BMI Calculator
Find your Body Mass Index from height and weight, with the healthy range for your height.
What it does
Body Mass Index, or BMI, is a simple ratio of weight to height. Doctors and researchers use it as a first screen for weight related risk, because it needs only two numbers and works the same way for everyone measured.
The tool above computes your BMI the moment you type. Pick metric or imperial first, then enter height and weight. The result shows your index, the band it falls in, and the weight range that maps to a BMI of 18.5 to 24.9 for your height.
BMI is a screening step, not a diagnosis. It cannot tell muscle from fat, so very muscular people can score high while carrying little fat, and older adults can score in the healthy band while carrying extra fat around the waist. Read the number together with waist size, activity, and, when it matters, a clinician view.
How to use
- Choose metric (cm, kg) or imperial (inches, pounds).
- Enter your height without shoes. For feet and inches, convert first: 5 ft 9 in is 69 inches.
- Enter your morning weight, after using the bathroom and before eating, for the most stable reading.
- Read the three results: the index itself, the band name, and the healthy weight span for your height.
Formula
- weight (kg)
- Morning body weight in kilograms.
- height (m)
- Height in meters, squared before dividing.
- 703
- Imperial unit fix so inches and pounds give the same answer.
Square the height first, then divide. The 703 in the imperial form only converts pounds and inches into the same scale as kilograms and meters, so both forms give the same answer. A BMI of 18.5 to 24.9 counts as the healthy band for most adults.
Worked example
Example: 70 kg at 175 cm
A person weighs 70 kg and stands 175 cm tall. Here is the full path to the answer.
- Convert height to meters: 175 cm is 1.75 m.
- Square it: 1.75 x 1.75 = 3.0625.
- Divide: 70 / 3.0625 = 22.9 after rounding.
- Read the band: 22.9 sits inside 18.5 to 24.9, so the healthy band.
- Healthy span for this height: 18.5 x 3.0625 = 56.7 kg at the low end, 24.9 x 3.0625 = 76.3 kg at the high end.
Reference table
| BMI | Band | What it signals |
|---|---|---|
| Below 18.5 | Underweight | Worth discussing with a clinician, mainly if weight keeps falling |
| 18.5 to 24.9 | Healthy range | Band linked with the lowest risk in large studies |
| 25.0 to 29.9 | Overweight | Risk starts to rise; waist size adds context |
| 30.0 and above | Obesity range | Higher risk; a clinician can grade it further |
Limits
- Adult screening only. Children need age based percentiles.
- Cannot separate muscle from fat, so athletes read high.
- Pregnancy and fluid shifts distort the number.
- One reading is noise. Trends across weeks count.
Common mistakes
- Measuring height with shoes on, which pushes BMI slightly down.
- Weighing late in the evening after meals and fluids, which pushes it up by a point or more.
- Mixing units, such as centimeters with the 703 imperial factor. Keep one system per calculation.
- Applying adult bands to children or teens, who need age based percentiles instead.
- Treating small day to day moves as fat change. Water and glycogen shift weight by a kilogram or more.
Try it yourself
Quick checks. Answers sit tucked under each question.
BMI at 80 kg and 180 cm?
1.8 squared is 3.24. 80 / 3.24 = 24.7, inside the healthy band.
Healthy weight span at 180 cm?
18.5 x 3.24 = 59.9 kg low end, 24.9 x 3.24 = 80.7 kg high end.
Questions
What is a good BMI?
For most adults, 18.5 to 24.9 is the band tied to the lowest risk in population studies. The middle of the band is not a personal target; any value inside it is in the healthy span.
Why does the imperial formula use 703?
It is a unit fix, not a medical constant. One kilogram is about 2.2 pounds and one meter is about 39.4 inches, and 703 folds both conversions into a single multiplier so the imperial answer matches the metric one.
Can athletes have a high BMI and still be lean?
Yes. Muscle is dense, so strength athletes often land in the overweight band with low body fat. In that case waist size and a body fat estimate say more than BMI alone.
Does BMI work for children?
Not with adult cutoffs. Children use BMI-for-age percentiles that compare against growth charts for the same age and sex. Ask a pediatrician for the right chart.
How often should I check?
Once a week under the same conditions is enough for most people. Daily checks mostly show water swings and can mislead.