How to use it
Choose cm / kg or in / lb, then enter your height and weight. Your BMI and category appear straight away — no calculate button. The marker on the scale shows exactly where you sit between underweight, normal, overweight and obese, the line below gives the weight range that keeps you in the normal band for your height, and the BMI chart plots your point on the full height-against-weight map so you can see how far the nearest category line is.
Tap Go advanced to set a target BMI (22 is the middle of the healthy range). You'll get the goal weight it works out to, how much to lose or gain to get there, plus BMI Prime and the Ponderal Index.
The BMI formula
BMI is a quick screen, not a diagnosis. It was designed to track weight across whole populations, so for one person it is a starting point — useful, but read it alongside other measurements.
What BMI misses
BMI uses only height and weight. It can't separate muscle from fat, and it says nothing about where fat is carried. A lean, muscular athlete can land in the overweight band; an older adult who has lost muscle can read as normal while carrying excess fat. A body fat estimate or a simple waist measurement fills that gap.
BMI and your calories
Once you know your category, our TDEE Calculator turns your height, weight and activity into daily maintenance calories and a target for losing or gaining weight at a sensible pace.
BMI categories (adults)
| Category | BMI |
|---|---|
| Underweight | < 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese — class 1 | 30.0 – 34.9 |
| Obese — class 2 | 35.0 – 39.9 |
| Obese — class 3 | 40.0 + |
Ranges follow the World Health Organization classification for adults 20 and over. Some health bodies use lower overweight cut-offs for people of South and East Asian descent, where risk rises at a lower BMI.
BMI Prime & Ponderal Index
BMI Prime is your BMI divided by 25, the top of the normal range — a value under 1 means you're inside it, and 1.15 means you're 15% over. The Ponderal Index divides weight by height cubed instead of squared, which stays more consistent for very tall or very short people, where plain BMI drifts.
Healthy weight range
For any height there's a span of weights that all sit in the normal BMI band. The calculator shows yours in your chosen units, so you can see how much room you have — or how far there is to go — without guessing.
Reading the BMI chart
The BMI chart maps height against weight, shaded by category. Your dot moves live as you type. Find your dot, then read straight down to the weight axis or across to the height axis to see exactly how many kg or lb separate you from the next band — more useful than a single number when you're tracking a change.
Common questions
How is BMI calculated?
BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). In imperial units it's pounds divided by height in inches squared, times 703. The calculator handles the conversion in either unit system.
What is a healthy BMI range?
The WHO classes below 18.5 as underweight, 18.5–24.9 as normal, 25–29.9 as overweight and 30 or above as obese. The tool also shows the exact weight range that keeps you in the normal band for your height.
Is BMI the same for men and women?
Yes — the formula and the adult categories don't change with sex. Because BMI ignores body composition, a very muscular person of either sex can read as overweight while carrying little fat.
Why can BMI be misleading?
It uses only height and weight, so it can't tell muscle from fat or show where fat sits. Athletes, older adults and some ethnic groups are common cases where BMI misclassifies. Pair it with a waist measurement or a body fat estimate.
Does this BMI calculator work for children?
No. It uses adult categories. For anyone under 20, BMI is read against age- and sex-based percentile charts rather than fixed cut-offs, so use a dedicated child and teen BMI percentile calculator.