom; conversion rounding is where small errors enter.
Height (cm) (example value 181) is a measured number in the unit named by the label; the formula does not convert for you. Keep every length field in the same unit; the formula assumes a consistency it cannot check for you.
Neck / waist circumference (cm) (example value 39) is a quantity rather than a category — precision here is cheap and worth having. Replace the example with the figure from your own records — the tool recomputes instantly and the printed steps follow your numbers.
Waist / hip circumference (cm) (example value 90) is a numeric input whose unit is fixed by the label; a wrong unit is a wrong answer wearing a plausible disguise. This input feeds the formula directly; a defensible figure here is worth more than false precision anywhere else.
Sex (example value Male) is a selector, not a measurement: the engine follows a different route for each option. This input feeds the formula directly; a defensible figure here is worth more than false precision anywhere else.
What each “Sex” choice actually does: Male → 25.5% · Female → 2.0% — each value above is the model’s own output for that option, so the choice can be made on evidence rather than on what a category feels like it means.
Body Fat (BIA) on the example inputs: the complete substitution
The tool prints its own working; this is the same arithmetic on the example values, reproduced so you can follow it by hand. Starting figures: Weight (kg), Height (cm), Neck / waist circumference (cm), Waist / hip circumference (cm). The computed body fat comes out at 25.5%, which the interpretation band labels “Above the healthy band — the trend matters more than today’s point”.
- Circumference difference waist−neck = 51.0 cm
- Navy formula (log₁₀ terms) → 25.5 %
- Fat mass = 125.0 × 25.5 % = 31.9 kg
Fat mass — 31.9 kg. If your inputs change, this is usually the first companion figure to move.
Lean mass — 93.1 kg. If your inputs change, this is usually the first companion figure to move.
BMI — 38.2. If your inputs change, this is usually the first companion figure to move.
Overwrite any input above and every line of this example updates with it — the substitution is live, not a screenshot of somebody else’s numbers.
How far Body Fat (BIA) can reasonably move
The honest version of any estimate is a corridor, and this one is cheap to produce: Body Fat (BIA) was recomputed at 5 points along the dominant input, with every other field left untouched.
| Waist / hip circumference setting | Body fat |
|---|---|
| 80 % of the example value | 9.2% |
| 90 % of the example value | 18.2% |
| example inputs | 25.5% |
| 110 % of the example value | 31.6% |
| 120 % of the example value | 36.8% |
That spread — 9.2% at the low end, 36.8% at the high — is what a careful read of Body Fat (BIA) looks like: a band you can state out loud, with the reason for each edge. Reproduce it in the form above by editing Waist / hip circumference alone; the substitution steps will follow your figures exactly.
What actually moves your Body Fat (BIA)
Generic advice says “it depends on your inputs”; this section says exactly how much. Each field was perturbed by ±10 % in isolation and the body fat recomputed, so the ranking below is measured rather than asserted. For Body Fat (BIA) the ranking is measured rather than guessed, and it belongs to these defaults: shift the starting point and the order can shift with it.
- Waist / hip circumference: ten percent up gives 31.6%, ten percent down gives 18.2% — ≈28.4 % of the body fat either way
- Nudge Height by ±10 % and Body Fat (BIA) lands between 22.6% and 28.7% (sensitivity ≈12.6 %)
- Neck / waist circumference +10 % → Body Fat (BIA) = 22.5% · −10 % → 28.3% (the body fat shifts ≈11.7 %)
Waist / hip circumference is the dominant lever here: ten percent either way moves the result by about 28.4 %, from 18.2% to 31.6%. Measure that input from a document rather than from memory, and treat Height as the second priority (≈12.6 % per ten percent). The remaining fields can sit at safe defaults without materially changing the answer — knowing which is which is what separates a careful calculation from an anxious one.
Reading the result: bands, zones and thresholds
Nobody acts on a bare figure — they act on a figure with a comparison attached. The verdict attached to the default calculation — “Above the healthy band — the trend matters more than today’s point” — is one band on a ladder your own figure may sit somewhere else on. How much of that band is luck? On Waist / hip circumference alone ten percent either way covers 18.2% to 31.6%. The practical reading rules: ask what would have to change to move you one band. If the answer is “a small, plausible input difference”, the band itself is not the finding — the uncertainty is.
