“What does the BMI on my health checkup actually mean?” or “what is the healthy weight for 170 cm?” - these questions come up as soon as you start managing your health.
In this article, we explain how to calculate your BMI: the formula, the Japanese classification, healthy weight, and the limits of BMI - with concrete examples.
What is BMI?
BMI (Body Mass Index) is an international indicator that evaluates your build from the balance of height and weight. It divides weight (kg) by the square of height (m):
BMI = weight (kg) ÷ height (m) ÷ height (m)Example: 170 cm and 65 kg
- 65 ÷ 1.7 ÷ 1.7 = 22.5
BMI was devised in the 1830s by the Belgian mathematician Adolphe Quetelet, so it is also called the Quetelet index. Its strength is comparing “how heavy” between people after removing the effect of height.
Key point: Taller people naturally weigh more, so comparing raw weight is unfair. BMI corrects for height, letting people of different heights be compared.
BMI classification
In Japan, the criteria of the Japan Society for the Study of Obesity are used:
| BMI | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to under 25 | Normal weight |
| 25 to under 30 | Obese (class 1) |
| 30 to under 35 | Obese (class 2) |
| 35 to under 40 | Obese (class 3) |
| 40 or above | Obese (class 4) |
Normal weight means 18.5 to under 25. The WHO uses the same 18.5 and 25 thresholds, though the obesity classes differ slightly from Japan’s.
Example weights at 170 cm
| BMI | Approximate weight |
|---|---|
| 18.5 | about 53.5 kg (lower end of normal) |
| 22 | about 63.6 kg (healthy weight) |
| 25 | about 72.3 kg (border of obesity) |
Calculating your healthy weight
Healthy weight is based on the Japan Society for the Study of Obesity position that “the BMI with the lowest disease risk is 22”:
Healthy weight = height (m) × height (m) × 22Example: 170 cm
- 1.7 × 1.7 × 22 = 63.6 kg
Knowing how far your weight is from the healthy weight makes diet and weight-gain goals much easier to set.
Goal-setting tip: If the gap to your healthy weight is large, don't aim for it all at once. Start with a small goal like "3 kg less than now" - it is far easier to sustain.
Limits of BMI
BMI is a simple indicator calculated from only height and weight. It can diverge from actual body fat in these cases:
- Muscular people: muscle is denser than fat, so people who train can score high BMI despite low body fat
- Older adults: bone and muscle loss lowers weight, so BMI may read lower than reality
- Children: different criteria apply during growth; infants use other indexes such as the Kaup index
- Pregnancy: weight gain reflects the baby and maternal changes
Caution: Do not judge your health by BMI alone. If you are concerned, measuring body fat percentage or consulting a medical professional is more reliable.
How to calculate your BMI
The Tools Hub BMI calculator shows your BMI, classification and healthy weight together - just enter height and weight.
Enter height and weight
Type your height (cm) and weight (kg).
Check BMI and classification
Your BMI and classification (underweight, normal, obese) are color-coded.
See the gap to healthy weight
The difference between your weight and the healthy weight for your height is shown too.
The tool introduced in this article
BMI Calculator
Follows the Japanese obesity criteria. All in your browser with nothing saved.
Summary
- BMI = weight (kg) ÷ height (m) ÷ height (m)
- Japanese criteria: below 18.5 is underweight, 18.5-25 is normal, 25+ is obese
- Healthy weight = height (m) × height (m) × 22
- BMI can diverge for muscular or older people
- Start with small goals rather than the full healthy weight at once
- The Tools Hub BMI calculator works entirely in your browser
Whenever you need to check your BMI, use the formulas above or the Tools Hub BMI calculator.
FAQ
What is the BMI formula?
Weight (kg) ÷ height (m) ÷ height (m). For 170 cm and 65 kg: 65 ÷ 1.7 ÷ 1.7 = 22.5.
What are the BMI cutoffs?
Under the Japanese criteria: below 18.5 is underweight, 18.5 to under 25 is normal, and 25 or above is obese.
How do I calculate my healthy weight?
Height (m) × height (m) × 22. For 170 cm: 1.7 × 1.7 × 22 = 63.6 kg.
Can someone with a high BMI still be healthy?
Yes. Muscular people can score high BMI with low body fat. Consider body fat percentage as well as BMI.
