BMI Calculator Guide: How to Calculate and Read BMI
By AllConvertor · · 2 min read
Body Mass Index (BMI) is one of the most widely used screening numbers in health. It takes only a height and a weight, which is why doctors, fitness apps and insurers all reference it. This guide explains how BMI works, how to read the result, and when you should look beyond it.
How BMI is calculated
BMI is your weight in kilograms divided by your height in metres, squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. If you use pounds and inches, multiply weight in pounds by 703 and divide by height in inches squared.
- Metric formula: BMI = kg ÷ m²
- Imperial formula: BMI = (lb ÷ in²) × 703
- Always measure height without shoes and weight at a consistent time of day for comparable results
BMI categories for adults
The World Health Organization groups adult BMI into broad bands. These bands are population-level screening ranges, not a diagnosis.
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25.0 to 29.9: overweight
- 30.0 and above: obesity (further split into classes I, II and III)
Some health bodies recommend lower cut-offs for people of South Asian and some other Asian backgrounds, because the risk of type 2 diabetes and heart disease can rise at a lower BMI. A commonly cited threshold for increased risk is 23 rather than 25.
What BMI does not tell you
BMI cannot distinguish muscle from fat. A strength athlete can land in the overweight range while having low body fat, and an older adult can have a normal BMI but little muscle and a high proportion of body fat. It also says nothing about where fat is stored, and belly fat carries more health risk than fat on the hips or thighs.
- Waist circumference and waist-to-height ratio add useful context
- Blood pressure, blood sugar and cholesterol matter more than any single number
- BMI is interpreted differently for children and teenagers, using age- and sex-specific percentile charts
- Pregnancy, certain medical conditions and very high muscle mass make adult BMI ranges unreliable
How to use a BMI calculator step by step
- Choose metric or imperial units.
- Enter your height and current weight accurately.
- Read the BMI value and the category it falls into.
- Note the healthy weight range for your height, and compare it with your current weight.
- Recheck every few weeks rather than daily, since water weight causes natural fluctuations.
Setting realistic goals
If your BMI is above the healthy range, even a 5 to 10 percent loss of body weight is associated with meaningful improvements in blood pressure and blood sugar. Aim for gradual change, and treat the calculator as a starting point for a conversation with a doctor or dietitian rather than a verdict.
This article is general information and not medical advice. Speak to a qualified healthcare professional about your personal situation.