BMI Calculator Guide: Formula & Worked Example
Short answer
Body Mass Index with WHO category and healthy weight range. In the fixed example below, bmi is 22.9 kg/m². Your answer depends on the inputs and assumptions you choose.
Formula
BMI = weight (kg) / height (m)²
Use the relationship above with consistent units. The inputs are units, weight (kg or lb), height (cm or inches). Use the input scale shown beside each field and keep time periods and units consistent. Only round the final result for display.
Worked example
- Units
- metric
- Weight (kg or lb)
- 70
- Height (cm or inches)
- 175
- BMI: 22.9 kg/m²
- Category: Normal
- Healthy weight range: 56.7 – 76.6 kg
- BMI Prime: 0.91
Understanding the result
Body Mass Index with WHO category and healthy weight range. Start with the base example, then replace each value with the figure that matches your situation.
Units
The example uses Metric (kg, cm) for units. This is an illustration, not a recommended value. Keep the unit and time period shown by the field.
Weight (kg or lb)
The example uses 70 for weight (kg or lb). This is an illustration, not a recommended value. Keep the unit and time period shown by the field.
Height (cm or inches)
The example uses 175 for height (cm or inches). This is an illustration, not a recommended value. Keep the unit and time period shown by the field.
What the comparison means
The comparison changes only weight (kg or lb), keeping every other example input fixed. This isolates that input's effect on the primary result. It does not predict future outcomes.
This is a screening or planning estimate, not a diagnosis. Age, pregnancy, medication and individual medical history can change how the result should be interpreted.
Adult BMI categories (CDC and WHO)
| BMI | Category | |---|---| | Below 18.5 | Underweight | | 18.5 – 24.9 | Healthy weight | | 25.0 – 29.9 | Overweight | | 30.0 and above | Obesity |
Some health authorities use lower cut-offs for people of Asian descent because health risks can rise at lower BMI values. Children and teens use age- and sex-specific BMI percentiles instead of these adult categories.
Imperial formula
BMI = 703 × weight (lb) ÷ height (in)². For 176 lb and 70.9 in: 703 × 176 ÷ 5,027 ≈ 24.6.
People also ask
Is BMI accurate for athletes?
Not always. BMI cannot tell muscle from fat, so muscular people can be labelled overweight. Waist circumference or a body fat estimate gives more context.
What is a healthy weight for my height?
Multiply your height in metres squared by 18.5 and by 24.9. For 1.75 m: 1.75² = 3.06, so roughly 56.7 kg to 76.3 kg.
Does BMI change with age?
The adult formula and categories are the same for all adults, but body composition changes with age, so doctors interpret BMI alongside other measures.
What should I do with my result?
Use it as a screening number. If your BMI is outside the healthy range, a doctor can assess waist size, blood pressure, blood sugar and other markers before recommending any change.
Comparison
| Scenario | Input | Result |
|---|---|---|
| Lower input | Weight (kg or lb): 56 | BMI: 18.3 kg/m² |
| Base example | Weight (kg or lb): 70 | BMI: 22.9 kg/m² |
| Higher input | Weight (kg or lb): 84 | BMI: 27.4 kg/m² |
Common mistakes
- Using the wrong units for weight (kg or lb).
- Comparing results while changing more than one input at a time.
- This is a screening or planning estimate, not a diagnosis. Age, pregnancy, medication and individual medical history can change how the result should be interpreted.
Checklist
- Confirm units.
- Confirm weight (kg or lb).
- Confirm height (cm or inches).
- Check the result against your source or provider.
Calculators Percentage Editorial Team
Calculator guides from the Calculators Percentage team. These introductory examples are educational, not professional advice.
Last reviewed:
This is a screening or planning estimate, not a diagnosis. Age, pregnancy, medication and individual medical history can change how the result should be interpreted.