Body Mass Index compresses your height and weight into a single number that correlates - roughly - with body fat across large groups of people. Doctors use it because it is free and instant; critics point out it cannot tell muscle from fat. Both are right. This guide covers the formula, the standard categories, and the limitations that matter, so the number from our BMI Calculator lands with the right amount of weight. (This article is educational only and is not medical advice - talk to a healthcare professional about your own situation.)
The Formula, Metric and Imperial
BMI = kg ÷ m² BMI = 703 × lb ÷ in²Weight divided by height squared - the 703 in the imperial version just converts pounds and inches into the same units. Two worked examples that describe the same person: 70 kg at 1.75 m tall gives 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9. In imperial, 154 lb at 69 inches gives 703 × 154 ÷ (69 × 69) = 22.7 - the tiny gap is unit rounding, not a different answer.
The WHO Categories
| BMI | Category | At 5'10" (178 cm), roughly |
|---|---|---|
| Below 18.5 | Underweight | Under 129 lb (58.6 kg) |
| 18.5 - 24.9 | Normal range | 129 - 174 lb (58.6 - 78.9 kg) |
| 25.0 - 29.9 | Overweight | 174 - 209 lb (78.9 - 94.7 kg) |
| 30.0 and above | Obese | Over 209 lb (94.7 kg) |
Notice how wide the "normal" band is: 45 pounds for one height. BMI was never designed to hand you a single target weight - it flags broad zones, and the healthy zone is genuinely broad.
Find Your Number and Your Range
Metric or imperial input, instant category, and the healthy weight band for your exact height
Open the BMI Calculator →What BMI Cannot See
BMI is a screening signal, not a diagnosis. Four blind spots account for most of its misfires:
- Muscle vs fat. A kilogram of muscle and a kilogram of fat count identically. Lean athletes routinely score "overweight"; a sedentary person can sit at 23 while carrying excess visceral fat ("normal-weight obesity"). BMI measures heaviness, not composition.
- Fat location. Abdominal (visceral) fat predicts metabolic risk far better than fat on hips and thighs, and BMI cannot tell them apart. Waist-to-height ratio - keeping your waist under half your height - is a stronger single check.
- Age. Muscle declines with age, so the same number means more fat at 70 than at 25, and slightly higher BMIs appear less harmful in older adults. Children and teens need percentile charts, not the adult table.
- Ethnicity. Risk thresholds differ across populations - South and East Asian adults face elevated metabolic risk from about BMI 23, which is why several countries apply lower cutoffs than the WHO defaults.
Use BMI as a first look, never a verdict
An out-of-range BMI is a reason to check better measures - waist circumference, blood pressure, blood sugar, lipids, fitness - not a diagnosis by itself. Two people with the same BMI can have completely different health profiles.
If you are tracking BMI over time, the trend matters more than any single reading - and expressing a change honestly ("down 6%" vs "down 4 points") is exactly the kind of arithmetic covered in our percentage change guide.
Frequently Asked Questions
Is BMI accurate for athletes and muscular people?▼
No - and this is its best-known blind spot. Muscle is denser than fat, so a lean, muscular person can register 27-30 and be classified "overweight" while carrying very little body fat. Many professional rugby players and sprinters are technically obese by BMI. For anyone who strength-trains seriously, waist measurement and body-fat estimates are far more informative.
What is a healthy BMI for my height?▼
The WHO healthy range of 18.5-24.9 translates to a weight band once you fix the height. At 5'10" (178 cm), it spans roughly 129-174 lb (58.6-78.9 kg). The 45-pound width of that band is the point: BMI defines a broad zone, not a single target weight.
Does BMI mean something different as I get older?▼
Somewhat. Older adults naturally lose muscle, so the same BMI reflects more fat than in a younger person - and some research finds slightly higher BMI (up to about 27) is not linked to worse outcomes past age 65. Standard adult categories also do not apply to children and teens, who are assessed with age- and sex-specific percentile charts.
Why do BMI cutoffs differ for some ethnic groups?▼
Health risks appear at different BMI levels in different populations. People of South and East Asian descent tend to develop type 2 diabetes and cardiovascular risk at lower BMIs, so bodies like the WHO have suggested action thresholds of 23 (rather than 25) for these groups. The categories are statistical conventions calibrated on European populations, not universal biological lines.
Should I worry if my BMI is slightly outside the normal range?▼
One point either side of a boundary is not a diagnosis - BMI cannot see whether the weight is muscle, where fat is distributed, or your blood pressure, lipids, and fitness. Treat an out-of-range number as a prompt to look at those better measures, ideally with a clinician, rather than as a verdict in itself.
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