How is BMI calculated?
Body Mass Index (BMI) estimates body fat from your weight relative to your height. The World Health Organization (WHO) formulas are:
Or, in imperial units:
Both formulas give the same result — pick whichever matches your units, or let the calculator do it for you.
BMI categories (WHO)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity — Class 1 |
| 35.0 – 39.9 | Obesity — Class 2 |
| 40.0 and above | Obesity — Class 3 |
Healthy weight ranges by height
The weight range corresponding to a BMI of 18.5–24.9, for common adult heights:
| Height | Metric | Imperial |
|---|---|---|
| 5′0″ (152 cm) | 42.7 – 57.5 kg | 94 – 127 lb |
| 5′3″ (160 cm) | 47.4 – 63.7 kg | 104 – 140 lb |
| 5′6″ (168 cm) | 52.2 – 70.3 kg | 115 – 155 lb |
| 5′9″ (175 cm) | 56.7 – 76.3 kg | 125 – 168 lb |
| 6′0″ (183 cm) | 62.0 – 83.4 kg | 137 – 184 lb |
| 6′3″ (191 cm) | 67.5 – 90.8 kg | 149 – 200 lb |
What BMI can't tell you
BMI is a useful first-screen at population level, but it has well-known limits. It may be misleading for:
- Athletic people — muscle weighs more than fat, so muscular people are often classified "overweight".
- Adults over 65 — healthy BMI ranges shift slightly upward with age.
- Children and teens — they need age-and-sex percentile charts, not the adult formula.
- Pregnant women — the formula does not apply.
- Ethnic groups — some populations face health risks at lower BMI thresholds.
For a complete picture, BMI should be combined with other measures such as waist circumference and body composition — and interpreted with a healthcare professional.
Frequently asked questions
How is BMI calculated?
BMI = weight (kg) ÷ height² (m²), or 703 × weight (lb) ÷ height² (in²). The calculator above applies these WHO formulas instantly.
What is a healthy BMI?
According to the WHO, a healthy BMI for adults is between 18.5 and 24.9. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above falls into the obesity classes.
Is BMI accurate for everyone?
No — it's a screening tool, not a diagnosis. It doesn't distinguish muscle from fat, and it isn't valid for children, pregnant women or the elderly without specific adjustments.
Is this BMI calculator free?
Yes — 100% free, unlimited, no sign-up. CalculUnits is funded by non-intrusive ads, never by paywalls.