Head-to-head
Ideal Body Weight vs Lean Body Mass
Ideal body weight is a target: a single number derived from height and sex, originally from mid-twentieth-century insurance tables. Lean body mass is a measurement: an estimate of everything in your body that is not fat, derived from your current weight and height.
One ignores your weight entirely; the other is built from it. That is the whole difference. Ideal body weight says what a person of your height should weigh. Lean body mass says how much of what you currently weigh is muscle, bone, organ and water.
The formulas were built for different jobs. Devine's 1974 equation was designed for drug dosing and ventilator settings, where a fixed reference weight beats actual weight in a patient carrying excess fat. Boer's 1932 equation came from body-density measurements and is used for protein requirements and fitness benchmarking. Reading the first as a personal goal, or the second as a health verdict, is where both go wrong: a 100 kg athlete can be obese by ideal body weight and perfectly healthy by body fat percentage. Use each for the job it was built for and neither becomes misleading.
Ideal body weight
How To Calculate Ideal Body Weight
Ideal body weight is a formula, not a fact about health. It was written in 1974 to help clinicians dose drugs, and every version of it returns a slightly different number.
Lean body mass
How To Calculate Lean Body Mass
Lean body mass splits a scale reading into the part that is fat and the part that is not, which is often the more useful half.
How They Differ
| Attribute | Ideal body weight | Lean body mass |
|---|---|---|
| What it estimates | A target total weight | The non-fat part of your current weight |
| Inputs | Height and sex | Weight, height and sex |
| Uses current weight | No | Yes |
| Origin | Devine, 1974, from insurance tables | Boer, 1932, from body density |
| Changes if you gain fat | No | Slightly, upward |
| Changes if you gain muscle | No | Yes, upward |
| Output | Kilograms | Kilograms, plus fat mass and FFMI |
| Clinical use | Drug dosing, ventilator tidal volume | Protein needs, fitness benchmarking |
| On the worked example | 75.0 kg | 62.2 kg |
| Common error | Treating it as a goal for everyone | Treating high lean mass as healthy regardless of fat |
The Same Numbers, Both Ways
A 180 cm man weighing 82 kg.
- Height
- 180 cm
- Weight
- 82 kg
- Sex
- Male
| Line | Ideal body weight | Lean body mass |
|---|---|---|
| Height in inches | 70.87 | 70.87 |
| Result | 75.0 kg | 62.2 kg |
| The other formula at this height | 72.6 kg under Robinson | 19.8 kg of fat |
| Share of the 82 kg actually weighed | 91.5% | 75.9% |
| Body fat implied | 17.0% | 24.1% |
| Fat-free mass index | Not reported | 19.2 |
At 180 cm the Devine formula puts ideal body weight at 75.0 kg, while the Boer formula puts lean mass at 62.2 kg out of an actual 82.0 kg — 19.8 kg of fat, or 24.1% body fat. Weighing the ideal 75.0 kg with that same lean mass would leave 17.0% body fat. Neither number is wrong; they answer different questions, and only one of them looks at the body you have.
When To Use Each
Ideal body weight
- You are calculating a drug dose or a ventilator tidal volume, where a height-based reference weight is standard.
- You are checking a clinical chart that quotes an ideal or predicted body weight.
- You need a reference point for someone whose actual weight is unreliable for dosing.
Lean body mass
- You are setting a protein target, where grams per kilogram of lean mass is the usual basis.
- You are tracking body composition across a training block rather than total weight.
- You want a fat-free mass index to compare muscularity at a given height.
Common Questions
Why are there several ideal body weight formulas?
Because they were fitted to different populations at different times. Devine, Robinson, Miller and Hamwi all share the same structure — a base weight at five feet plus a per-inch increment — but with different bases and increments. For a 180 cm man they span roughly 71 to 75 kg, which is an honest measure of how precise the idea really is.
Is lean body mass the same as muscle mass?
No. Lean mass includes muscle, bone, organs, skin and body water. Skeletal muscle is a bit over half of it. That is why a lean mass estimate can rise on a day when you have gained water rather than tissue.
Can lean body mass exceed ideal body weight?
Easily. A 100 kg athlete at 12% body fat carries 88 kg of lean mass, well above the 75 kg Devine assigns at that height. It is the clearest demonstration that ideal body weight is a dosing reference rather than a health target.
How accurate is the Boer formula?
Within a few kilograms for most people, and less accurate at the extremes of height and adiposity. It is a prediction from weight and height, not a measurement. DEXA or hydrostatic weighing will disagree with it, sometimes by several kilograms.
Should I use ideal body weight to set a goal?
Rarely. It ignores frame size, muscularity and age, and it was never designed as a target. A body fat percentage or a waist-to-height ratio says more about whether your current weight is actually a problem.
More Comparisons
- BMI vs Body Fat Percentage
BMI is weight against height squared; body fat measures composition directly. Three people at a BMI of 26.2 can come out at 19.8%, 26.4% and 32.2% body fat — which is the gap BMI cannot see.
- BMR vs TDEE
BMR is what you burn at complete rest; TDEE multiplies it by an activity factor. For a 30-year-old man at 80 kg that is 1,780 versus 2,759 calories a day.
- Protein Intake vs Macros
For a 75 kg person eating 2,000 kcal, protein by bodyweight at 1.6 g/kg is 120 g — 24% of calories. A 30/40/30 macro split on the same 2,000 kcal gives 150 g. Same person, same day, 30 g apart.
Sources
- Lean body mass — measurement and estimation — Wikipedia
- Human body weight — reference weights and indices — Wikipedia
- Body fat percentage — methods and typical ranges — Wikipedia
Both methods sit in Health.