Your BMI
Body Mass Index compares your weight to your height with a single formula. It does not measure body fat directly. Health bodies use it as a fast, no-equipment screen to flag whether a weight might warrant a closer look, not as a diagnosis. The calculator above rounds to one decimal place and reports the category your number lands in.
BMI is your weight in kilograms divided by your height in metres squared (kg/m²). Switch to imperial and the calculator converts pounds and inches before applying the same formula, so the result is identical either way. A worked example: 70 kg at 175 cm is 70 ÷ (1.75 × 1.75) = 22.9. In imperial, a person at 5'10" weighing 160 lb reads 23.0, and at 180 lb reads 25.8. The cut-offs below follow the World Health Organization categories, which the CDC also uses for adults.
To make those numbers concrete, take someone 5'10" (177.8 cm). They cross from underweight into the healthy range at about 129 lb, from healthy into overweight at about 174 lb, and into the obese range at about 209 lb. A shorter or taller person hits each line at a different weight, which is the whole point of dividing by height.
BMI counts only total weight, so it cannot tell muscle from fat. A lean, heavily trained athlete can read 27 or 28 — technically "overweight" — while carrying very little fat, because muscle is dense. Read the other direction, an inactive person at a normal BMI can still carry a high fat percentage, sometimes called normal-weight obesity.
BMI is also blind to where fat sits. Weight carried around the abdomen (visceral fat) is more closely tied to metabolic risk than the same weight on the hips and thighs, and two people with the same BMI can have very different waistlines. That is why clinicians often pair BMI with a simple waist measurement or a waist-to-height ratio rather than reading the number alone.
Finally, the thresholds were drawn largely from European-descent populations. Risk can appear at a lower BMI in some groups — several health bodies use about 23 as a caution point for people of South Asian descent — and the standard cut-offs are not validated for children, pregnancy, or some older adults. Treat your number as one signal among several, not a verdict.
Each BMI point corresponds to a fairly small weight change. For most adult heights it is roughly 2.5 to 3.7 kg (about 5.5 to 8 lb): closer to 5.6 lb if you are around 5'3", and closer to 8 lb if you are 6'4". Taller people need a larger change to move the same single point, because the height term is squared. Knowing your per-point weight makes a target easier to read — moving from a BMI of 27 to 24 at average height is roughly a 9 to 11 kg (20 to 24 lb) change, not a vague goal.
Is BMI different for men and women?
The formula and the standard ranges are the same for adult men and women. Body composition differs on average between the sexes, which is one reason BMI is a screen rather than a precise body fat reading.
Does BMI work for children and teens?
Not with these adult ranges. For people under 20, BMI is read against age and sex percentile charts instead, so a child's number should be checked with a paediatric chart or a clinician.
What is a healthy BMI for older adults?
Some research suggests a slightly higher BMI can be protective later in life, but there is no single official older-adult cut-off. The standard ranges still apply as a general guide.
How much weight changes my BMI by one point?
Roughly speaking, one BMI point is about 2.5 to 3.5 kg (5 to 8 lb) for most adult heights. Taller people need a larger change to move the same amount.
Last reviewed June 2026. Ranges follow the WHO BMI categories. For general guidance, not medical advice.