The comparison worth making is not with somebody else’s Body Fat (BIA) but with the version of your own situation measured last quarter — same formula, same conventions, different inputs.
Why Body Fat (BIA) results go wrong — and how to prevent it
Hydration and recent meals shift centimetres — measure fasted, morning, same spot. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Treating the example inputs as a recommendation. They are a demonstration of the mechanism — a decision-grade Body Fat (BIA) begins the moment your own figures replace them.
Attention is also misallocated between fields. Having measured this engine, we can say where it belongs: Waist / hip circumference first, everything else second.
The subtlest error is answering the wrong question precisely. Confirm that body fat is actually the quantity your decision needs — the arithmetic is flawless either way, which is what makes the mistake invisible.
Body Fat (BIA) verification checklist
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- Note the date and the inputs beside the number so the figure can be audited later.
- Re-verify Waist / hip circumference against a source document — ten percent of error there moves the headline by ≈28.4 %.
- Recompute once with the dominant input deliberately set low, and see whether the decision changes.
- State, in one sentence, which convention each rate, unit or period in the form is using.
- Repeat the ±10 % test on the inputs that matter and see whether the decision — not just the number — still holds.
A routine for decision-grade Body Fat (BIA)
Using a calculator well sounds trivial, but the difference between a decision-grade figure and a throwaway one lives entirely in the routine. The routine for Body Fat (BIA) takes a minute to learn and applies to every input above.
Step 1 — start from evidence. The inputs behind Body Fat (BIA) are weight (kg), height (cm), neck / waist circumference (cm), waist / hip circumference (cm), sex. Write them down with their source next to each one; if you cannot name where a figure came from, that is the input most likely to be wrong. If the verdict changes with a ten-percent nudge, report the nudge alongside the number.
Step 2 — baseline before exploration. Get one defensible Body fat on screen before you start nudging numbers. Without a baseline, every variation looks interesting and none of them means anything. Rounding early is the quietest way to introduce a permanent error.
Step 3 — change one input at a time. Nudge a single field by ten percent, watch the headline and its companions move, then undo it. Two or three fields will dominate — the sensitivity table above already ranks them for this engine. A calculation you cannot explain to somebody else is a calculation you cannot defend.
Step 4 — compute a corridor, not a point. Three runs — worst, likely, best — turn Body Fat (BIA) from a snapshot into a range. Plan against the pessimistic end and treat the optimistic end as the upside story. The inputs are the part of the work that nobody else can do for you.
Step 5 — write the one-line memo. “Body fat = 25.5% on this date, from these inputs.” That line is the entire deliverable; everything else on this page is how you earned the right to write it. Note the assumption you are least sure about; that is where the real risk lives.
From calculation to decision: where Body Fat (BIA) earns its keep
In Body Measurements the figure computed here is usually the first one somebody asks to see. Having it — with the formula behind it — changes your position in the conversation from participant to reference. Say the answer out loud before you save it; if it sounds wrong, the model did not change, your understanding did.
Professionals in Body Measurements use this style of calculation as a screening filter before spending real money or effort: a quick Body Fat (BIA) check eliminates weak options in minutes, and the survivors earn the deeper, slower analysis. When in doubt, prefer the boring input: the interesting one is usually the one you misread.
There is a personal-administration use people overlook: periodic self-checking. Body Fat (BIA) quietly drifts as life changes — rates move, bodies change, costs compound — and re-running it a few times a year catches drift while correction is still cheap. Numbers are cheap to produce and expensive to retract; the extra minute is the cheapest insurance available.
The questions that come up most about Body Fat (BIA)
What affects Body Fat (BIA) the most?
Body Fat (BIA) is driven above all by Waist / hip circumference. Perturbing only that field by ten percent either way produced 31.6% against the 25.5% baseline — a ≈28.4 % response, while Height manages ≈12.6 %.
Which matters more: height or waist / hip circumference?
For Body Fat (BIA), Waist / hip circumference wins the head-to-head (≈28.4 % against ≈12.6 % per ten-percent change). Knowing which input dominates is the difference between careful work and merely slow work.
How much does the “Sex” choice change the answer?
