What is a BMI calculator?
A BMI calculator estimates body mass index, a simple ratio between weight and height. BMI is often used as a broad screening number for adult weight categories, but it does not directly measure body fat, fitness, muscle mass, or individual health.
Use the result as a general reference, not as a medical diagnosis. For personal health decisions, talk with a qualified healthcare professional.
How to use the BMI calculator
Choose metric units for kilograms and centimeters, or US units for pounds, feet, and inches. The calculator converts the values into the standard BMI formula, squares height, and divides weight by that squared height.
BMI formula
Metric: BMI = weight(kg) / height(m)^2; US: BMI = 703 x weight(lb) / height(in)^2Example calculation
If someone weighs 70 kg and is 175 cm tall, their height is 1.75 m. The BMI is 70 divided by 1.75 squared, which is about 22.86. In US units, a person who weighs 154 lb and is 5 ft 9 in tall has a very similar BMI using the 703 multiplier.
How to interpret BMI
BMI categories are commonly used to describe broad adult weight ranges. A result below 18.5 is often described as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obesity range. These categories are population-level screening references, not personal diagnoses.
The calculator also shows an estimated healthy weight range for the entered height. This gives more context than the BMI number alone, but it still does not account for muscle mass, body composition, medical history, pregnancy, age, or individual health goals.
BMI calculation notes
Small changes in height or weight can change the result, so enter current measurements as accurately as possible. If you use rounded values, treat the result as an estimate.
BMI can be useful for quick comparison, but it should not be the only measure used to evaluate health. For medical guidance, nutrition planning, or weight management, consult a qualified professional.
Why BMI is only a screening number
BMI is popular because it is easy to calculate and compare, but it has limits. It does not know whether weight comes from fat, muscle, water, or bone structure. Two people can have the same BMI and very different body composition. This is why BMI should be treated as a broad screening number rather than a complete health measurement.
The healthy weight range shown in the result panel is based on standard adult BMI category limits. It can provide context, but it should not replace personal medical advice, especially for athletes, pregnant people, older adults, teenagers, or anyone with a health condition.
Using BMI with other measurements
If you are tracking health or fitness, BMI can be one of several reference points. Waist measurement, body fat estimate, strength, blood pressure, lab results, activity level, and medical history can all provide additional context. A BMI calculator is useful for a quick first check, but better decisions usually come from multiple measurements.
For best results, enter weight and height carefully. Small height errors can change the BMI because height is squared in the formula. If your measurement is approximate, treat the result as approximate too.
Frequently asked questions
Is BMI accurate for everyone?
No. BMI is a general screening tool and can be less useful for athletes, children, older adults, and some body types.
What is a normal BMI?
For many adults, a BMI from 18.5 to 24.9 is commonly described as the normal range.
Does BMI measure body fat?
No. BMI uses height and weight only.
Is this medical advice?
No. This calculator is for general information only.