BMI
calculator
Body mass index from your height and weight, with the WHO categories — plus two things most BMI calculators skip: the weight at every category line for your height, so you can see how far you are from the next one, and why the number reads a muscular athlete as "overweight". Imperial or metric.
BMI is your weight divided by your height squared, sorted into the WHO categories: under 18.5 is underweight, 18.5 to 24.9 is normal, 25.0 to 29.9 is overweight, and 30 and up is one of three obese classes. It takes two numbers and returns one, which is its strength as a population screen and its weakness for any individual — it can't tell muscle from fat. This page adds the part most calculators leave out: the exact weight at each category line for your height, and what to do when the category and your body composition disagree.
How the math works
BMI is a weight-for-height ratio. In metric it's weight in kilograms over height in metres squared. In imperial it's the same identity with the unit factors folded in: weight in pounds times 703, over height in inches squared.
The ratio was proposed by Adolphe Quetelet in the 1830s and named "body mass index" by Ancel Keys in 1972, who found it the best of the simple weight-for-height indices for tracking body fatness across a population (Keys et al., 1972). That last word carries the whole caveat: it was built to describe groups, not to measure the fatness of one person in front of you.
WHO categories
| Category | BMI (kg/m²) |
|---|---|
| Underweight | < 18.5 |
| Normal | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese class I | 30.0 – 34.9 |
| Obese class II | 35.0 – 39.9 |
| Obese class III | 40.0 + |
Because a category is just a BMI threshold, you can run the identity backwards to find the bodyweight at each line for a fixed height — which is what the reference strip in the calculator does. That turns "you're overweight" into "you're 6 lb over the line," which is a more useful number to have.
Worked example
Take a 30-year-old, 5'5" (165 cm), 140 lb (63.5 kg) subject. Imperial and metric give the same answer:
- Imperial: 703 × 140 / 65² = 703 × 140 / 4,225 = 23.3
- Metric: 63.5 / 1.651² = 23.3
- Category: Normal (18.5–24.9).
Now the case the muscle caveat is about. A 35-year-old lifter, 6'0" (183 cm), 200 lb (90.7 kg), at 12% body fat:
- Imperial: 703 × 200 / 72² = 140,600 / 5,184 = 27.1
- Metric: 90.7 / 1.829² = 27.1
- Category: Overweight (25.0–29.9).
BMI calls this lifter overweight. His body fat says otherwise: 12% sits inside the athlete range of 6–13% (ACE). Same number, two verdicts — and the body-fat one is describing the thing you actually care about. BMI didn't malfunction; it did exactly what a height-and-weight ratio does when the weight is muscle.
When this calculator is wrong
BMI misclassifies muscular people as overweight, and it's whole categories, not a rounding error. The lifter above computes to 27.1 — "overweight" by WHO — while carrying athlete-range body fat (6–13%, ACE). The mistake is systematic for muscular builds, because BMI was built as a population screen, not an individual body-fat measure, and it counts a pound of muscle exactly like a pound of fat.
The counter-case is the general, non-athletic adult. For someone who doesn't carry unusual muscle, BMI tracks body fatness well enough to be a useful first-pass screen — which is what it was designed for. The misclassification bites at the edges: the heavily muscled read high, and people who've lost muscle (older adults, the long-term ill) can read "normal" while carrying too much fat for their frame. A quick sanity check: if BMI puts you in "overweight" but a tape at the navel and a body-fat estimate both look lean, trust the body composition. If it agrees with the mirror and the tape, it's probably right.
What to do with the result
Use BMI as a screen, then measure the thing it's a proxy for. Pair it with two cheap numbers: waist circumference (abdominal fat carries most of the health signal BMI is reaching for) and a body-fat estimate from tape or calipers. When all three agree, you have a clear picture; when BMI disagrees with the other two, the other two win.
Then use the boundary weights, not just the label. The calculator shows the weight at each WHO line for your height, so a goal becomes concrete — the top of the healthy range at your height is a real number in pounds or kilos, not an abstract "24.9". Recompute when your weight moves more than about 10 lb (4.5 kg), and remember that if you're gaining muscle on purpose, your BMI is supposed to go up.
Common questions
- How do I calculate my BMI?
- Weight divided by height squared. Metric: kilograms over metres squared. Imperial: pounds times 703, over inches squared. A 200 lb person at 6'0" (72 in) is 703 × 200 / 72² = 27.1. Both unit systems land on the same number.
- What is a healthy BMI?
- The WHO normal range is 18.5–24.9. Below 18.5 is underweight; 25.0–29.9 is overweight; 30 and above is obese. The calculator converts that range into the actual weight band for your height, so you can see the top of "normal" as a number on the scale rather than a BMI value.
- Why does my BMI say I'm overweight when I'm not fat?
- Because BMI can't separate muscle from fat — it only sees total weight. A muscular 6'0", 200 lb lifter computes to BMI 27.1 ("overweight") while sitting at 12% body fat, inside the athlete range (6–13%, ACE). If your body fat is low and your waist is lean, the body-composition number is the honest one, not the BMI category.
- Is BMI accurate?
- As a population screen, yes — it correlates with body fatness well enough to flag risk across large groups, which is what Keys built it for in 1972. As an individual measure, it has known blind spots: it reads high for muscular builds and low for people who've lost muscle, and it says nothing about where fat sits. It's a starting point, not a verdict.
- Is BMI different for men and women?
- The calculation and the WHO adult cut-points are the same for both. At a given BMI, women on average carry more body fat than men, so the same number can mean different body composition — another reason to read BMI alongside a body-fat estimate rather than on its own. (Children and teens use age- and sex-specific percentile charts, not these fixed adult cut-points.)
- What's a better measure than BMI?
- For body composition, a body-fat estimate (tape method, calipers, or DEXA) tells you the muscle-vs-fat split BMI can't. For health risk, waist circumference and waist-to-height ratio capture abdominal fat, which carries more of the risk signal. BMI is the fast screen; these are what you measure when the screen flags something or disagrees with the mirror.