BMI Calculator
Use this tool to calculate Body Mass Index in metric or imperial units.
Quick answer: how to do BMI on a calculator
You can calculate BMI (Body Mass Index) with a basic calculator in under a minute. The formula depends on the unit system you use:
- Metric: BMI = weight (kg) ÷ [height (m)]²
- Imperial: BMI = 703 × weight (lb) ÷ [height (in)]²
Once you get the number, compare it with the BMI categories to interpret your result.
Step-by-step: BMI in metric units (kg and cm)
Formula
BMI = kg / (m × m)
How to do it manually
- Write down your weight in kilograms.
- Convert your height from centimeters to meters (divide by 100).
- Multiply height in meters by itself (square it).
- Divide weight by that squared height.
Example: 70 kg, 175 cm
175 cm = 1.75 m
1.75 × 1.75 = 3.0625
BMI = 70 ÷ 3.0625 = 22.9
Step-by-step: BMI in imperial units (lb, ft, in)
Formula
BMI = (703 × lb) / (in × in)
How to do it manually
- Write down your weight in pounds.
- Convert your height to total inches (feet × 12 + inches).
- Square your total inches.
- Multiply your weight by 703.
- Divide by squared inches.
Example: 154 lb, 5'9"
Total inches = (5 × 12) + 9 = 69
69 × 69 = 4761
703 × 154 = 108,262
BMI = 108,262 ÷ 4761 = 22.7
How to interpret your BMI result
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity (Class I) |
| 35.0 to 39.9 | Obesity (Class II) |
| 40.0 and above | Obesity (Class III) |
Common calculator mistakes to avoid
- Using cm as m: 175 must become 1.75 in the metric formula.
- Not squaring height: Height must be multiplied by itself.
- Mixing units: Don’t combine kg with inches or lb with meters.
- For imperial: Always convert full height to total inches first.
- Rounding too early: Keep decimals until the final answer.
Is BMI enough on its own?
BMI is useful for quick screening, but it is not a full health diagnosis. It does not directly measure body fat, muscle mass, or fat distribution. Athletes, older adults, and people with high muscle mass can get a BMI that doesn’t fully reflect overall health. For better context, pair BMI with waist measurement, blood pressure, lab work, and guidance from a healthcare professional.
FAQ
Can I calculate BMI on a phone calculator?
Yes. Just follow the same formula and unit steps shown above.
What is a good BMI number?
For most adults, 18.5 to 24.9 is considered the healthy range.
Do children use the same BMI chart?
No. Children and teens use BMI-for-age percentiles, not adult categories.
Bottom line
If you’re wondering how to do BMI on calculator, the process is straightforward: use the correct formula, keep units consistent, and compare your result to the standard BMI ranges. Use the calculator above for a fast result, then track trends over time instead of obsessing over a single number.