Body Mass Index (BMI)
What it is, how to calculate it, and how useful it is for men and women
What is BMI
Body Mass Index (BMI) — Indicates whether your weight is normal, insufficient, or excessive, signaling potential health risks such as obesity, diabetes, or heart problems.
Important: BMI does not directly measure fat and does not show where fat is located (subcutaneous vs visceral). Therefore, for men (especially those who train), it can be inaccurate in both directions.
How to Calculate BMI
Formula:
Calculation example:
Height 1.80 m, weight 85 kg
1.80² = 3.24
85 / 3.24 ≈ 26.2 (overweight)
WHO Classification
For adult men, the threshold values are generally the same as for women (the difference is often not in the thresholds, but in interpretation due to body composition).
| BMI | Classification |
|---|---|
| < 16.0 | Severe Thinness |
| 16.0 – 17.0 | Moderate Deficit |
| 17.0 – 18.5 | Underweight |
| 18.5 – 24.9 | Normal Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity Class I |
| 35.0 – 39.9 | Obesity Class II |
| ≥ 40.0 | Obesity Class III |
Important: For some ethnic groups (especially in Asia), risks may start at lower BMI, so separate recommendations may have different threshold values.
Is BMI Useful in Practice
Yes, but as a "first filter", not as a diagnosis.
✓ Where BMI is Useful
- • Fast and simple: you only need height and weight.
- • Linked to risk: on average, very low or high BMI is associated with more health problems (type 2 diabetes, hypertension, etc.).
- • Useful for tracking trends: if your BMI changes over time, it can be a signal (for better or worse).
✗ BMI Limitations
- • Doesn’t distinguish fat and muscle: an athlete with high muscle mass can have a “high BMI” even if body fat is low.
- • Doesn’t show fat distribution: the most “dangerous” fat for the heart and metabolism is visceral fat around the abdomen, and BMI doesn’t capture it.
- • Works worse for some groups: older adults (less muscle and risks can be higher at the same BMI), pregnant women, and different ethnic groups (risks at the same BMI can differ).
- • The cutoffs are conditional: “normal/overweight” is a simplification — real life is more complex.
BMI Alternatives
BMI is the simplest but worst method for assessing body composition (it cannot distinguish muscle from fat), so most body composition assessment methods are superior to BMI when measured correctly.
Below are the most commonly used options (from the simplest to the more "accurate" but expensive).
Categorically Superior to BMI
Often Better (with caveats)
Simple Methods (can be done at home)
A) Waist Circumference (WC)
What it measures: central (abdominal) obesity; better than BMI, reflects "belly" and associated risks.
Pros: very simple metric; often adds information about cardiometabolic risk on top of BMI.
Cons: threshold values depend on ethnicity/guidelines and measurement method; it's important to measure consistently (in the same place and under similar conditions).
How to measure (briefly): on exhale, tape parallel to the floor, without pulling in the stomach; use one standard (e.g., midpoint between the lower rib and iliac crest) and stick to it.
How much better/worse: WC is better than BMI alone, captures central obesity. Often use risk zones (for Caucasian populations): ~94/80 cm (m/f) as "elevated risk" and ~102/88 cm as "substantially elevated risk" (wording may differ in various recommendations).
Best answers: "Do I have abdominal fat and is the risk elevated?"
B) Waist-to-Height Ratio (WHtR)
Formula: waist / height (in the same units, e.g. cm/cm).
Simple screening: "waist less than half of height" (WHtR < 0.5 — commonly used threshold for elevated risk).
Interpretation (commonly used): 0.4–0.49 — no elevated risk; 0.5–0.59 — elevated risk; ≥0.6 — high risk of central obesity (in some guidelines for adults, usually at BMI < 35).
Pros: often better than BMI at identifying people with early cardiometabolic risks (especially with "normal" BMI).
Cons: not a direct % body fat; thresholds/classifications may vary between recommendations.
Best answers: "Is there central obesity and elevated risk, even if weight seems normal?"
C) Waist-to-Hip Ratio (WHR)
What it shows: fat distribution ("apple" vs "pear").
Pros: strongly associated with risk because it reflects fat "centrality", not total weight.
Cons: two measurements → more errors; hips can change the indicator due to anatomy/training.
Best answers: "Where is fat deposited (abdomen vs hips) and is the profile associated with risk?"
Guidelines (commonly cited): abdominal obesity: ≥0.90 in men and ≥0.85 in women (WHO/similar thresholds in sources).
