| Metric | Value | Meaning | Copy |
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| Method | LBM (kg) | LBM (lb) | Lean (%) | Vs consensus | Why use it | Copy |
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| {{ row.label }} | {{ formatMass(row.lbmKg) }} | {{ formatMass(row.lbmLb) }} | {{ formatPercent(row.leanPercent) }} | {{ formatSignedMass(row.deltaVsConsensusKg) }} kg | {{ row.note }} |
Lean body mass is your body weight minus fat mass. It includes skeletal muscle, organs, bone, blood, and body water, so it answers a different question from scale weight alone. When body weight changes, lean body mass helps you ask whether that shift is more likely to reflect muscle and other lean tissue, stored fat, or a mix of both.
This calculator estimates lean body mass from weight, height, sex, and a selected route. The default headline is the consensus median of four built-in formulas: Boer, Hume, James, and Janmahasatian. You can keep that median, switch to one named formula, or add a body-fat based cross-check and promote that route to the headline if it fits your use case better. Alongside the main result, the page shows fat mass, lean share, BMI context, derived FFMI, formula spread, and a short recommendation line.
The results area is designed for comparison rather than just a single answer. Snapshot gives the quick read, Method Table shows how each route differs from the consensus median, Method Lens turns that spread into a chart, Lean Split shows the active lean-versus-fat balance, and JSON provides a structured record for logging or later review. CSV, DOCX, chart-image, and JSON exports are built in.
This is still an estimate, not a scan. Hydration shifts, edema, pregnancy, pediatric use, very high muscularity, and serious illness can all move a formula-based result away from direct measurement. When you already have a trusted body-fat reading, the calculator's cross-check routes are most useful as a reality check against the height-and-weight formulas rather than as proof that one number is final.
The calculator accepts weight in kilograms or pounds and height in centimeters, meters, inches, or feet plus inches. It converts measurements into a common internal format before running the formulas, then reports the chosen lean-mass result in kilograms and pounds. From that same headline route it derives fat mass, lean percentage, fat percentage, BMI, and FFMI.
| Route | Calculation path | How the calculator uses it |
|---|---|---|
| Consensus median | Median of Boer, Hume, James, and Janmahasatian | Default headline because one outlier formula does not get to control the result. |
| Boer | Male: 0.407 x W + 0.267 x H - 19.2Female: 0.252 x W + 0.473 x H - 48.3 |
Useful as a straightforward height-and-weight estimate and one of the routes included in the consensus headline. |
| Hume | Male: 0.32810 x W + 0.33929 x H - 29.5336Female: 0.29569 x W + 0.41813 x H - 43.2933 |
A long-standing clinical estimate included in the formula comparison and consensus median. |
| James | Male: 1.10 x W - (128 x W^2 / H^2)Female: 1.07 x W - (148 x W^2 / H^2) |
Included for comparison, but the calculator warns that this route can become less stable as BMI rises. |
| Janmahasatian | Male: 9270 x W / (6680 + 216 x BMI)Female: 9270 x W / (8780 + 244 x BMI) |
A BMI-aware route that often stays useful when body size is well outside average ranges. |
| Known body fat | Lean mass = weight x (1 - body fat / 100) |
Turns a trusted body-fat reading directly into lean mass for comparison with the formula set. |
| BMI plus age screen | BF% = 1.2 x BMI + 0.23 x age - 10.8 x sex - 5.4, then lean mass is derived from body weight |
Quick adult screening route when you do not have a direct body-fat measurement. |
| Navy-style tape estimate | Uses neck and waist values for males, plus hip for females, then converts the estimated body-fat percentage into lean mass | Provides a circumference-based comparison route when tape measurements are taken carefully and consistently. |
The page does not treat every body-fat cross-check as equally trustworthy. Each option carries a built-in disagreement allowance, measured in kilograms, before the calculator starts warning that the direct route and the formula median are too far apart to treat casually.
| Cross-check source | Default spread | What that means in practice |
|---|---|---|
| DEXA / scan-grade | 1.0 kg | Close agreement is expected. A larger gap deserves a second look. |
| Calipers / coached estimate | 1.8 kg | Good repeat field work can be useful for trend tracking, but landmark consistency still matters. |
| Smart scale / BIA | 3.0 kg | Hydration and timing can move the reading enough to shift lean-mass estimates. |
| Visual estimate | 4.5 kg | Best treated as a broad comparison rather than a precise check. |
| Mixed / rough source | 6.0 kg | Useful only as a loose range until a stronger measurement is available. |
| BMI plus age screen | 4.0 kg | A screening route, not a body-composition measurement. |
| Navy-style tape estimate | 2.5 kg | Works best when the same landmarks and tape tension are repeated each time. |
The Navy wording needs one extra note. The calculator uses the familiar neck-waist and neck-waist-hip circumference equations commonly described as a U.S. Navy method. Current official U.S. Navy Guide-4 guidance dated December 2025 uses a different two-step process based on waist-to-height ratio and then waist, height, and weight tables. That means this calculator's tape route is best read as a Navy-style comparison estimate, not as an official pass-or-fail Navy BCA result.
All calculations and exports are generated in the page from the values you enter. There is no server-side lean-mass processing in this tool.
