BMI is your weight in kilograms divided by your height in meters squared. The WHO normal range is 18.5–24.9. The index works well for large groups of people, but it can't tell fat from muscle, so for an individual it can miss the mark.Example: at 1.75 m (5'9") and 70 kg (154 lb), BMI = 70 / (1.75 × 1.75) ≈ 22.9 — the middle of the normal range.
Body mass index is the best-known and most argued-about number in health. Doctors calculate it at your first visit, insurers put it on forms, and lifters in comment sections point out that "according to BMI, I'm obese." In this article we'll break down what the index actually does well, where its limits are, and what to look at if you want a more honest assessment.
01. What BMI is and how to calculate it
The formula is simple: BMI = weight (kg) / height² (m). The basic version makes no adjustment for sex, age, or frame size — which is exactly why it caught on: anyone can calculate it in ten seconds. The height-squared denominator exists so people of different heights can be compared: without it, every tall person would automatically look "heavier than normal."
It was invented almost two hundred years ago for population statistics, not for diagnosing individuals — and that's worth remembering. BMI is genuinely good at describing populations, and on average it does correlate with health risk: people with a BMI above 30 have higher rates of type 2 diabetes, hypertension, and heart disease.
02. The WHO ranges: category table
| BMI | WHO category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Normal |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Obesity class I |
| 35.0 and above | Obesity class II–III |
A couple of reference points for someone 170 cm (5'7") tall: the normal range is roughly 53–72 kg, overweight starts at 72 kg, and obesity at 87 kg. The normal band is wide — nearly 20 kg — and that's fine: bodies within it can look very different. So a BMI of 27 is a reason to check your waist and habits, not a reason to panic.
One important note: these cutoffs don't apply to children and teens — they use age-based percentiles instead, so never apply the adult table to a child.
03. When BMI lies
The index sees only height and weight, so it regularly misses in specific cases:
- Athletic people. Muscle is heavier than fat at the same volume. A 180 cm lifter weighing 95 kg gets a BMI of 29.3 — "nearly obese" at 12% body fat.
- Older adults. Muscle declines with age, so someone with a "normal" BMI can carry excess fat and too little muscle at the same time.
- Slim people with a belly. A BMI of 23 doesn't rule out visceral fat around the organs — the kind most strongly tied to heart and metabolic risk.
- Very tall and very short people. The height-squared formula slightly penalizes the tall and flatters the short.
In all of these cases the number is technically "wrong" — but not because the formula is bad. It simply answers a different question: about the ratio of weight to height, not about body composition.
add one metric to your BMI — waist circumference. The rule of thumb is simple: your waist should be less than half your height. At 170 cm, that's under 85 cm. Above that, risk is elevated even with a normal BMI.
04. What to track alongside BMI
Three simple tools give you the real picture. First, waist circumference once a month: it catches the visceral fat BMI can't see. Second, your weight trend rather than any single number: the direction over months says more than one weigh-in ever could. Third, how you actually feel and function: strength, endurance, sleep, and blood work at your annual checkup.
If you want more precision, body fat percentage can be estimated with bioimpedance scales or calipers, but those methods carry real error margins too. For most purposes, the combination of BMI + waist + weight trend is more than enough.
Watch the direction of the index itself, too: moving from 32 to 29 is real progress, even if you're technically still in the "overweight" category. And if your BMI stays above 30, bring it up with your doctor at a routine visit — together with blood work, the picture becomes complete.
BMI is a decent screening tool: fast, free, and easy to understand. It honestly answers "how does my weight compare to my height" — but not "is my body in good shape." Treat it as a starting point, add your waist measurement, and watch the trend. In Sportygram that takes a couple of minutes a week, and the picture you get is far more honest than any single number.
This material is for informational purposes and does not replace medical advice.