D) Relative Fat Mass (RFM) — body fat % estimation by waist and height
What it is: simple formula that estimates body fat % better than BMI (in several studies/validations) — but it's still an estimate.
Formula: RFM = 64 − 20 × (height/waist) + 12 × sex, where sex: 0 = male, 1 = female (height and waist in the same units).
Pros: convenient at home; closer to the question "how much fat" than BMI; in the original work gave fewer "false-normal" cases of fat obesity than BMI.
Cons: poor for the question "where exactly is the fat (visceral/subcutaneous)"; accuracy depends on population and waist measurement accuracy.
Best answers: "Approximately what is my body fat %?"
E) Body Roundness Index (BRI) — "body roundness" (waist + height)
What it is: geometric body shape index (waist + height), associated with total and abdominal adipose tissue.
Pros: may reflect central obesity better than BMI; there are modern large studies on the relationship between BRI and CVD risk and mortality.
Cons: usually calculated with a calculator; thresholds are less standardized, and the relationship with outcomes is often nonlinear (not always "the more — the better/worse").
Best answers: "How pronounced is central obesity (body shape) and potential risk?"
F) BIA — bioimpedance (smart scales, body fat %)
What it is: body composition assessment by electrical resistance (indirect model).
Pros: convenient at home; better suited for tracking trends (where you're heading) than for "accurate body fat %".
Cons: result noticeably depends on water/food/alcohol/training/temperature; "absolute" numbers may be biased.
Best answers: "Is my body composition changing over time?"
How to use to be useful: always measure under the same conditions (e.g. in the morning after the toilet, before food/coffee/training) and watch the dynamics for 2–4 weeks, not one measurement.
Advanced Methods (for experienced users)
G) Skinfold Calipers
What it is: assessment of subcutaneous fat by fold thickness at standardized points.
Pros: cheap; with skill, well suited for subcutaneous fat dynamics.
Cons: strongly depends on technique; works worse with obesity; doesn't show visceral fat.
Best answers: "Is subcutaneous fat (folds) changing over time?"
I) ABSI (A Body Shape Index) — body shape/risk considering BMI
What it is: index that normalizes waist by height and BMI; designed as a marker of "shape" and risk, partially independent of BMI.
Formula (classic): ABSI = WC / (BMI^(2/3) × height^(1/2)) (WC and height in meters).
- • Adds "central fat" context beyond BMI: two people with the same BMI can have different waist/shape and different risk signals.
- • Uses measurements you already have for BMI + a tape measure (no special device).
- • In population studies, can add risk information (including mortality) partly independent of BMI.
- • Hard to interpret: many studies use age/sex-specific z-scores; universal "good/bad" cutoffs are not standardized.
- • More error-prone in practice: small mistakes in waist measurement (and height/weight) can noticeably change the result.
- • Doesn't answer "what is my body fat %" and rarely changes the next step compared to simpler metrics (waist circumference, WHtR).
Best answers: "Is there additional risk due to shape/waist at the same BMI?"
Clinical Methods
H) DXA (densitometry) / clinical body composition assessment
What it is: clinical assessment of fat/muscle/bone mass; widely used as a reference for body fat % in studies.
- • Separates body composition: shows fat mass and lean/muscle mass — not just “weight for height”.
- • Regional assessment: you can see fat and muscle distribution by segment (trunk/legs/arms) and asymmetries.
- • Works across ages and sexes.
- • Cost; not always necessary for everyday decisions.
- • Sensitive to conditions: hydration, recent food intake, edema, and some conditions can bias lean mass estimates.
- • Shows “how much”, not “quality”: DXA is weaker for muscle quality (fat infiltration) — MRI/CT can be better here.
Best answers: "How much fat and muscle do I have (most reliable available in clinic)?"
J) MRI/CT visceral fat (most accurate imaging)
- • Measures what matters: visualizes visceral fat (inside the abdominal cavity), which is more strongly linked to insulin resistance, type 2 diabetes, dyslipidemia, and CVD than overall weight/BMI.
- • Separates different “body components”: distinguishes visceral from subcutaneous fat, and can assess muscles and other tissues (especially with MRI).
- • Not fooled by “muscle”: in athletes/highly muscular people BMI often overestimates fatness, while MRI/CT shows the real fat picture.
Cons: Cost and availability; time and “logistics”; CT = radiation exposure.
Role: usually for clinical tasks/research, or when you need very precise estimates.
Practical Assessment Scheme
- 1Calculate BMI (quick reference)
- 2Add waist or WHtR — to catch abdominal risk
- 3If you're a strength athlete or have doubts — assess body composition (BIA or DXA)