For a first pass, leave the headline on the consensus median. That gives you a center point from four formula estimates and reduces the chance that one formula choice shapes the whole story. If the method spread is narrow, choosing Boer, Hume, James, or Janmahasatian instead usually changes the headline only a little. If the spread is wide, the calculator is telling you that route selection matters and that smaller differences should not be over-read.
Use a named formula when you have a reason to stay with that exact equation. That might be a comparison against an older record, a study workflow, or a repeated personal check-in where consistency matters more than finding a single universal "best" formula. If BMI is high, the calculator's own warning system is worth respecting: James can drift lower than the other routes and may not be the safest headline.
The advanced panel is most helpful when you already know how your body-fat number was obtained. A scan-grade reading is a strong cross-check. Calipers can be useful if the tester and landmarks stay consistent. BMI plus age is fast but broad, and the calculator treats it as a screening estimate rather than a precise measurement. The tape route is practical when you measure carefully, but it is sensitive to neck, waist, and hip placement.
Start with the meaning of the quantity itself. Lean body mass is not the same as muscle mass. It includes muscle, but it also includes water, organs, bone, and other non-fat tissue. A higher lean-mass estimate can reflect more muscle, but it can also reflect hydration and body-size differences that have nothing to do with training progress.
Next, read the formula spread before you act on the headline. The calculator labels spread bands directly, and those labels are a useful shortcut for deciding how cautious you should be.
| Formula spread | Badge shown | How to read it |
|---|---|---|
| 0 to 1.5 kg | Tight spread | The equations cluster closely, so a single-formula headline is less likely to distort the picture. |
| More than 1.5 to 3.0 kg | Usable spread | The routes are still reasonably aligned, and the consensus median remains a stable summary. |
| More than 3.0 to 5.0 kg | Wide spread | Method choice is starting to matter. Small changes in lean mass should be interpreted cautiously. |
| More than 5.0 kg | High disagreement | The equations are far enough apart that a direct body-fat route or a stronger measurement method is worth adding. |
A direct body-fat row is most useful when you compare it against the consensus median, not when you treat it as automatic truth. If the gap is small for the quality preset you chose, the formula set and the cross-check are telling a similar story. If the gap is wider than the preset allows, the calculator is pointing to a measurement problem, a route mismatch, or a body type that the simple formulas do not capture well.
BMI and FFMI are supporting context. BMI gives a familiar size label but does not separate fat from lean tissue. FFMI adjusts lean mass for height, which makes tall and short users easier to compare. Both are useful, but neither fixes a weak lean-mass estimate underneath them.
For a male entry of 70 kg and 175 cm, the calculator builds a consensus headline of 55.94 kg and a formula spread of 3.71 kg. That falls into the calculator's Wide spread band, which means the formulas are still usable but not interchangeable.
| Route | Lean mass | Vs consensus | What it suggests |
|---|---|---|---|
| Consensus median | 55.94 kg | 0.00 kg | Good default headline because it sits in the middle of the four formula rows. |
| Boer | 56.02 kg | +0.08 kg | Very close to the median, so using Boer instead of consensus barely changes the story. |
| Hume | 52.81 kg | -3.12 kg | The lowest row in this example and the main reason the spread is not tight. |
| James | 56.52 kg | +0.58 kg | Still close here because BMI is moderate rather than very high. |
| Janmahasatian | 55.86 kg | -0.08 kg | Nearly identical to the consensus median in this case. |
| BMI plus age screen (age 30) | 57.31 kg | +1.37 kg | Higher than the formula median, but still comfortably inside the calculator's 4.0 kg screening spread. |
A second example shows why source quality matters. For a female entry of 68 kg and 165 cm, the formula consensus is 46.34 kg. If you add a known body-fat reading of 28%, the direct route becomes 48.96 kg, which is 2.62 kg above the consensus median. That gap is much wider than the calculator's 1.0 kg scan-grade spread, so a scan-quality preset would trigger a stronger warning. The exact same 28% entered as a visual estimate would still sit 2.62 kg above the median, but the page would treat that disagreement more cautiously because the preset already assumes low precision.
That is the main lesson of the examples: the calculator is not asking you to chase one perfect formula. It is helping you judge whether several reasonable routes point to the same rough lean-mass range.
No. Muscle is part of lean body mass, but lean body mass also includes body water, bone, organs, blood, and other non-fat tissue. A change in lean body mass should not be read as a one-for-one change in muscle.
The consensus median is the safest default because it reduces single-formula bias. Switch to a named formula only when you need continuity with a previous method or when you have a specific reason to follow that exact equation.
James uses a squared weight term, so it can become less stable at higher BMI. The calculator flags that situation directly and suggests comparing James against Boer, Janmahasatian, or the consensus median before using it as the headline.
No. The tool uses the familiar circumference equation with neck, waist, and, for female entries, hip measurements. Current official U.S. Navy Guide-4 guidance dated December 2025 uses a different body-composition workflow based on waist-to-height screening and waist, height, and weight tables. Use the tool's tape route as a comparison estimate, not as an official BCA score.
The page will still calculate a number, but the built-in formulas are mainly adult formulas and normal hydration assumptions matter a lot. For children, teenagers, pregnancy, edema, or major illness, age-specific or clinical body-composition methods are more appropriate.
No. The calculator performs the lean-mass work in the browser and creates the table, chart, and JSON exports from the values already on the page.