What Is BMI and What Does Your Number Actually Mean?

BMI — or Body Mass Index — is one of the most commonly used health measurements in the world. Doctors, gyms, diet apps, and health check-up reports all reference it. Yet many people who know their BMI number are uncertain about what it actually means, how it is calculated, or whether it is the right tool for assessing their health. This article explains BMI clearly, including how to interpret it in the Indian context and where its limitations lie.

What Is BMI?

Body Mass Index is a numerical value calculated from a person's height and weight. It was developed in the 19th century as a simple way to classify body weight at a population level. Over time it has become one of the most widely used tools for initial screening of weight-related health risks in clinical and public health settings.

BMI is not a direct measurement of body fat. It is a ratio that provides a rough estimate of whether a person's body weight is in a range associated with good health, or in a range associated with increased risk of certain health conditions.

How Is BMI Calculated?

BMI is calculated by dividing your weight in kilograms by the square of your height in metres.

• Formula: BMI = Weight (kg) ÷ Height (m) × Height (m)
• Example: A person who weighs 70 kg and is 1.65 m tall has a BMI of 70 ÷ (1.65 × 1.65) = 70 ÷ 2.72 = 25.7

You do not need to calculate this manually. Online BMI calculators — including the one available on MeriDiet — do this instantly when you enter your height and weight.

What Do BMI Categories Mean?

BMI values are grouped into categories that correspond to different levels of health risk. It is important to note that the standard international categories — set by the World Health Organisation — use different cutoff points than the guidelines recommended for South Asian populations, including Indians. Research has shown that Indians tend to carry a higher proportion of body fat and face a greater risk of metabolic conditions at lower BMI values compared to people of European descent.

The following ranges reflect the guidelines recommended for Asian and Indian populations:

• Below 18.5 — Underweight
• 18.5 to 22.9 — Normal weight
• 23.0 to 27.4 — Overweight
• 27.5 and above — Obese

For comparison, the standard WHO categories define overweight as 25.0 and above, and obesity as 30.0 and above. The lower cutoffs for Indians reflect the fact that health risks — particularly those related to diabetes, heart disease, and metabolic syndrome — tend to appear at lower body weights in South Asian populations.

What Does Your BMI Number Tell You?

Underweight (Below 18.5)

A BMI below 18.5 may indicate that a person is carrying less body weight than is associated with good health. This can be linked to nutritional deficiencies, low muscle mass, or underlying health conditions. Being underweight has its own set of health risks, including weakened immunity, bone density loss, and fatigue.

Normal Weight (18.5 to 22.9)

A BMI in this range is generally associated with a lower risk of weight-related health conditions for most Indians. This does not guarantee good health — lifestyle factors, diet quality, physical activity, and other markers all matter — but it suggests that body weight is likely in a healthy range.

Overweight (23.0 to 27.4)

A BMI in this range suggests excess body weight that may begin to increase the risk of conditions such as type 2 diabetes, high blood pressure, and cardiovascular disease, particularly in the Indian context. This range is often where early lifestyle intervention — including dietary changes and increased physical activity — can be most effective.

Obese (27.5 and Above)

A BMI of 27.5 or above in Indian adults is associated with a meaningfully higher risk of serious health conditions including type 2 diabetes, heart disease, sleep apnoea, joint problems, and certain types of cancer. Medical evaluation and structured lifestyle intervention are generally recommended at this level.

What BMI Cannot Tell You

BMI is a useful screening tool, but it has well-recognised limitations. Understanding these limitations helps you use the number appropriately rather than treating it as a definitive assessment of your health.

• BMI does not measure body fat directly: Two people with identical BMIs can have very different body compositions. A person with high muscle mass may have a BMI in the overweight range despite having a low body fat percentage. Conversely, someone with a normal BMI may carry a relatively high proportion of body fat with little muscle.
• BMI does not account for fat distribution: Where fat is stored on the body matters for health risk. Fat stored around the abdomen — sometimes called visceral fat — is associated with greater health risk than fat stored in other areas. BMI gives no information about fat distribution.
• BMI does not distinguish between muscle and fat: Muscle is denser and heavier than fat. Athletes and people who exercise regularly often have higher BMIs due to muscle mass, not excess fat.
• BMI is a population-level tool: It was designed to describe patterns across large groups of people, not to provide a precise individual health assessment.
• BMI does not account for age-related changes: Body composition changes with age — muscle mass typically decreases while fat mass may increase even if weight remains stable. BMI does not capture these shifts.

Other Measurements That Give a Fuller Picture

Because of its limitations, BMI is most useful when considered alongside other measurements. The following are worth knowing:

Waist Circumference

Waist circumference is a practical indicator of abdominal fat. For Indian adults, health organisations generally suggest that a waist circumference above 90 cm in men or above 80 cm in women is associated with increased health risk. This measurement is easy to take at home with a measuring tape and provides information that BMI alone does not.

Waist-to-Height Ratio

Dividing your waist circumference by your height gives a waist-to-height ratio. Research suggests that keeping this ratio below 0.5 — meaning your waist should be less than half your height — is a reasonable target associated with lower cardiometabolic risk. This is a simple measure that many health professionals consider a better predictor of health risk than BMI alone.

Body Fat Percentage

Body fat percentage directly measures the proportion of your body that is fat, as distinct from muscle, bone, and water. This can be estimated using methods such as bioelectrical impedance analysis (available in some weighing scales and fitness centres) or more accurate clinical methods. While harder to measure than BMI, body fat percentage provides more meaningful information about body composition.

How to Use BMI When Setting Weight Loss Goals

If your BMI indicates that you are in the overweight or obese range, it is a useful signal that working towards a lower body weight may reduce your health risk. However, BMI alone should not be your only metric for progress.

As you lose weight through a combination of dietary changes and physical activity, you may notice improvements in energy levels, waist circumference, blood pressure, blood sugar, and overall fitness — sometimes before your BMI moves into the next category. These improvements matter even if your BMI number changes slowly.

A realistic and sustainable weight loss goal for most people is 0.5 to 1 kilogram per week. At this rate, a meaningful reduction in BMI typically takes several months of consistent effort. This is normal and expected — slow, steady progress is generally more sustainable than rapid weight loss.

Final Thoughts

BMI is a simple, accessible, and reasonably useful tool for getting an initial sense of whether your body weight is in a healthy range. For Indians, using the lower Asian cutoff values gives a more accurate picture of health risk than the standard international categories.

At the same time, BMI is one piece of information — not a complete health assessment. Your diet quality, physical activity level, waist circumference, blood markers, sleep, and stress levels all contribute to your overall health in ways that a height-and-weight ratio cannot capture. Use BMI as a starting point for awareness, not as a final verdict on your health.