What BMI is — and what it is not
Body Mass Index (BMI) is a single number that relates your weight to your height. It has one job: to give a fast, cheap screen of whether your weight might be a health concern. Because it needs nothing but a scale and a tape, it is used everywhere from doctors’ offices to national health surveys.
But BMI has a famous blind spot: it cannot tell muscle from fat. That is why a lean, muscular man can be labelled “overweight” while carrying single-digit body fat. Use BMI for what it is good at — a quick first look — and lean on body fat percentage for the real picture.
How BMI is calculated
The formula is simple: BMI = weight (kg) ÷ height (m)². A 180 cm man weighing 85 kg has a BMI of 85 ÷ (1.8 × 1.8) ≈ 26.2. In imperial units, it is weight (lb) × 703 ÷ height (in)². This calculator does the conversion whichever units you use.
The BMI categories
- Under 18.5 — Underweight: may signal under-nutrition or low muscle mass.
- 18.5–24.9 — Normal: the healthy range for most adults.
- 25–29.9 — Overweight: a common screen flag; context matters (muscle vs fat).
- 30+ — Obese: associated with higher risk of metabolic disease; worth acting on.
These bands come from population data used by bodies like the CDC. They describe risk across large groups, not any individual’s health with certainty.
See your number and healthy range
Enter your height and weight above for your BMI, category, and personalised healthy weight range.
Use the calculatorWhy BMI can mislead — especially for men who train
Muscle is denser than fat. Build a meaningful amount of it and your weight — and therefore your BMI — rises, even as you get leaner and healthier. This is not a minor quirk: a large share of fit, strong men sit in the “overweight” BMI band. The reverse also happens — someone can have a “normal” BMI yet carry too much fat and too little muscle (sometimes called “skinny-fat”), which BMI completely misses.
For men over 35, three numbers beat BMI for tracking real health:
- Body fat percentage — how lean you actually are.
- Waist circumference — a strong marker of harmful visceral fat.
- Strength — a powerful, independent predictor of healthy ageing.
How to use your BMI sensibly
- Treat it as a screen, not a verdict. A flag in the overweight range is a prompt to look closer, not a diagnosis.
- Pair it with body fat.If your BMI is high but your body fat is low, you’re likely fine. If both are high, that is a clearer signal.
- Use the healthy weight range as a rough target. It is a reasonable long-term goalpost for most non-lifters.
- Focus on what moves health. Strength, daily steps, protein, and sleep do more for you than chasing a specific BMI.
Where to go from here
If your BMI flagged something, the fix is not dramatic — a modest calorie deficit, enough protein, and consistent strength training. Start with your calories in the TDEE Calculator, set your protein, and follow a simple plan like our free 3-day program. Progress on the mirror and the tape will tell you more than BMI ever could.