A great deal, on these inputs: Body Fat (BIA) comes out at 2.0% on the lowest option (“Female”) and 25.5% on the highest (“Male”). Each option is listed with its computed value so you can see what the choice is worth before you make it.
What is a typical Body Fat (BIA)?
With the example inputs on this page the result is 25.5%, labelled “Above the healthy band — the trend matters more than today’s point”. “Typical” varies with situation, which is why every result here ships with its verdict band and its sensitivity list: they turn one figure into a range you can locate yourself inside.
How do I calculate Body Fat (BIA) by hand?
Three steps: write the formula (US Navy: BF% = 86.010·log₁₀(waist−neck) − 70.041·log₁₀(height) + 36.76 (men); +1.08·…−84.52 women), insert your values, and simplify. The worked example section shows exactly that chain for the default inputs; replicating it once on paper teaches you more about Body Fat (BIA) than any number of calculator runs.
What happens to Body Fat (BIA) if waist / hip circumference changes by 10 %?
On these inputs a ±10 % change in waist / hip circumference takes Body Fat (BIA) to 31.6% against the 25.5% example — around 28.4 % of the headline. The same test on every other field is in the sensitivity section.
Is the Body Fat (BIA) result of 25.5% good or bad?
The tool answers “what”, not “whether”: 25.5% carries the verdict “Above the healthy band — the trend matters more than today’s point” for the default case, which is a starting interpretation rather than a verdict on your situation. Re-run with your own figures and compare the bands.
Does Body Fat (BIA) change with age?
Yes — reference bands shift deliberately across the lifespan, which is why this tool prints an interpretation rather than a bare number. Re-read the bands each time inputs like Weight (kg) change materially.
How accurate is Body Fat (BIA) for me personally?
The formula encodes averages; body-composition and measurement error typically shifts results by a few percent in either direction. Use ranges and trends, and treat any single 25.5% as approximate.
How accurate is tape-meat body fat?
±3 points versus DEXA in lean subjects, better in groups; the trend is reliable even when the level is soft.
Which formula is best for me?
Consistency beats formula choice — the same method, same conditions, every time.
Can this tool see the numbers I type?
It cannot. There is no submission step: the calculation happens in your browser and the result disappears when you close the page. Treat the fields above as a scratchpad rather than a form.
Key terms on this page
- Fat mass
- 31.9 kg on the example inputs — a companion figure printed beside the headline of Body Fat (BIA) so that no result is read in isolation.
- Lean mass
- 93.1 kg on the example inputs — a companion figure printed beside the headline of Body Fat (BIA) so that no result is read in isolation.
- BMI
- 38.2 in the default Body Fat (BIA) calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Body fat
- The primary output of this tool: 25.5% for the default inputs, computed exactly from US Navy: BF% = 86.010·log₁₀(waist−neck) − 70.041·log₁₀(height) + 36.76 (men); +1.08·…−84.52 women.
- Worked example
- Every substitution in order, from the example inputs to 25.5% — the audit trail for the headline figure.
- Verdict band
- The interpretation attached to a result (“Above the healthy band — the trend matters more than today’s point” on these inputs), which turns a bare number into a position on a scale.
- Sensitivity (±10 %)
- How far the Body Fat (BIA) moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Where the Body Fat (BIA) model comes from
There is no black box on this page — the engine evaluates US Navy: BF% = 86.010·log₁₀(waist−neck) − 70.041·log₁₀(height) + 36.76 (men); +1.08·…−84.52 women. Source of record: US Navy Body Composition Program method. Specialist software layers extra coefficients on top of the same algebra; none are hidden here, and that transparency is what makes a disagreement traceable to a specific input instead of a black box.
Treat the output as a precise answer to the question you typed: exact arithmetic, transparent formula, published source, no hidden rounding. The design constraint was set by usefulness rather than by comfort: an estimate you can interrogate beats an answer you have to trust.
Next steps after the calculation
If the verdict band sits far from your usual range, take the printed inputs and result to a professional — this tool screens, it does not diagnose.
The audience here is people checking a personal health figure before acting on it. Everything you need to defend the result — formula, substitution, sensitivity, source — is printed above, so the only remaining variable is the honesty of your inputs.