BMI Calculator: Formula, Chart & Healthy Weight Range
Body Mass Index (BMI) is still the most searched quick check for whether weight sits in a healthy range for height. A free BMI calculator applies the standard adult formula in seconds — metric or imperial — and maps the result to WHO categories.
This guide covers the formulas, the adult BMI chart, how to read a healthy weight range, and when BMI misleads (athletes, older adults, pregnancy). It is educational, not medical advice.
Check adult BMI in metric or imperial units
Free BMI calculator — category, healthy weight range, no signup.
BMI Formula Explained
BMI is weight divided by height squared. Use kilograms and meters, or the imperial form with the 703 constant:
Imperial: BMI = 703 × lb ÷ in²
Worked example
Height 170 cm (1.70 m), weight 70 kg → BMI = 70 ÷ (1.70²) = 70 ÷ 2.89 ≈ 24.2 (normal). Same person at 5′7″ and 154 lb uses the imperial formula and lands in the same band.
Adult BMI Chart (WHO)
| Category | BMI |
|---|---|
| Underweight | < 18.5 |
| Normal (healthy) | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | ≥ 40.0 |
Some Asian health guidelines use lower overweight/obesity thresholds. Always follow guidance from your clinician or local public-health standards.
Check adult BMI in metric or imperial units
Free BMI calculator — category, healthy weight range, no signup.
Healthy Weight Range
For a given height, multiply m² by 18.5 and 24.9 to estimate the kilogram band that maps to a “normal” BMI. The online BMI calculator shows both kg and lb ranges so you do not have to recompute by hand.
- Use morning weight and accurate height for consistency.
- Track trends over weeks, not a single day's number.
- Combine with waist measure and fitness goals for context.
When BMI Is Misleading
BMI ignores body composition. Strength athletes, people with high muscle mass, and some older adults with low muscle can be misclassified. Pregnancy needs obstetric care, not adult BMI charts. Children need percentile charts, not this adult tool.
Not a diagnosis. BMI is a population screening tool. Discuss results with a qualified health professional before changing diet or medication.
Related Tools on ToolMars
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Check adult BMI in metric or imperial units
Free BMI calculator — category, healthy weight range, no signup.
Frequently Asked Questions
How do you calculate BMI?
kg ÷ m², or 703 × lb ÷ in² for imperial.
What is a healthy BMI?
Usually 18.5–24.9 for adults under WHO cutoffs.
Is BMI accurate for athletes?
Often not alone — muscle can inflate BMI without excess fat.
Is the BMI calculator free?
Yes on ToolMars — no signup.